Basics Of MRI Physics PDF

▪ Only certain nuclides with an odd number of neutrons and BASICS OF MRI PHYSICS protons are magnetic neutrons and protons that make up Delivered by: Dr. Eda Mae S. Agustin, MD...

▪ Only certain nuclides with an odd number of neutrons and BASICS OF MRI PHYSICS protons are magnetic neutrons and protons that make up Delivered by: Dr. Eda Mae S. Agustin, MD, DPBR a nucleus had an intrinsic angular method or spin. ▪ Pairs of neutrons and protons lie in situating that are space-cancelled. However, when there is an odd number of protons or neutrons in odd mass numbers, some of the spins will not be cancelled and the total nucleus will have a net spin characteristic. ▪ Net spin characteristic is a spinning characteristic of a particle with an electric charge the nucleus that produces a magnetic probably known as the magnetic moment. ❖ Certain materials such as tissue are placed in a strong magnetic field, two things happen wherein the material got a resonant characteristic and they become magnetized. ❖ Resonance can absorb materials and re-radiate radiofrequency radiation at a specific frequency (e.g., radio receiver transmitter) o It is the nuclei of the atoms that resonate which Has odd number of protons → Net spin characteristic states the general phenomenon of nuclear Spinning characteristic → produces a magnetic moment magnetic resonance (NMR). ❖ The resonant frequency material such as tissue is typically ATOMIC MASSES OF FIRST 30 ELEMENTS in the radiofrequency range so that the emitted radiation is in the form of radio signals. Atomic Elements & Atomic Mass (After Number Symbols Rounding Off ❖ Characteristics of RF signals emitted by materials or 1 Hydrogen (H) 1 determined by certain physical and chemical characteristics of the material. 2 Helium (He) 4 ❖ The RF signals produced by the NMR process can be 3 Lithium (Li) 7 displayed either in the form of images (MRI) or as a graph 4 Beryllium (Be) 9 depicted chemical composition (Magnetic Resonance 5 Boron (B) 11 Spectroscopy [MRS]) 6 Carbon (C) 12 7 Nitrogen (N) 14 BASIC PHYSICS 8 Oxygen (O) 16 The concentration of chemical elemental tissue covers a considered range ❖ MRI signal comes from H+ nuclei in water and fat because depending on a tissue type and such factors as metabolic or pathologic state magnetic signals can only be emitted from free-flowing substances or substances that exhibit Brownian motion ▪ 4 elements that make up 99% of tissue mass: o Brownian motion is the extrinsic motion of all o Hydrogen (The only one that would release the atoms of the body. It is usually seen in free-flowing more isotopes that has strong magnetic nucleus) objects as it is in water and fat. o Carbon o Nitrogen WHY HYDROGEN? o Oxygen ▪ Nucleus at the hydrogen-1 atom is a single proton. ▪ Materials that participate in the MR process must nuclei ▪ Among the all the chemical elements, hydrogen (H or themselves in specific method and compromise in order H+) is unique that it occurs in relatively high to interact with the magnetic field and decline themselves to concentrations in most tissues and the most abundant small magnets and have a magnetic property or magnetic isotope (H-1) has a magnetic nucleus. moment. ▪ The magnetic characteristics of an individual in determined by its neutron-proton composition. Kan | 1 ❖ Subatomic particles possess spin or spin angular moment. →↓ ⇉ The spin gives the proton a magnetic moment. No magnetic field: No Net Magnetization Vector Each hydrogen proton behaves like a tiny magnet or defector. When a material containing magnetic modalities placed in a magnetic field (e.g., MRI machine), the nuclei experience a torque that encourages them to align in the direction of the field. However, in the human body, thermal energy averagely condemned since most of them are aligned or direction of the magnetic field. Alignment along the direction of the magnetic field (z-axis denotes B0 which is the low energy plane) requires low energy and thus, nuclei preferentially take on this H+ proton 'spin' → magnetic moment configuration. ❖ Magnetic moment is represented by small arrow o Z-axis is longitudinal down the bar of the magnet. passing through the nucleus. External magnetic field (i.e., MRI machine): ❖ If you think of the nucleus as a small conventional o Net Magnetization Vector Aligns magnet, the magnetic moment arrow corresponds to the south pole-north pole direction of the magnet. RADIOFREQUENCY (RF) PULSE ❖ Hydrogen is an odd number which gives a dipole ❖ It translates the net magnetization into an MRI signal and characteristic wherein its end has a positive and negative is designed to tip the net magnetization away from the z- charge due to polarity. This polarity allows it spin known axis. as the magnetic moment. ❖ It is essentially a smaller, varying magnetic field applied at a specific frequency in a direction perpendicular to the NUCLEAR ALIGNMENT main magnetic field. Net magnetization vector in MRI is the summation of ❖ It is not a radio wave but it is an electromagnetic energy all magnetic moments of the individual hydrogen nuclei. with frequency that is in the radio wave spectrum. In the absence of external magnetic field, all the time ❖ In order to tip the magnetization away from the z-axis, magnets within the nucleus of an echo are pointed in all the RF must be at the frequency that matches the spin of different directions. what is trying to do (i.e., flip). The number of opposing protons is more or less equal in the work and since they are opposing, they also cancel each other out. Therefore, net magnetization vector in tissue is zero. ⇅⇆ Net magnetization vector = 0 EXCITATION In MRI, RF pulse is used to flip that flips some of the nuclei in the transverse plane of the magnetic field. In this excited state, the precession is performed in the spinning of the nucleus around the axis of the magnetic field. Precession is the turning motion that has a dipole system. o However, since it’s in the magnetic field, it can no longer turn around its own axis. Instead, it will go around the magnetic field Kan | 2 RADIOFREQUENCY PULSE (B1) – emitter 1. Tips the net magnetization into the XY plane, perpendicular to the Z axis (i.e., 90-degree flip angle). o How much it tips away from the z-axis is called the flip angle. 2. Causes precession of the H+ nuclei. In this excited state, the precession is transformed into a spinning motion of the nucleus around the axis of the magnetic field. It should be noted that the spinning motion is an enhanced precession which is different from the intrinsic spin of the nucleus around its own axis. RF tips the net magnetization away from the Z-axis and into the XY plane How The significance of a magnetic nucleus spinning around much it tips away from the Z-axis → FLIP ANGLE (arbitrarily 90֯) → XY or the axis of the magnetic field is that its motion transverse plane is indicated by B1 (transverse to B0 or the Z-axis) generates an RF signal. It is the signal from many nuclei that is collected to from PRECESSION AND RESONANCE the MR image wherein protons within the system functions as both the receiver and emitter of the RF pulse/signals. When an external magnetic field is applied to protons, they align in the direction of the magnetic field or along the z-axis which requires low energy. When the spinning magnet nucleus aligns the magnetic field, it is now at fix. The nucleus precesses, or oscillates a body axis at the magnetic field. Precession is physical phenomenon that results in an interaction between the magnetic field and the spinning momentum of the nucleus and is akin to a wobbling motion of the spinning top. If the pulse of RF energy when the frequency o At this point, protons are precessive along the same corresponding to the nuclear precession rate is applied axis and are therefore, precessive in sync with each to the material, some of the energy will absorb by the other. individual nuclei. The significance of the nuclear precession is that it causes o This absorption of energy by a nucleus flips its the nucleus to be extremely sensitive or true to the RF energy alignment away from the direction of the magnetic that has a frequency identical with the precession frequency. field This increase in energy places the nucleus in unnatural RESONANCE or excited state. Basis of all MR procedures and is fundamental to RF Pulse has two things: absorption and emission of energy by many objects or device. Kan | 3  However, the MR signal rapidly placed due to independent processes that reduce transverse magnetization and thus, cause a return to a stable state present before excitation (i.e., spin-lattice interaction and spin-spin interaction) 2 MECHANISMS: a) Longitudinal relaxation - Longitudinal magnetization recovery nuclear precession - causes nucleus to be extremely sensitive, or tuned, to RF b) Transverse relaxation - Transverse magnetization decay energy that has a frequency identical with the precession frequency (rate) → condition is known as resonance → basis for all MR procedures T1: LONGITUDINAL RELAXATION o NMR is the process in which a nucleus resonates Transverse magnetization decays and magnetic moments or “tunes in” within a magnetic field gradually realign with the z-axis of the main magnetic field Objects are most effective in exchanging energy at their B0. own resonant frequency. ➔ As transverse magnetization decays, the longitudinal The resonance of an object or device is determined by magnetization (Mz), the projection of the certain physical characteristics. magnetization vector onto the z-axis is lowly distort. Frequency during proton precession motion is called This process is known longitudinal relaxation. Larmor Frequency. The nuclei can return to the ground state only by dissipating their excess energy to their surroundings. ➔ The surrounding of the nuclei is reverted at the lattice (i.e., spin-lattice relaxation) Frequency (f) is determined by the magnetic field B0 When flipping the vector of the protons at 90o to its xy plane and it goes back to the z-axis (TI Relaxation), the time that the glide The stronger the magnetic field, the faster the protons precess in occurs is very specific from high to low energy and the process is sync after the RF pulse is applied called TI relaxation. Magnetic Field Strength = Independent Variable F0 α B 0 Larmor frequency of proton = 42MHz at I Tesla, o 127 MHz at 3 Tesla The higher is the tesla strength, the higher is the frequency which T2/T2*: TRANSVERSE RELAXATION results to better images Phase is a position of a magnetic moment on its circular RELAXATION precessional path. It expresses as an angle. ❖ It is a return to equilibrium of net magnetization. IMMEDIATELY AFTER EXCITATION ❖ During relaxation, the electromagnetic energy is retransmitted to RF emission called as the NMR signal. Part of it spins precess synchronously that have a phase of 0o and are said to be "in phase". This state is called IMMEDIATELY AFTER EXCITATION: phase coherence. Magnetization rotates in the xy-plane → transverse magnetization (Mxy) Transverse relaxation results from spins getting out of Rotating transverse magnetization that gives rise to phase. As spins move together, their magnetic fields the MR signal in the receiver coil. interact. This is called the spin-spin interaction wherein interactions are temporary and random. Kan | 4  Spin-spin interaction → slightly modify precession rates ❖ They are substances or elements that have (+) unpaired electrons (odd numbers) which gives them a positive Spin-spin relaxation causes a cumulative loss in phase magnetic susceptibility (magnitude = 0.1% of that of causing transverse magnetization. ferromagnetic materials) ❖ They are slightly magnetic compared ferromagnetic and When applying the RF, the protons or particles are precessing they can be used for imaging or in the clinical setting. around the axis of the magnetic field in sync. ❖ Paramagnetic materials are of no danger and pose no risk As the particles peer and interact with each other, the to the safety of both the medical professionals and motions become out of sync wherein they cannot do this patients/clients since its magnetic properties are not synchronized motion indefinitely and momentarily. strong. GADOLINIUM CONTRAST DIFFERENT TISSUES HAVE DIFFERENT RELAXATION TIME (1.5 TESLA) It is currently the only widely used contrast medium for Tissue T1 (ms) T2 (ms) MRI and has (+) 7 unpaired electrons which is the largest Fat (adipose) 250 50 number of electrons that is available. Liver 500 45 It shortens spin-lattice relaxation time which causes Kidney 650 60 brighter T1W images due to better or enhanced White matter 800 90 contrast Grey matter 800 100 o Having an enhanced contrast can to better CSF 2,400 280 delineation of the images or structures within that Different relaxation times is due to different chemical properties image caught. It is given intravenously in the extracellular space (i.e., Each of the chemical properties has a characteristic look blood vessels) due to hyperfusion in order to accumulate on a different sequence. it. o T1-weighted image has an imaged produced by T1 Detection of the gadolinium would determine well- relaxation. perfused structures. Each of the tissue types look different on T1 and T2. COMPONENTS OF THE MRI MACHINE MRI is very ideal for soft tissue assessment due to having good soft tissue conference ❖ Superconducting Magnet ❖ Gradient Coils ❖ Radiofrequency Coils ❖ Computer System When having protons inside an MRI, they are in the z- axis and RF pulse is given which would change the precession along the axis of the magnetic field. It would tilt into 90o which causes to be resonant that would give off signals which is acquired and absorb by the RF coils. After that, the computer system would generate or translate into images. PARAMAGNETIC MATERIALS ❖ Include O2, ions of various metals (Fe, Mg, Gd) The computer system is not the part of the machine Kan | 5 MAGNET Magnets in MRI uses superconducting magnets and it is the heart of the MRI. Almost all magnets used for individual imaging are superconducting magnets of 1.5 Tesla (T) (low field strength) or 3.0 T (high field strength) Superconducting magnets consists of a series of coils wound on a cylindrical form within a bath of liquid helium When having different procedures on different parts of the body, different coils are (He) enclosed in a cryostat. required for the specific need of a certain procedure. Hence, the coils basically take ➔ Helium provides a pooling mechanism for the images in pre-dimensional ways. magnet. As long as the magnet is in the pool property, the MAGNETIC FIELD GRADIENTS: current flowing through the superconducting coils can maintain a magnetic field strength. z-axis - slice selection gradient Y-field gradient - produces phase encoding within the slice x-field gradient - produces frequency encoding within the slice. RADIOFREQUENCY COILS These are used to send RF pulses and receive the signal back from the patient's body. Helium is important due to the MRI machine that is always on When putting the patient in the MRI, the protons are aligned in order to flip to a 90o angle into a pulse. GRADIENT COILS RF coils gives off the pulse that starts the cascade of MR Its primary function is to allow spatial encoding of the MR process and it receives RF pulse or signals that is given signal. off by the protons. Therefore, it is both an emitter and a o Gradience produce an additional magnetic field that transmitter. varies in strength along its along direction that is superimposed on the main magnetic field PRODUCING AN IMAGE FROM MR SIGNALS REQUIRES: 1. Choosing a specific slice within the patient's body to be examined. 2. Determining the voxels to be designated within the slice. Kan | 6 COMPUTER SYSTEM The multiple computer system invented in an MRI scanner have a range of functions: 1. Control the RF and gradient pulses. o It determines the pulses and inputs in the computer on the how much is the frequency applied in the procedure. 2. Collect the data. 3. Process and display the generated image. MRI SAFETY: CONTROLLING ACCESS Without the computer system, wrong pulses may occur and no Zone l: General Public angle images may be seen. Zone II: Unscreened MRI Patients ➔ Parents/ folks of the patients can enter BASICS OF MRI SAFETY Zone III: Screened MRI patients and trained personnel ➔ A control or computer room MRI SAFETY Zone IV: Screened MRI patients under constant direct supervision of trained MR personnel ❖ 2019 update (American College of Radiology [ACR]) ➔ It houses the magnet (MRI scanner) "...the guiding principles of MR safety remain. MR personnel must be appropriately educated, be vigilant in their awareness of a dynamic environment, and apply that knowledge to screening before, and fulfilling patient and staff member safety during their time in the MR suite." LEVEL I VS LEVEL II PERSONNEL Level 1 Personnel: Those who have passed minimal safety educational efforts to ensure their own safety as they work within Zone III. They do not have access to ZONE II Zone IV, but can access Zone IV with permission and by It is the interface between publicly accessible, escort of a Level II trained individual. uncontrolled Zone 1, and strictly controlled Zone III. Usually where patients are screened (e.g., conducting Level I Personnel should complete screening form every time they interviews, determining of patient can proceed with the enter Zone IV procedure [tattoos, piercings]). Level Il Personnel: Those who have been more extensively trained and educated in the broader aspects of MR Safety issues, including, for example, issues related to the potential for thermal loading or burns and direct neuromuscular excitation from rapidly changing gradients. These Individuals have access to Zone IV (i.e., MR Technologists; MR Radiologists) ZONE III The MR Medical Director shall identify those Individuals who Located directly outside the scanner room. qualify as Level II MR personnel. Serious injury or death can result in this area for unscreened patients due to interactions between the “MRI is ALWAYS ON” individuals or equipment and the MR scanners particular environment. Magnetic field extends beyond the room housing the It is a restricted area hence, should have a controlled machine. access. o The layers mitigate the movement or spread of the Employees entering Zone III need training. magnetic field. Kan | 7  FERROMAGNETIC MATERIALS They are strongly attracted to a static magnetic field and it poses the greatest danger to patients and staff in MRI environment. Iron and its alloys - most commonly used metal ZONE IV It is a scanner room wherein the door of the room must always be closed or monitored. If door left open, a "caution" barrier is recommended o Could be easily adjusted straps or plastic chains Technologist should position/approach patient from far side of scanner to view door (and be out of the way of projectiles). Direct supervision over level 1 personnel by level 2 personnel. Any ferromagnetic material that is attracted to the “Screened Patients only” machine would just zoom towards the machine. When that happens, it is called a projectile. All personnel that enter Zone IV must be screened. Getting in the path of the projectiles gets a frequency of a When entering Zone IV, all hazards of the MRI can certain velocity that is going further. Therefore, it is a big already be applied. enough mass that would pose serious physical injury or even, death. MRI ROOM WALL Magnetic Shield Radiofrequency Shield STATIC MAGNETIC FIELDS ❖ No permanent bio-effects at clinical strengths ( 3 Tesla can stimulate peripheral nerves. ❖ Allows clear visualization of the ligamentous, neural, ➔ Mild Cutaneous Sensations discal, muscular, and other soft tissue features along with ➔ Involuntary Muscle Contraction the osseous structures typically required in clinical ➔ Cardiac Arrythmias decision making. FDA LIMITS RADIOFREQUENCY POWER DEPOSITION ❖ Views are usually obtained with slices in sagittal and axial sequences. By measuring Specific Absorption Rate (SAR) o Patients will have to stay in the MRI scanner room Radiofrequency Heating can occur in conductive for a long period in order to get the accurate image materials or MRI safe objects such as tattoos (ink), bone or slice. implants, and ECG leads. Kan | 9 HOW DOES MRI WORK? T1 VS T2 IMAGES Magnetic resonance imaging (MRI) uses the body’s natural T1 images better for viewing anatomy. magnetic properties to produce detailed images from any T2 images better for pathology since these images part of the body. accentuate still fluid. For imaging purposes, the hydrogen nucleus (a single proton) is used because of its abundance in water and fat. Signal intensity T1 - Weighted T2 - weighted (i.e., very sensitive to fluid) High intensity (white) Fat Still fluid (inflammation) The hydrogen proton can be likened to the planet earth, spongy bone Tumor ↓ spinning on its axis, with a north-south pole. Under still fluid Fluid normal circumstances, these hydrogen proton “bar Cartilage Muscle magnets” spin in the body with their axes randomly Tumor Cartilage aligned. Muscle Spongy bone Low intensity o With the use of MRI, it creates a magnetic field Fluid subQ fat using the scanner that would allow the protons to line up. When the body is placed in a strong magnetic field, such T1 T2 as an MRI scanner, the protons’ axes all lineup. Tissue with high water content will Tissue with high water content will This uniform alignment creates a magnetic vector appear dark (grey) e.g., fat, edema, appear white/ brighter e.g., bone infection oriented along the axis of the MRI scanner. Tissue with low water content will Tissue with low water content will appear darker (grey) MRI scanners come in different field strengths, usually appear white/brighter Water is white on T2 (Pneumonics: between 0.5 and 1.5 tesla. Bone, Lungs World War II) HOW ARE MR IMAGES PRODUCED? The radio pulses are stopped, the absorbed energy is released and measured by the computer detector. This info is converted to an image. o The fluid is being measured or scanned Unique images are produced because each tissue has a different amount of hydrogen and relax at different rates. The length and sequence of the pulses produces different quality images of the same tissues. Repetition time (TR) is the time that elapses between two consecutive radio wave pulses CLINICAL USES OF MRI Echo Time (TE) is the time selected to wait after the a) MRI is very sensitive for detecting changes and variations start of the TR to receive the signal or “echo” from the in bone marrow. patient. b) MRI excels in the display of soft tissue detail. (i.e., best tool to assess tissue pathologies) Varying the TR and TE will accentuate different tissues c) MRI is the best modality for differential diagnosis Using a shorter TE will produce a T1 weighted image between disk herniations and other causes of nerve root Using a longer TE will produce a T2 weighted image or Impingement. STIR (Short tau inversion recovery) both. d) MRI has the ability to stage neoplasms in bone and soft tissues as well as evaluate the extent of tissue invasion, MR sequences that suppress fat and show pathology – prior to surgery. increase in water bright and fat suppressed. o MRI is the only imaging that can be used to T1 and T2 images are different for each tissue and determine if a fracture is either benign or produce different intensities (degrees of whiteness) of pathologic in nature. the same tissue. e) It is more sensitive than bone scan for detecting bone metastases, although bone scan is more effective as a screening technique. (i.e., scaphoid fractures) Kan | 10 Advantages Limitations CLINICAL THINKING POINTS It allows to visualized superior soft Limitations lie in the imaging of cortical tissue visualization, especially muscles, bone because of its low signal intensity. CLINICAL THINKING POINT 1: BONE BRUISE—THE tendons, ligaments, nerves, articular cartilage and menisci Length of time needed to produce an FOOTPRINT OF INJURY image Okay for spongy bone (high fat content) High cost Bone marrow contusions are frequently identified with No ionizing radiation, no known harm Cannot use with patients who work MRI following musculoskeletal injury due to sensitivity to Best imaging for multiple sclerosis in with metal shavings, metal pacemakers, order to see the black formations. certain types of fixative devices (ferrous inflammation. metals) Very sensitive in stroke These are injuries that typically do not involve permanent Patients should wear clothes that is 100% cotton or hospital gown changes to osseous structures. It cannot be recommended in Bone marrow contusions, which have aptly been called emergency cases the “footprints of injury,” may be the result of traction injury to ligaments, direct blow to the bone or compression forces at joint surfaces during injury. CONTRAINDICATIONS AND HEALTH Frequently an injury that is not visible on radiographic CONCERNS examination or CT is discovered because it leaves a “footprint” in the form of bone marrow edema visible on Ferromagnetic surgical clips can be displaced. In the case MRI. of brain aneurysm clips, such displacement can cause fatal hemorrhage. Orthopedic hardware can cause image distortion, but it generally does not represent a health hazard. (e.g., cemented implants) Other concerns are the following: ➔ Pacemakers may malfunction within or near the magnetic field. ➔ Claustrophobia, which affects about 10% of patients. ➔ The need to sedate patients (such as children) who may not be able to stay still for the duration of the examination. CLINICAL THINKING POINT 2: MR IMAGING OF CT VS. MRI STRESS FRACTURES Stress fractures, which most commonly involve the bones of the lower extremity, are the result of repeated subliminal trauma. This process starts with accelerated turnover and remodeling of bone, which may progress to a stress fracture if the stress continues o Common in the military and marathon runners Radionuclide bone scan – Gold standard for the diagnosis of stress fractures; fastest to diagnose stress fractures about 48 hours. CT is the most accurate diagnostic modality for stress fx in the navicular bone. MRI CT MRI is the most sensitive method for femoral neck stress Better for: Better for: fx Spongy bone Cortical bone Stress fractures is also common in female athletic Soft tissue (ligaments, tendons) Subtle and complex fractures runners due to hormonal changes and long term can Articular Cartilage Calcifications in any tissue develop early osteoporosis causing more risk of stress Hemorrhagic strokes (emergency case) fractures. Kan | 11  Multiple MRI findings (five or more) was associated with mildly greater pain severity at baseline (0.84; 0.50 - 1.17) and greater increase in pain-severity over 6 years in those pain free. CONCLUSION: MRI degenerative findings we examined, individually or in combination, do not have clinically important associations with LBP, with almost all effects less than one unit on a 0 to 10 pain scale. Figure 5-15 Stress fracture of the calcaneus in a 23-year-dd female runner. The posteroinferior half of the calcaneus shows an area of decreased signal intensity on SUMMARY OF KEY POINTS Tl (left); a slightly larger area of increase in signal intensity is seen on T2. Both images show an oblique line of low signal intensity on extending across the posterior 1. MR images are made on the basis of energy emitted by talus, representing the fracture line. (Image courtesy of John C Hunter, MD, protons during their re-alignment with the main magnetic University of California, Davis School of Medicine) field. 2. Diagnosis is often based on the differences between T1- MRI has the value, over the other imaging modalities, of weighted and T2-weighted images. being able to demonstrate often considerable soft tissue 3. T1-weighted images demonstrate great anatomical abnormalities adjacent to the fractured bone. detail and tend to highlight structures rich in fat, while MRI investigations of the progression of stress fractures T2-weighted images are grainier and emphasize have revealed that not all stress reactions visible on MRI give structure with high free-water content and inflammation. rise to stress fractures. 4. Sequences, such as SE Sequences (T1 and T2, as well as o Other stress fractures can be called as bone proton density) and GRE sequences, are different methods conclusions due to sensitivity. for capturing the MR signal. 5. Protocols refer to the choice of imaging planes and combinations of sequences used for certain clinical conditions. 6. Contrasts (e.g., gadolinium) can be used intravenously for the purpose of highlighting structures or pathology with rich blood supply, or be used in intra-articular injections (MR arthrography). 7. Open and upright scanners reduce the problem of claustrophobia and offer the possibility for imaging in a METHODS: weight-bearing position, but are associated with lower field strength and longer imaging times. Participants (n+3369) from a population-based cohort 8. Structures of high density, like cortical bone, ligaments, study were imaged at study entry, with LBP status menisci, and tendons, are dark (have low signal intensity) measured at baseline and 6-year follow-up. MRI scans on all MRI sequences, while most other structures show were reported on for the presence of a range of MRI different signal intensities on T1-weighted images, as findings. LBP status was measured on a 0 to 10 scale. compared to T2. 9. MRI excels at detecting changes in bone marrow, RESULTS: displaying soft tissue detail, and demonstrating areas of inflammation. MRI findings were present in persons with and without 10. Advantages of MRI over CT include no use of ionizing back pain at baseline. Higher proportions were found in radiation, greater contrast resolution, and greater ability to older age groups. 76.4% of participants had a least one image structures surrounded by bone. MRI finding and 8.3% had five or more different MRI 11. Disadvantages of MRI include long imaging times and findings. expense. In the longitudinal analyses, most MRI findings were not associated with future LBP severity regardless of the presence or absence of baseline pain. Kan | 12 BASICS OF CT SCAN Delivered by: Dra. May Connie Joy Leda-Panaligan, MD, FPCR, FUSP COMPUTER TOMOGRAPHY (CT) The tube in the CT scan contains detectors that spins very fast around it. As it spins, it takes pictures in 360o of the patient. Unlike the x-ray, it gets the picture in only one direction (i.e., 900 or 450) depending angulation or request. HOUNSFIELD UNIT (H) Every chemical property has its own Hounsfield unit. This unit is named after Sir Godfrey N. Hounsfield who invented the CT. Water is assigned a value of 0 to 20 The number of slices in CT scan is upgraded for 1 second Bone +400 to +1000 ❖ Provides radiographic image of slices of a living patient. Soft tissue +40 to +80 ❖ Displays each imaged slice separately, without the Fat -60 to -100 superimposition of blurred structures. Air -1000 ❖ A narrow, well collimated beam of x-rays is generated on These units are important because it will help in identifying what structures is being one side of the patient, the x-ray beam is attenuated by identified. absorption and scattered as it passes through the patient. A lesion that has a bony, fatty, and soft tissue component is called (i.e., same mechanism with the x-ray and how its images is teratoma. generated) ❖ Unlike an x-ray machine, the CT scan generates its images rapidly. According to the picture, the highest degree of whiteness is the cortical bone. It is important to know the contrast media in order to identify the structures being identified especially in blood vessels if it has calcifications or just contrast media. ❖ Sensitive detectors on the opposite side of the patient Contrast media has a usual attenuation of 100 to 500 HU measure x-ray transmission through a slice. These ADVANTAGES measurements are systematically repeated many times from different directions while the x-ray tube is pulsed as 1. Rapid acquisition it rotates 360o around the patient. o How fast can we do a CT scan ❖ The tube or round thing in the CT scan is known as the o It can be used for emergency cases such as stroke gantry. due to disoriented patients (i.e., Radiologist cannot give instructions because the patient is not able to follow what he/she is trying to say) Kan | 13  o Patients who had fractures due to vehicular NEUROANATOMY accidents can also do CT scan. 2. Superior bone detail CRANIAL o Temporal bone is most common cranial fracture. 3. Superior demonstration of calcifications RADIATION DOSE CT now accounts for more than 40% of all radiation exposure to patients from diagnostic imaging. Radiation given to the patient is very minimal. CONTRAST ADMINISTRATION (Axial view of the cranial CT scan) This kind of view that is usually given ▪ Intravenous iodine-based contrast agents are administered in CT to enhance density differences ❖ Brain is bilaterally symmetrical. Deviations in sizes should between lesions and surrounding parenchyma, to not be significant. demonstrate vascular anatomy and vessel patency, and to ❖ It is considered to be significant of it is not more than 3 characterize lesions by their patterns of contrast mm or 0.3 cm. enhancement. ▪ Contrast media is given in order to further enhance lesions. (e.g., tumors, inflammations) o A patient that has a tumor, inflammation, and other form has high vascularity wherein there is an increased uptake of the contrast media. o 15 HU is needed to increase the enhancement of a lesion in order to be significant. (e.g., granuloma) CONTRAST AGENTS ▪ Iodinated Contrast Agents Ionic Contrast Agents Non-ionic Contrast Agents High osmolality – Can cause significant Low osmolality – Significant decrease in hemodynamic, cardiac, and subjective adverse reactions effects Used recently The black form in the picture are the mastoid cells Axial view or Angiogram procedure of the Circle of Willis (Cranial CT scan with contrast media in order to enhance vascular structures) Kan | 14  Contrast media is used to enhance vascular structures especially during hemorrhage (i.e., stroke secondary to aneurism) Patients are selected to undergo angiogram procedures due to its cost. Sagittal View of the Cervical Spine (X-ray [L], CT scan [R]) Image of the angiogram Axial View of the Cervical Spine SPINE Trachea (anterior to the cervical spine) is the marker 7 Cervical in knowing you are still in the cervical/neck area. 12 Thoracic Cervical spine is posterior to the trachea. 5 Lumbar 5 Sacral 1 Coccyx Axial view of the Thoracis spine Axial view of the Lumbar spine Lumbar spine has a big vertebral body compared to other parts of the spine The spinous process is the marker to know that you are still in the lumbar area. Kan | 15 LOOKING AT THE BRAIN In the image, no sulci are present or it is compressed. It is the duty of the medical professional to assure the patients. MIDLINE The sulci (left picture) are not extending towards the inner table of the skull. The ❖ It should be in the middle of the patients’ head and the sulci are non-liminar and there is emplacement which means the sulci pattern two sides should be symmetrical. (i.e., no deviation should can no longer be appreciated. Both the cistern and sulci (right picture) on the left be seen) side of the brain cannot be seen. ❖ Any shift of midline structures is presumed to represent a mass lesion on the side or laterality from which the VENTRICLES midline is displaced. Overall size is assessed (i.e., enlarged, small, or cannot be seen at all) Symmetry or shift of the ventricles may be the only sign of intracranial pathology. Composed of: o Lateral Ventricles o 3rd Ventricle o 4th Ventricle LATERAL VENTRICLE Midline of the brain (right picture) is deviated towards the right side due to a mass or lesion that compresses the structures of the left towards the right SYMMETRY ❖ Generally, sulcal pattern should be symmetric ❖ Anterior hemispheric fissure should be visualized The anterior horn of the left lateral ventricle (left picture) is compressed and midline is minimally shifted towards the right side. ❖ Sulci should extend towards the inner table of the skull Kan | 16  If there is blood in the 4th ventricle and temporal horns of the lateral ventricle and enhancement of the sulci in the cisterns are seen in the CT scan, the patient is developing hydrocephalus or there is obstruction in the subarachnoid. (i.e., expect hydrocephalus and dilatation of the 3 ventricles) NEUROIMAGING OPTIONS The picture demonstrates all the ventricles present in one slice. The image demonstrated is present in patients with hydrocephalus which is difficult to ❖ Ultrasound may be used as the first test in infants. manage. Hydrocephalus due to satarac with hemorrhage o Cranial ultrasound is done in infants and not in adults THIRD VENTRICLE due to the presence of the anterior Fontanelles which is open up to 3 years old. ❖ As a general rule in brain imaging, CT is performed in acute neurological illness. ❖ If the CT or MRI suggests a primary vascular lesion, do a CTA or MRA. However, CT and MR are being studied first due to financial considerations. ❖ If the CT or MRI, suggests a tumor, do a contrast enhanced study. ❖ If CT or MRI fails to demonstrate an acute infarct and symptoms would suggest a transient ischemic attack, do a The picture demonstrates a normal 3rd ventricle in a normal, non-atrophic patients (elderly patients). carotid Dopple ultrasound, MRA or CTA. o In a stroke patient, most of the findings in the first The 3rd ventricle is slit-like and beside it is the thalamus. 4-6 hours is normal. However, pathologies will be Thalamic lesions can obstruct the 3rd ventricle. seen in the CT scan after 12 hours. FOURTH VENTRICLE Clinical CT CT WITH MRI MRI WITH presentation (PLAIN) CONTRAST (PLAIN) CONTRAST TRAUMA XX STROKE SEIZURE INFECTION X XX MASS ACUTE XX HEADACHE CHRONIC HEADACHE XX DEMENTIA In a normal non-atrophic patient, temporal horns of the COMA XX lateral ventricles cannot be seen. However, if it is seen in XX- BEST STUDY; X - ACCEPTABLE non-elderly patients, a possible pathology might occur. ANALYSIS OF ABNORMALITY MASS Recognized by displacement of the normal structures away from the abnormality. Normal midline structures may be shifted contralateral to the mass and the sulci adjacent to the mass may be effaced. The picture demonstrates the cisterns are filled with hyperdensities. These Ipsilateral ventricle may be compressed, thus causing hyperdensities are blood wherein patient has a sub-tumor subarachnoid asymmetry. hemorrhage with intravesical or intraventricular extension. Kan | 17  An atrophic brain in CT scan imaging demonstrates The picture demonstrates a lodged mass present on the right side of the brain widening of the lateral ventricles, enlargement of 3rd which causes the ventricles to be displaced towards the left. The sulci on the right side are asymmetrical to the left side. There is compression of the right lateral ventricle and sulci (i.e., can be seen in elderly patients, ventricle and midline is shifted towards the left. chronic drug users, alcoholics, and as early as 20+ old patients) Sulci is widened and ventricles increase in size. The picture (right) demonstrates a lesion that causes deviation of the midline towards the left side. The white midline structure is the falc cerebri which separates the left and right ventricles. There is compression in the right ventricles and the sulci is in displacement. INTRA-AXIAL OR EXTRA-AXIAL? In the left image, there is slight deviation of the midline structure because of the mass present in the right side. The ventricles are compressed and there is INTRA-AXIAL EXTRA-AXIAL replacement of the sulci. Inside the brain and expanding it Outside the brain and compressing it Usually surrounded by the brain. Usually has a broad surface In the supratentorial region, the adjacent In the posterior fossa, the most reliable gyri are expanded and CSF spaces are sign is widening of the ipsilateral compressed subarachnoid space. In the posterior fossa (cerebellum), Buckling of the gray and white matter demonstrates a narrow ipsilateral interface subarachnoid space. The cerebellum and the brainstem are displaced away from the bony margins are of the calvarium by the mass. Example of cauditrus enhancement. The picture demonstrates a plain cranial study where in a definite mass cannot be seen. However, the sulci are displaced, decreased density of the white matter, compression of the anterior horn of the lateral ventricle associated with mild deviation of the midline structure. Patient has metastases in the posterior portion. When looking for pathologies for stroke, look at the The midline structure is deviated towards the contralateral side and ventricles are vascular territories such as the blood supply. compressed. In the posterior fossa mass, the ventricles are displaced to the left and ATROPHY there is obliteration of the subarachnoid space. (A pathology in the subarachnoid space indicates that the ▪ Recognized by widening of the ipsilateral sulci or widening of temporal horns of the lateral ventricles are affected). the ventricle adjacent to the lesion. ▪ No shifting of the midline structures Kan | 18  MUST KNOWS IMAGING TIME COURSE AFTER BRAIN INFARCTION TIME CT MRI Minutes No changes Absent flow void Arterial enhancement High signal (DWI) 2-6 hours Hyperdense artery sign Brain swelling (T1WI) Insular ribbon sign Subtle T2WI hyperintesity MRI, lesions tend to enhance in a ring- 6-12 hours Sulcal effacement T2WI hyperintensity like manner or irregular fashion Decreased attenuation In general, intra-axial mass tend to have more surrounding edema 12-24 hours Decreased attenuation T1 hypointensity To distinguish, the interface between the mass and the surrounding brain 3-7 days Maximal swelling 3-21 days Gyral enhancement (peak Gyral enhancement (peak 7- 14 days) 3-21 days) SOLITARY OR MULTIPLE Petechial methemoglobin Single or solitary lesion is more likely to be the result 30-90 days Encephalomalacia of isolated primary cerebral disease. Loss of enhancement Multiple lesions, are more likely manifestations of Resolution of petechial blood systemic disease. In stroke patients whose pathologies were not seen in an initial CT scan are requested to have a daily CT scan after 12 hours. It is important to know and identify if the patient has stroke due to massive swelling that occurs at 3 – 7 days (i.e., 5th or 6th day). EPIDURAL VS SUBDURAL HEMATOMA Subdural Epidural After contrast study, there is only one lesion. However, after contrast enhance study, multiple can be seen other areas of the brain. CONTRAST ENHANCEMENT Enhancement of the brain parenchyma means that the blood brain barrier has been broken and that the process is biologically active o If there is contrast enhancement or lesion, the Follows inner layer of dura Follows outer layer of dura (periosteum) blood supply affected develops a lesion that has the "Rounds the bend" to follow falx or Crosses falx or tentorium tentorium tendency to grow and spread which requires Limited by sutures of skull (typically) Not affected by sutures of skull immediate treatment to the patient. Tendency for lentiform shapes Tendency for crescentic shapes Typical source of EDH: skull fracture with More mass effect than expected for arterial or sinus laceration their size Seen in trauma patients Typical source of SDH: cortical vein Seen in elderly and chronic patients The left picture demonstrates no lesion was identified, However, lesions on the right picture were presented or identified after contrast studies. Kan | 19  CT-SCAN PT APPLICATIONS Prepared by: Diane Jessica Naoe, PTRP, DPT PRINCIPLES OF COMPUTED TOMOGRAPHY a) CT merges x-ray technology with the computer to provide detailed digital cross-sectional images of the body relatively free from superimposition of the different tissues. (i.e., radiographic technology) b) The radiodensities of the body tissues are represented in the image as shades of gray. c) CT creates images based on cross-sectional (axial) slices, created by up to 1,000 projections from different angles. ELEMENTS OF CT SCAN SUBARACHNOID HEMORRHAGE A CT scanner has three components: o The gantry, into which the patient slides during the examination Densities in the cisterns associated with global aphasement of the sulci and o The operator’s console temporal horns of the lateral ventricles o The computer(s) GANTRY The gantry contains the x-ray tube, its high-voltage generator, a collimator assembly, a detector array, a data acquisition system, and a couch where the patient will lie into. WHAT IS THE DIAGNOSIS? X-RAY SOURCE Hemorrhage Global aphasement Hydrocephalus Subarachnoid hemorrhage CT employs a high-intensity x-ray tube in order to Visualization of the temporal lobe Temporal horns are dilated provide uniform penetration of the tissues and reduce Compression of the 4th ventricle Sulci are displaced attenuation by bone relative to soft tissue. Hypertensive Kan | 20  OPERATOR CONSOLE AND CT COMPUTER The operator console is the desk from which the CT COLLIMATORS technologist controls the scanning process and selects slice thickness, reconstruction algorithms, and other The fan-shaped x-ray beam must be tightly collimated. The specifications during the scanning process. collimators are apertures through which the x-rays pass on the way to the patient and serve the following functions: o Control radiation scatter o Create a narrow, fan-shaped beam of x-rays, determining the field of view o Determine the slice thickness MAKING THE CT IMAGE SCANNING PROCESS ❖ X-ray tube and detectors rotate around the patient, with the axis of rotation running from the patient’s head to toe. ❖ Detectors measure the average linear attenuation DETECTORS coefficient, µ, between the tube and detectors. ❖ Attenuation coefficient reflects the degree to which X-rays are attenuated by body tissues and then exit the the X-ray intensity is reduced by the material it passes patient’s body as remnant radiation, similarly to through. conventional radiography. ❖ 2D measurement are taken in a helical or spiral manner Up to 1,000 detectors are arranged in an array, encircling all around the patient. the patient, for the purpose of measuring this remnant ❖ Attenuation data is summed up from thousands of angles radiation. Modern units commonly have 4 to 16 rows of used in a process called reconstruction. detectors. ❖ When a patient is undergoes CT scan, a contrast dye is sometimes used to make the internal organs more visible DATA ACQUISITION SYSTEM in the image. ❖ Bone appears white; gases and liquids are black; tissues It amplifies the signal from the detectors. are gray. The incoming signal is in the form of a varying electrical ❖ Measurements taken in Hounsfield units (Hu). current, known as an analog signal. ❖ The same study data can show bone structure or soft The data acquisition system converts it from analog to tissue detail, simply by altering the window and leveling digital form and then sends it to the computer for the (i.e., which Hu range will the 0-255 greyscale values will operator to view the scan. correspond to) Kan | 21  CONE BEAM COMPUTED TOMOGRAPHY (CBCT) CBCT acquires all the data in a single sweep of the scanner employing a large, cone-shaped x-ray beam matched with a flat-panel detector for volume acquisition of data. Images in a CBCT scanner are not constructed from a large number of slices but rather based on one volume of data. It requires shorter scanning times and lower radiation exposure for patients. DIFFERENT FORMS OF CT SCAN The spatial resolution is greater than with conventional THREE-DIMENSIONAL CT SCAN scanners. 3 CBCT scanners have been used in areas suited for this Creates three-dimensional (3D) presentations of body type of scanning, such as dentistry— including imaging of parts that can be rotated in “in space” on the computer the temporomandibular joint—and imaging of the breast. screen; a process called multiplanar reconstruction (MPR). Able to convey complex anatomic information in a manner that is not previously possible. Can be challenging for radiologist, and other clinicians used to two-dimensional imaging studies, with many challenges. These images are not adequately viewed in the printed format (hard copy). They reveal their potential only when viewed in the digital format (soft copy), using software that allows free CLINICAL USES OF CT 3D rotation of the examined body part. WHAT DOES CT IMAGE BEST? LIMITATIONS For evaluating bone, CT is usually the CT has limited capabilities for CT MYELOGRAM imaging modality of choice. At present: determining the histological makeup of the imaged tissues because it CT is best for identifying subtle identifies tissues primarily on the basis Using CT, the radio dense column created by contrast fractures and/or complex fractures of radiodensity. material in the spinal fluid can be visualized in every CT is best for evaluating degenerative Different tissues may be assigned the changes, such as spinal arthritic same shade of gray if their plane while simultaneously gathering accurate changes and osteoarthritic changes. radiodensities are similar. For example, information about what structures impinge on the thecal CT may be the first imaging choice in a tumor that has the same radiodensity serious trauma, since multiple injuries as the muscle surrounding it may be sac and nerve roots. (i.e., best imaging for viewing the spine) to both osseous and soft tissue missed because the computer assigns CT myelography is better able to distinguish between structures can be determined from one the same shade of gray to the tumor as imaging series. (i.e., Patients with TBI have to the muscle. (i.e., Not an effective osteophytes, ligament infolding, and annular material brain or skull fractures that can already be diagnosis tool for conditions such as seen by the CT scan and possible brain Parkinson’s disease and brain tumor) than is MRI. damage can also be seen) High radiation exposure Where there is history of symptomatic lumbar For patients who undergo a contrast stenosis, CT myelography is considered the definitive type of CT scan, a severe allergic preoperative investigational imaging. reaction with the diving injected case