MRI - PT106 Finals PDF

PT106 Finals MRI Liquid helium is needed to serve as a cryogen to maintain superconductivity of the magnet Basic...

PT106 Finals MRI Liquid helium is needed to serve as a cryogen to maintain superconductivity of the magnet Basic Physics of MRI Wired temperature rises accidentally: it looses its MRI emits non-ionizing radio waves which would not superconducting property and the energy stored in damage our DNA unlike x-rays and CT scan which emits the magnetic field makes the liquid helium boil that ionizing radiation that has the potential to damage our results to a magnetic quench DNA  In oder to generate field strength of several Teslas, Electromagnetic Spectrum superconducting magnet is used and with the help of the superconducting coils  Most diagnostic imaging magnetic field strength: 1.5 and 3 Tesla  4-7 Teslas are used for research Gradient Coils - used to code spatial location of the MR sginals - in order to localize the MR signals, the three gradients are switched on and off: Slice selection -The Z gradient that runs along the long axis produces axial images -The Y gradient that runs along the vertical axis Non-ionizing Radiation: AM, FM, TV, Cellphones, produces coronal images Radar, TV Remote, MRI (do not have the potential to -The X gradient that runs along the horizontal axis damage DNA) Ionizing Radiation: Sun, X-Ray Machine, produces the sagittal images Radioactive Elements. CT Scan (have the potential to Phase encoding damage DNA) Frequency encoding - magnet gradients are essential in generating MRI Components of MRI Machine images Superconducting - magnetic field gradients inside the machine: X,Y, Z Magnet orientations - is wired by the superconducting coils and bathed with the liquid helium Gradient Coils Radiofrequency Coils Computer System - where the MRI machine is connected Shim Coils used for magnetic shimming to improve the magnetic field uniformity Axial (Z) Gradient - produced by using Hemholtz foils Superconducting Magnet - has a magnetic field Sagittal and Coronal (X and Y) Gradients - the magnetic field has strength and is measured by - produced by Saddle coils Tesla in honor of Nikola Tesla Earth - 50 microtesla  Magnetic gradient cause different locatinos to have MRI - 1.5 Tesla (minimum magnetic field strength for magnetization precision frequencies diagnostic imaging)  Gradients may use to switch or need to switch on and - kept cool by the liquid helium because constant current off rapidly about less than 500 microseconds is used at all times that is why the MRI machine is always  Gradient coils are responsible for the different MRI hot sounds or noises that may be heard during the MRI - a satisifactory amount of liquid helium which serveas a procedure coolant which prevents the heating of the MRI machine is needed Liquid helium is below critical level: MRI machine is not allowed to operate 1  PT106 Finals Radiofrequency Coils Zoning - electromagnetic radiation with frequencies ranging from - observed to prevent non authorized personnel to have 1 megahertz to 10 gigahertz an access to the MRI magnet; safeguard the MRI to avoid freak accidents - have transmitter and receiver radiofrequency coils Tranmitter Coils 4 Facilities: - used to send radiofrequency pulses Zone 1 - public area (accessible to all) Zone 2 - Reception/Waitingroom and Patient Receiver Coils Preparation/Holding room - used to detect signals from the patient Zone 3 - Control room and Computer room (radtechs - may be physically separate from the transmitter coils or manipulate the console) maybe the same coils switch from the transmit mode to Zone 4 - MRI scanner (room where MRI magnet receive mode located) - placing it closer to the body being imaged may improve detected radiowave signals or increases the signal noise ratio (example: knee and head coil) Magnetic Flux Lines - from main magnetic field which extends far distance from the main magnet - peripheral magnetic field or fringe field can affect magnetically sensitive devices such as credit cards, watches, and pacemakers - magnetic sensitive devices are prohibited inside the MRI room to prevent malfunction of these dvices MRI Room Wall Computer System - magnetic and radiofrequency sheilding is important - frequncies convey image information by changes in Magnetic Shielding frequency amplitude and phase to the computer system - is embedded in the walls of the MRI room - digital signals are stored K space and process and - prevents the fringed field coming from the magnet inside transform into a digital image in the MRI monitor by the the MRI machine extended beyond the scanner room application of the mathematical formula called the Fast preventing the malfunction of the magnetically-sensitive Fourier Transform devices Radiofrequency Shield - or Faraday cage - made of copper which is also embedded in the MRI room wall prevents the radio signals caused from the radio broadcast coming from the outside of the room getting in to the coils that causes background noise which affects MRI quality Basic MRI Safety - prevents radiofrequency from escaping the MRI room interfering the outside eelctronic equipments Missile Effect - greatest potential hazard of MRI Iron - is pulled towards the magnet - ferromagnetic materials will pull towards the magnet Signage - should be in the doors of the MRI scanner room to always remind of the dangers and prohibitions Patients may experience: Mild cutaneous sensations Involuntary muscle contraction Cardiac arrhythmias This floor plan must be considered before constructing the facility. The facility is consist of 4  FDA limits to 3 Tesla for diagnostic MRI zones >3 Tesla can stimulate peripheral nerves 2  PT106 Finals  FDA limits radiofrqeuency power deposition By measuring SAR (specific absorption rate) Radiofrequency Heating - can occur in conductive materials or MRI safe objects such as tattoos, bone implants, and ECG leads  Heat protection is mandatory (earplugs) Magnetic field is cancelled in paired spin up and spin Summary down protons  MRI consistsof a very strong magnet and radiofrequency coils  Potential hazards include missile effect and heating of tissues respectively  Hearing loss and peripheral nerve stimulation were reported  Knowledge of MRI safety is mandatory to prevent potential injury or death Remaining spin up protons or unpaired Physics of MRI Signals protons in low energy state produces the - based on nuclear magnetizaiton resonance net nuclear magnetization in the direction - nuclear magnetic resonance physics need a lot of of the external magnetic field which has a imagination vector facing up Hydrogen Atom  Hydrogen atom moves like a - has a nucleus and has orbits spinning top or precession motion - protons are located in the nucleus and electrons are  Frequency during precession located in the orbits motion is called Larmor Frequency Hydrogen Proton - atoms with odd proton have nuclear magnetization: Larmor Frequency unpaired proton behave like magnets - (gyromagnetic ratio of a particle or system is the ratio of  Human body = 70% water (H2O) its magnetic moment to its angular momentum)  1cm3 tissue = > 1022 H protons Magnetic Field Strength = Independent Variable - precess/wobbles at different angles and moves randomly → (B) = the only independent variable which is the → If they move randomly, they have no net nuclear magnetic field strength, the rest are constant magnetization → F0 α B0 → The larmor frequency is directly proportional to the Relative number of magnetic field strength = meaning MRI machines with Tissue mobile protons % (Spin 3 Tesla has increased nuclei larmor frequency than density) 1.5 Tesla White matter 100 → Increasing larmor frequency = increases MR signals Fat 98 Larmor frequency of proton = 42MHz at 1 Tesla Gray matter 94 127 MHz at 3 Tesla Liver 91 Bone ~5 Resonance Lung ~3 - occurs when radiofrequency electromagnetic field that is generated by the radiofrequency coils and interacts with hydrogen photons in the body that is subjected to a strong magnetic field - protons in the body produces MR signals with the help of the gradient coils that localizes signals - MR signals or echoes are then received by the radiofrequency coils and are digitized to the computer system projected as MR images The protons processing randomly once the body is ubjected to a strong magnetic field, this hydrogen protons align or spin to the direction of the primary magnetic field Spin Up Protons - have low energy Spin Down Protons - have high energy 3  PT106 Finals Vectors - a 180⁰ radiofrequency pulse reorients 180o to the direction it had prior - when radiofrequency pulse is applied, the radiofrequency excitation causes the net magnetization vector to flip by a certain angle and this produces to net magnetization components Longitudinal and Transverse Magnetization - imagine always the Z, X, and Y axis when talking about vectors - vector has a direction and a magnitude - hydrogen protons wobbles or processes creating a vector when they are placed in a strong magnetic field - orientation is in Z-axis - occurs when radiofrequency electromagnetic field that is generated by the radiofrequency coil interacts with the hydrogen proton inside the body Longitudinal Magnetization - applied radiofrequency pulse must be at larmor - the component of net magnetization vector parallel to the frequency and perpendicular to the main magnetic field main magnetic field causing the magnetization vector to rotate from Z to XY - taken to a point in z-axis axis - when proton vector faces up in the direction of magnetic field Transverse Magnetization - the component perpendicular to the main magnetic field - taken to be in x y-axis - when RF is applied, the vectors rotates facing X-Y axis  When a 90⁰ RF pulse is applied to the longitudinal magnetization When radiofrequency pulse is turned on, the magnetization vector rotates from z-axis to x y-axis The rotation of magnetization continues while the radiofrequency pulse is switched on 90⁰ Pulse  Result to Transverse magnetization - rotating magnetization can be detected as a used - a 90⁰ radio frequency pulse reorients the magnetization voltage in a radiofrequency coil vector to a direction 90o perpendicular to the prior vector - radiofrequency coil transmits and detects signals orientation - the detected voltage is free induction decay signal so - there is phase coherence of protons or the protons are they are detected and digitized and used to produce MR in-phase images - the little blue arrows which represents the individual hydrogen protons that are in-phase when radiofrequency Relaxation pulse is applied 180⁰ Pulse 4  PT106 Finals This illustration emphasized the direction of the hydrogen - T1 relaxation basically is the returning to longitudinal vector in Z and XY axis magnetization  Longitudinal magnetization is oriented in Z axis  Transverse magnetization is oriented in XY axis Relaxation - happens when radiofrequency pulse is off - the protons will undergo T1 and T2 relaxation When proton is subjected to a strong magnetic field, the proton vector faces up in the direction of magnetic field in Longitudinal magnetization - after 90⁰ radiofrequency pulse is on, the magnetization When radiofrequency applied, the vector would face vector rotates in a direction of XY-axis. During this time in phase to XY axis in Transverse Magnetization there is phase coherence or inphase of the individual hydrogen protons T1 Relaxation - when the radiofrequency is turned off, the protons lose - vector is parallel to the magnetic field their phase coherence (the little blue arrows are out of - return to longitudinal magnetization phase). This is T2 relaxation a.k.a spiin-spin relaxation. - Spin-Lattice Relaxation - transverse magnetization decays exponentially with time - (Protons that is previously excited by the radiofrequency - with time, longitudinal magnetization also begins to pulse returns to Longitudinal Magnetization where the recover simultaneously with transverse relaxation after the vector is parallel to the magnetic field, this is called Spin- radiofrequency pulse is off. However, the return to Lattice Relaxation) equilibrium conditions occurs over a longer period of time. T2 Relaxation Different tissues have different relaxation time (1.5 - transverse magnetization decays Tesla) - transverse Relaxation - Inphase to Outphase - Spin-Spin Relaxation Tissue T1 (ms) T2 (ms) - (The protons that is previously excited by the Fat (adipose) 250 80 radiofrequency pulse that are inphase or inphase Liver 300 45 coherence loses coherence or termed outphase, this is Kidney 650 60 called Spin-Spin Relaxation) White Matter 800 90 - (In time transverse magnetization decays simultaneously Grey Matter 900 100 with recovery of longitudinal magnetization until protons Cerebrospinal reaches equilibrium) 2400 280 Fluid Protons will undergo T1 and T2 relaxation Liquid: Csf - has long T1 & T2 time - substance with more water has rapidly rotating molecules – longer time to relax or return to equilibrium Viscous Or Solid Materials: Fat - have short T1 and T2 relaxation time - substance with more water has rapidly rotating molecules therefore they have longer time to relax or return to equilibrium Return To Equilibrium Of Longitudinal Magnetization For Two Tissues With Different T1 Relaxation Times -i n T1 relaxation, hydrogen protons placed in magnetic field produce a net magnetization with vector directed parallel to the external magnetic field or in z-axis - (Left Image) - Longitudinal magnetization when radiofrequency pulse in on. The applied vector is erected/directed to x y-axis - (Right Image) - when the radiofrequency pulse is off, the longitudinal magnetization recovers releasing energy back to the lattice until full equilibrium is reached. This is called spin-lattice relaxation or T1 relaxation. 5  PT106 Finals - graph Tissues with short T1 relaxation time have higher signal intensities compared to tissues with long T1 Phase encoding relaxation time - causes a phase shift in spinning photons - in T1 weighted MR images, fat with shorter relaxation time appears brighter than cerebrospinal fluid which has Frequency encoding longer T1 relaxation time - tissues with shorter T1 relaxation time - composes precessional frequencies of shift protons - reach the equilibrium faster than long T1 relaxation time - also called “readout gradient” ex: Fat or Adipose tissue - composite signals are received by the radiofrequency - shorter T1 relaxation time coil and digitized - has high intensity signals than CSF During acquisition in the common imaging planes: axial/ Loss Of Transverse Magnetization Due To T2 coronal/ sagittal planes Relaxation (Signal localization requires magnetic field gradients with the help of gradient coils. Gradient coils inside the MR machine are oriented in the X, Y, and Z axis. They’re used for Section Selection, Phase and Frequency Encoding during the acquisition in the common imaging planes: the Axial, Coronal and Sagittal) - tissues with high signal intensity has long T2 relaxation time or has a long time to decay or reach the eclipse equilibrium - Helmholtz Coils – axial (Z) gradients - in T2 weighted MR images, cerebrospinal fluid which has - Saddle Coils – sagittal and coronal (X & Y) gradients a long T2 relaxation time, appears brighter than fat tissue which has a short T2 relaxation time and cerebrospinal Gradient coils are oriented in XYZ axis adjacent to the fluid hydrogen protons has a longer time to return to main magnetic field equilibrium compared with hydrogen protons of fat tissue Graph: Signal densities of two tissues in short and long T2 relaxation time - tissues with high signal intensity has long T2 relaxation time or long time to decay or reach the equilibrium ex: CSF - appears brighter than fat - long time to decay Element Of Time - there is always an element of time in relaxation of - each location of the gradient has a unique field strength protons when we talk about signal intensities of the that produces a specific larmor frequency different tissues in the body that are projected in MR Typical SPIN echo pulse sequence diagram images Signal localization - to come up with Coronal Imaging plane, Gradient Y is Gradient Coils used for slice selection, Gradient X is used for phase - sgnal localization requires magnetic field gradients with encoding and Gradient Z is used for frequency encoding the help of gradient coils - to come up with Axial Imaging plane, Gradient Z is used - gradient coils inside MRI machine are oriented in XYZ for slice selection, Gradient X for phase encoding and axis; Used for: Gradient Y for frequency encoding. Section selection/ slice selection - selects section to be images 6  PT106 Finals - to come up with Sagittal Imaging plane, Gradient X is Raw imaging data in j-space must be fourier used for slice selection, Gradient Y for phase encoding transformed (which is based on a mathematical and Gradient Z for frequency encoding formula) to obtain final mr image Application of Gradients in a Typical Spin-Echo Pulse Sequence Review Resonance occurs when radiofrequency electromagnetic field is generated from the radiofrequency coil interacts with the hydrogen proton in the body that is subjected to a strong magnetic field Protons in the body produces MR signals with the help of Gradient coils that localize signals MR signals/echoes are then received by radiofrequency coil and digitize it to the computer system by fast fourier transform Projected as MR images Summary RF: Radiofrequency pulse SSG: Slice selection gradient: PEG: Phase encoding Basic physics of magnetic resonance signal is based FEG: Frequency encoding: on nuclear magnetization DAQ: Data acquisition mr signals are produced by strong magnetic field and radiofrequency pulses applied in the body What happens in a spin echo pulse sequence? Protons undergo changed in magnetization vectors SSG (Slice Selection Gradient) - is on synchronous and released energy tissue to the tissues and with the application of the 90 degree radiofrequency produced an echo which in concert would create an pulse mri signal PEG (Phase Encode Gradient) - is on after the gradients are used to localized mr signals by slice application of 90 degree radiofrequency pulse and selection , phase and frequency encoding slice selection gradient K-space where raw imaging data is transformed by 180⁰ degree pulse is applied mathematical process (fourier transformed) to obtain FEG (Frequency Encode Gradient) - is on after the final mri image application of the 180 degree radiofrequency pulse, slice selection gradient and phase encode gradient Basic Acquisition Parameters Composite echo signals are encoded by the - acquisition Parameters & MRI Sequence frequency encoding gradient and received by the - key to creation of images radiofrequency coil - consist of repetition time or TR K-Space echo time or TE Repetition time (TR) - measured in milliseconds - is the time between the application of a radiofrequency excitation pulse and the start of the next radiofrequency pulse - MR signals or echoes are then stored in K-Space K - symbol of wavenumber in a matrix of axels within which raw imaging data are stored 7  PT106 Finals Echo Time ( TE) - also measured in milliseconds - time between the application of the radio frequency pulse and the peak of the echo detected - MR signals are most often obtained in the form of echoes - from transverse magnetization to MR, echoes occur at time echo or TE - under operator control and can be selected to be long or short  Remember the position of acquisition times in this SPIN echo pulse sequence Spin Echo Pulse Sequence - in T1 weighted technique, tissues with long T1 relaxation time (e.g. CSF) do not recover their longitudinal magnetization in short repetition time. - therefore, it contributes to little signal or has a low signal intensity. This explains why the signal intensity of the cerebrospinal fluid in T1 weighted images is dark - only tissues with short T1 relaxation time values (e.g., fat or fat tissues) fully recover their longitudinal magnetization and short repetition time and contribute to a signal or has a high signal intensity - this explains why the signal intensity of fat tissue (e.g., Repetition time and Echo time subcutaneous fat tissue) in T1 weighted image is bright - affect contrast on MR images because they provide like the subcutaneous fat in the MR image. varying levels of sensitivity to differences in relaxation between the various tissues Contrast - darkness and brightness of the tissues in image Controlled by Operator/Already set - can be adjusted to emphasize a particular type of contrast - eX: T1, T2 and proton density weighting technique T1 weighted Technique= short TR and short TE. T2 weighted = long TR and long TE. Proton density weighting = long TR and short TE. T1 weighted (short TR and TE) Summary: Tissues with a long T1 do not recover in short Remember this graph and table when we are talking repetition time and therefore contribute little signal. Only about time over petition and T1 weighting: tissues with short T1 values fully recover their longitudinal magnetization in short repetition time and contribute a signal 8  PT106 Finals T2 Weighting (Long TR and TE) 1. Short TE & short TR Remember this graph and table when we are talking - T1W (T1 weighting) about echo time or TE and T2 weighting - CSF in T1 weighting appears dark; fat in the scalp appear bright 2. Long TE & long TR - T2W: (T2 weighting) - with long repetition and echo time, the CSF appears brighter than the fat in the scalp 3. Short TE & long TR - PDW (proton density weighting) - long TR results to a bright CSF signal - long TR results to bright subcutaneous fat signals - short TE means that this brightness is less than the signal in T2 weighted image - short TE means that the signal is brighter than T2 weighted image - therefore, the signal of CSF is in the middle Long TE values will result in big loss of transverse 4. Short TR & Long TE magnetization (i.e., big T2 decay; loss of signal or - exhibit poor contrast brightness). - not useful in diagnostic imaging T2 weighted technique, long repetition and echo time are applied. Since it is T2 weighting, we only think of the graph and echo time (TE) In long echo time (TE) Tissues with long T2 decay value (such as CSF) have high signal intensity than tissues with short T2 decay values (such as fat) That is why the CSF appears brighter than fat in a T2 weighted image Long TE values will reduce the intensity of transverse magnetization for tissues with short T2 much more than tissues with long T2 Table of the different tissue contrasts in axial brain Basic MRIPulse Sequences MRI images in different weighting based on the acquisition parameters. 2 Fundamental types of MR Pulse Sequences: Spin Echo (SE) Gradient Echo (GRE) - ll other NR sequences are variations of these 2 fundamental sequences with different parameters added on. Routine Brain MRI Sequences: T1W T2W FLAIR (variations of SPIN echo pulse sequences) DWI/ADC GRE (variations of GRADIENT echo pulse sequences) 9  PT106 Finals As mentioned, sequences with: Routine Spine MRI Sequences Short TR and TE are used to obtain T1W T1W Long TR and TE are used to obtain T2W T2W Long TR and short TE are used to obtain PDW STIR (Short Tau Inversion Recovery) - variations of SPIN echo pulse sequences Spin Echo Pulse Sequence Commencement - 90⁰ radio frequency (RF) pulse is on together with a slice Fast SPIN echo variant technique (FSE) selection gradient (SSG) - variant of Spin echo pulse sequence - recall: when a 90⁰ pulse is applied, the magnetization - technique which shortens acquisition time by generating vector will rotate to transverse plane or xy-axis multiple phase encoding steps during each time of - in transverse plane, the magnetization rapidly de-phases repetition or out-phases when the phase encode gradient is on - a single 90⁰ pulse is applied to flip the magnetization vector SPIN Rephasing - after which, multiple 180o radio frequency pulses are - is achieved by applying 180o RF pulse to generate a applied SPIN echo time (TE) when the frequency encode gradient - in result it rephases pulses; each of which create an or without gradient is on echo/echo train - SPIN echo sequence of a 90o and1800 radio frequency - the acquisition of data is greatly reduced with the use of pulse is repeated at time of echo (TE) the fast SPIN echo sequence as opposed to conventional SPIN echo sequence SPIN echo pulse sequence - changes in repetition (TR) and echo times (TE) to Echo Train emphasize T1 differences and/or T2 differences of tissues - all echoes together - data acquisition difference (Brain) T1W T2W PDW - are dependent on the changes in TR and TE whether it Conventional SE will be short or long - 7 minutes - this is already programmed in the computer system Fast SE (FSE) -34 Seconds 10  PT106 Finals - radiologist usually suggest fast SE to finish the procedure faster Inversion Recovery - variation of spin - 2 important clinical implementations: 1. Fluid-attenuated inversion recovery (FLAIR) 2. Short tau inversion recovery (STIR) The same principle is applied in STIR FLAIR The signal of fat rather than water in STIR is nulled - inversion time value set to eliminate the cerebrospinal fluid (CSF) signal MR images if the hips STIR - inversion time value selected to null or suppress the signal of fat T1W image - bilateral proximal femurs - pelvic bones - lumbar spine - subcutaneous tissue is hyperdense (bright) STIR - suppresses fat signals In inversion recovery: - subcutaneous tissue appears hypodense (dark) - the FLAIR sequence image is T2 - only remaining bright signal is water - but the signal of the CSF is nulled - water in the urinary bladder and inside of the intestines - the pulse sequence of this commences when a 180o RF Note: STIR sequence best detects edema which indicates pulse is applied to flip the magnetization vector to 180o pathology - imagine vectors z, x, and y axis – there is nulling of signal from a particular entity (such as water) with the vector in 180⁰ plane - when the radiofrequency pulse ceases, the spinning nuclei begin to relax - while magnetization vector of water passes at transverse plane which is a null point for tissues, the 90o RF pulse is applied and then the phase and coil gradient is on to diphase the protons - 180⁰ RF pulse is on to refocus the protons in phase - frequency gradient is on to read the MR signals - this sequence is continuous as before, nulling the signal of water - therefore, the signal in the CSF is nulled in this pulse sequence 11  PT106 Finals Gradient Echo Sequence (GRE) Left - GRE sequence of brain without hemorrhage Right - Brain with hemorrhage, exhibiting blooming artifact or magnetic susceptibility signal Diffusion Weighted Imaging (DWI) - produces signal differences based on the mobility and directionality of water - uses fast gradient recalled echo sequence and 2 equal gradients are applied on each side of the 180o RF pulse (purple) - if no net movement of hydrogen protons occur between the applications of these gradients, the first gradient will dephase the spins and the second gradient rephases them - therefore high intensity signal is seen Gradients - if there is net movement (hydrogen protons move) such - used to diphase and rephase transverse magnetization as in normal tissues, the protons are not affected by both - uses less than 90o RF pulse that partly flips the gradients magnetization vector into the transverse plane - they may undergo through dephasing but NOT rephasing - GRE uses short repetition time which permits fast or vise versa acquisition - therefore, the signal intensity is decreased - this sequence is also called T2 star weighted - one of the major uses of DWI is the diagnosis of a recent - T2* = T2W + magnetic inhomogeneity brain infarct -they are T2 but do not refocus field inhomogeneities; which is why GRE sequences are more sensitive to Brain Infarction magnetic field inhomogeneity - causes cell edema or cytotoxic edema which limits the - secondary to magnetic susceptibility differences between movement of water in the interstitial tissue tissues - if there is restricted diffusion of water, the infarcted part - so magnetic susceptibility related signal loss or of the brain will appear bright in DWI susceptibility artifact is caused by a magnetic field - will appear dark in ADC map inhomogeneity - can be described in terms of T2 star signal decay DWI - applied in conjunction with apparent diffusion coefficient or ADC map - together with ADC allow to determine the age of brain infarct Summary: Basic Acquisition Parameters: Relaxation Time (TR) and Echo time (TE) T1 Weighting: short TR and TE T2 Weighting: long TR and TE Proton Density Weighting: long TR and short TE 2 fundamental MRI Pulse Sequences: Spin Echo and GRE Gradient Echo. The rest are variations - sensitive to magnetic inhomogeneity secondary to FLAIR: suppresses CSF Signal magnetic susceptibility differences between tissues STIR: Suppresses Fat Signal - used for detection of hemorrhage as iron and GRE: Detects hemorrhage hemoglobin becomes magnetized locally and produces DWI together with ADC: Detect acute infarct and magnetic field this dephases the spinning nuclei determine age of infarct 12  PT106 Finals Echo Time (TE) is the time selected to wait after the Magnetic Resonance Imaging (PT Implications) start of the TR to receive the signal or “echo” from the patient. Magnetic Resonance Image (MRI) Varying the TR and TE will accentuate different - is a cross-sectional imaging technology that uses a tissues magnetic field and radiofrequency signals to cause Using a shorter TE will produce a T1 weighted image hydrogen nuclei to emit their own signals, which then are Using a longer TE will produce a T2 weighted image converted to images by a computer. or STIR (Short tau inversion recovery) both - allows clear visualization of the ligamentous, neural, MR sequences that suppress fat and show pathology diskal, muscular, and other soft tissue features along with – increase in water bright and fat suppressed the osseous structures typically required in clinical T1 and T2 images are different for each tissue and decision making. produce different intensities (degrees of whiteness) of - views are usually obtained with slices in sagittal and the same tissue axial sequences. T1 vs T2 Images How does MRI work? T1 images better for viewing anatomy Magnetic resonance imaging (MRI) uses the body’s T2 images better for pathology since these images natural magnetic properties to produce detailed accentuate still fluid images from any part of the body. For imaging purposes the hydrogen nucleus (a single Signal Intensity T1-Weighted T2-Weighted proton) is used because of its abundance in water High intensity SubQ fat Still fluid and fat. (white) Spongy bone (inflammation) The hydrogen proton can be likened to the planet Still fluid Tumor earth, spinning on its axis, with a northsouth pole. Cartilage Fluid Under normal circumstances, these hydrogen proton Tumor Muscle “bar magnets” spin in the body with their axes Muscle fluid Cartilage randomly aligned. Spongy bone The hydrogen proton can be likened to the planet Low intensity SubQ fat earth, spinning on its axis, with a northsouth pole. Under normal circumstances, these hydrogen proton Clinical Uses of MRI “bar magnets” spin in the body with their axes MRI is very sensitive for detecting changes and randomly aligned. variations in bone marrow. When the body is placed in a strong magnetic field, MRI excels in the display of soft tissue detail. such as an MRI scanner, the protons’ axes all lineup. MRI is the best modality for differential diagnosis This uniform alignment creates a magnetic vector between disk herniations and other causes of nerve oriented along the axis of the MRI scanner. root impingement. MRI scanners come in different field strengths, MRI has the ability to stage neoplasms in bone and usually between 0.5 and 1.5 tesla. soft tissues as well as evaluate the extent of tissue invasion, prior to surgery. How are MR Images produced? It is more sensitive than bone scan for detecting bone The radio pulses are stopped, the absorbed energy is metastases, although bone scan is more effective as released and measured by the computer detector. a screening technique. This info is converted to an image. Unique images are produced because each tissue Advantages has a different amount of hydrogen and so relax at Superior soft tissue visualization, especially muscles, different rates tendons, ligaments, nerves, articular cartilage and The length and sequence of the pulses produces menisci different quality images of the same tissues. Okay for spongy bone (high fat content) Repetition time (TR) is the time that elapses between No ionizing radiation, no known harm two consecutive radio wave pulses Echo Time (TE) is the time selected to wait after the Limitations of MRI start of the TR to receive the signal or “echo” from the The limitations of MRI lie in imaging of cortical bone patient. because of its low signal intensity. Varying the TR and TE will accentuate different Length of time needed to produce an image tissues High cost Using a shorter TE will produce a T1 weighted image Cannot use with patients who work with metal The length and sequence of the pulses produces shavings,metal pacemakers, certain types of fixative different quality images of the same tissues. devices (ferrous metals) Repetition time (TR) is the time that elapses between two consecutive radio wave pulses Contraindications and Health Concerns 13  PT106 Finals Ferromagnetic surgical clips can be displaced. In the Critical Thinking Point 2: MR Imaging of Stress case of brain aneurysm clips, such displacement can Fractures cause fatal hemorrhage. Orthopedic hardware can cause image distortion, but Stress fractures, which most commonly involve the it generally does not represent a health hazard. bones of the lower extremity, are the result of Other concerns are the following: repeated subliminal trauma. ○ Pacemakers may malfunction within or near the This process starts with accelerated turnover and magnetic field. remodeling of bone, which may progress to a stress ○ Claustrophobia, which affects about 10% of fracture if the stress continues patients. Radionuclide bone scan - gold standard for the ○ The need to sedate patients (such as children) diagnosis of stress fractures who may not be able to stay still for the duration CT - is the most accurate diagnostic modality for of the examination stress fx in the navicular MRI - is the most sensitive method for femoral neck CT vs MRI stress fx CT better for MRI better for Cortical one Spongy bone Subtle and complex Soft tissue (ligaments, fissures tendons) Calcifications in any tissue Articular Cartilage Hemorrhagic strokes Clinical Thinking Points Clinical Thinking Point 1: Bone Bruise—The Footprint of Injury Bone marrow contusions are frequently identified with MRI following musculoskeletal injury. These are injuries that typically do not involve Critical Thinking Point 3: MR Imaging of Stress permanent changes to osseous structures. Fractures Bone marrow contusions, which have aptly been MRI has the value, over the other imaging modalities, called the “footprints of injury,” of being able to demonstrate the often considerable May be the result of traction injury to ligaments, direct soft tissue abnormalities adjacent to the fractured blow to the bone or compression forces at joint bone. surfaces during injury. MRI investigations of the progression of stress Frequently an injury that is not visible on radiographic fractures have revealed that not all stress reactions examination or CT is discovered because it leaves a visible on MRI give rise to stress fractures. “footprint” in the form of bone marrow edema visible on MRI. Association of Lumbar MRI Findings with Current and Future Back Pain in a Population- based Cohort Study Methods: Participants (n+3369) from a population-based cohort study were imaged at study entry, with LBP status measured at baseline and 6-year follow-up. MRI scans were reported on for the presence of a range of MRI findings. LBP status was measured on a 0 to 10 scale Results: MRI findings were present in persons with and without back pain at baseline. Higher proportions were found in older age groups. 76.4% of participants had a least one MRI finding and 8.3% had five or more different MRI findings In the longitudinal analyses,most MRI findings were not associated with future LBP severity regardless of the presence or absence of baseline pain 14  PT106 Finals Multiple MRI findings (five or more) was associated with mildly greater painseverity 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. Summary of Key Points 1. MR images are made on the basis of energy emitted by protons during their re-alignment with the main magnetic field. 2. Diagnosis is often based on the differences between T1-weighted and T2-weighted images. 3. T1-weighted images demonstrate great anatomical detail and tend to highlight structures rich in fat, while T2- weighted images are grainier and emphasize structure with high free-water content and inflammation. 4. Sequences, such as SE Sequences (T1 and T2, as well as proton density) and GRE sequences, are different methods 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 weight-bearing position, but are associated with lower field strength and longer imaging times. 8. Structures of high density, like cortical bone, ligaments, menisci, and tendons, are dark (have low signal intensity) on all MRI sequences, while most other structures show different signal intensities on T1-weighted images, as compared to T2 9. MRI excels at detecting changes in bone marrow, displaying soft tissue detail, and demonstrating areas of inflammation. 10. Advantages of MRI over CT include no use of ionizing radiation, greater contrast resolution, and greater ability to image structures surrounded by bone. 11. Disadvantages of MRI include long imaging times and expense. 15