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Magnetic Resonance Imaging - The number of excess protons is proportional with B0 - That is also the reason why 1.5T systems make better TOS 1.1 E...

Magnetic Resonance Imaging - The number of excess protons is proportional with B0 - That is also the reason why 1.5T systems make better TOS 1.1 Explain the nuclear physical principles applied in images than systems with lower field strengths - When the scanner excites these protons with a radio magnetic resonance imaging. frequency timed to the frequency they are wobbling, it knocks them out of alignment directly opposite to the Historical Development (NMR) direction of magnetic field pole - When the scanner turned off this radio frequency, within - In 1930 Magnetic fields associated with atoms and nuclei milliseconds they spiral back into the direction of strong was first described magnetic field pole giving out with a faint radio signal of - Otto Stern-for his contribution to the development of the their own (FID) free induction decay MHz molecular ray method and his discovery of the magnetic - In order to see what happens with this net magnetization moment of the proton" in an easy way, the scientific community came up with the - Isador Rabi-described how nuclei could be induced to brilliant idea to visualize it by means of vectors flip their principal magnetic orientation by an oscillating - The Z-axis is always pointing in the direction of the main magnetic field. magnetic field, while X and Y are pointing at right angles from Z. Historical Development (MRI) - The net magnetization is now called Mz or longitudinal magnetization. - Primary credit (and the Nobel Prize) for MR imaging was awarded to Sir Peter Mansfield and Paul Lauterbur. However, Raymond Damadian must also be recognized for his innovation and construction of the first full-body human MR scanner. - Lauterbur's Contribution: Projectional NMR Tomography - Mansfield's Contribution: Use of a field gradient for slice selection - In 2003 Lauterbur and Mansfield shared the Nobel Prize in physiology and medicine “for their discoveries concerning magnetic resonance imaging.” - Zeugmatography A technique in which the addition of carefully controlled inhomogeneous magnetic fields enables measurements of nuclear magnetic resonance (NMR) to be made at selected regions of heterogeneous samples Historical Development (NMRI-MRI) TOS 2.1 Apply the terms relevant to magnetic resonance - 1970’s the name was changed from NMRI to MRI due to imaging the negative connotations associated with the word “nuclear”. Many patients thought that the exam would Relevant Nomenclatures expose them to radiation. - RF-non-ionizing radiation - Hydrogen atom - It is very abundant in the human body-Hydrogen makes up 80% of all atoms found in the human body, making Electromagnetic Spectrum hydrogen extremely useful for MRI - Visible light - Its solitary proton gives it a relative large magnetic - 400nm(blue) moment - 700nm(red) - Diagnostic x-ray-20 keV-150 keV - RF- 3 kHz- 3 GHz - MRI RF- 10-300 MHz Fundamental Concepts - Spinning like tops hydrogen atom normally points in random direction/ oriented, net magnetization is zero - But inside the scanner’s strong magnetic field they align themselves in the direction of the fields poles,creating net - Magnetic moment- A force created when a magnetic magnetization (M) dipole is in a magnetic field. (T) - But even in alignment, however they wobble, or precess - Precession- the wobble of the rotational axis of a at a specific rate or frequency, the stronger the magnetic field the greater the frequency spinning body about a stationary axis that describes a - The excess amount of protons aligned parallel within a cone 0.5T field is only 3 per million (3 ppm = parts per million) - Resonance- vibration on a mechanical or electrical - 1.0T system there are 6 per million system cause by a relative small periodic stimulus with a - 1.5T system there are 9 per million.  frequency at or close to a natural frequency of the body parts. Computer analysis of the radio-wave data is system required. PRIMARY MAGNETS PERMANENT MAGNETS - 1964 EMI (England) developed the CT scanner (by Mr.Hounsfield) - The earliest commercial magnets are made from ferrite, in 1930’s an alloy called alnico(aluminum,nickel and cobalt) was developed, bricklike ceramics are the new material used today - There are no shim coils in this type of magnet - It is done by mechanically adjusting the two finely machined pole pieces MRI Parameters - Spin density - T1 relaxation time - T2 relaxation time - X-ray attenuation coefficient of soft tissue differs by < 1% RESISTIVE MAGNET - Spin density and T1 relaxation time of the same tissue - Using several large loop of coils of roughly 1.5 m in differs by 20% to 30% diameter, the coils must be precisely positioned to meet - T2 relaxation time differs as much as 40% required field homogeneity - MRI is superior low-contrast resolution SUPERCONDUCTING MAGNET TOS 2.2 Illustrate the processes involved in magnetic - 1964 EMI (England) developed the CT scanner (by resonance imaging, its instrumentation and image Mr.Hounsfield) acquisition. - Superconductivity is the property of some metals,that allows them to reach infinite conductivity - Niobium-titanium (NbTi) is the metal used in most Magnetic Resonance clinical electromagnets - A process by which certain nuclei, when placed in a - Outermost layer-liquid nitrogen- temp.77K magnetic field, can absorb and release energy in the form - Interior container is filled with liquid helium- temp. 4.2K - Dewar is the giant container that houses the cryogen of radio waves. This technique can be used for chemical liquid analysis or for the production of cross sectional images of  - If the temperature of the magnet rises, the system looses Z gradient coils its superconducting properties and the resultant power - Z gradient coils are used for selection of a transaxial slice dissipation causes the magnet to quench - Axial gradient (z) are produced using Helmholtz coils - Symbolized as Bz or Bss Y gradient coils - Y gradient coils are used for selection of coronal slice - Y gradient is also termed the phaseencoding gradient - Symoblized as Bγ or BΦ Which way does the main magnetic field point in an MR X gradient coils scanner? - X gradient coils are used for the selection of sagittal slice - depends on the type of magnet and its configuration. - X gradient conventionally is termed the - The vast majority of MR scanners sold today are of frequency-encoding gradient cylindrical (tube) design and have their main magnetic - Symbolized as Bx or Br fields directed along the bore of the scanner. - x and y gradients are normally produced by saddle coils - All permanent and dipolar electromagnet scanners have their fields directed vertically or horizontally. Secondary Coils Shim Coils - A principal requirement of the imaging magnet is magnet is field homogeneity. - Shimming is the process of making the B0 field homogeneous. - Magnetic field homogeneity is specified as the ppm over a specific spherical volume 1. The Gz gradient selected an axial slice. 2. The Gy gradient created rows with different phases. 3. The Gx gradient created columns with different frequencies. Gradient Coils Remark: Phase Encoding can only be done one row at the time. - Are made-up of electric coils that produces gradient In order to scan the whole slice the entire process of slice magnetic field. encoding, phase encoding and frequency encoding has to be - The strength of gradient magnetic field and the shape of repeated as many times as specified by the parameter Matrix the RF pulse determine the width of the slice selected. phase encoding (Mxpe). - Gradients strength range from 1-10 mT/m(0.1 t0 1 This also explains the necessity of the scan parameter Repetition G/cm) Time (TR).  - Energizing the three coils simultaneously will result in TOS 2.2 Illustrate the processes involved in magnetic oblique slice resonance imaging, its instrumentation and image acquisition. Antenna - device used for transmitting or receiving radio waves. MRI Parameters - Spin density - T1 relaxation time - T2 relaxation time - X-ray attenuation coefficient of soft tissue differs by < 1% - Spin density and T1 relaxation time of the same tissue differs by 20% to 30% - T2 relaxation time differs as much as 40% - MRI is superior low-contrast resolution RF coils / RF probe - It must accommodate high frequency AC current 10-80 MHz - Initial signal from the RF probe comes from the device called frequency synthesizer - Saddle coil is the most widely used probe design in early MRI - Quadrature coils are the new developed probe design Spin Density - The concentration of nuclei in tissue processing at the Larmor frequency and contributing to the MRI signal. - proton density,HD - SD weighted scans have no contrast from either T2 or T1 decay, the only signal change coming from differences in the amount of available spins (hydrogen nuclei in water).mobile hydrogen - Pulse sequence-spin echo or sometimes a gradient echo sequence, with short TE and long TR. - Radiofrequency(RF) according to Faraday a Radio Frequency wave has an electric and a magnetic component, which are at a right angles from one another, have a 90º phase difference and both move in the same direction with the speed of light - In MRI it is the magnetic component(RF) in which we are interested because that induces the current in the receive coil. - The only way to achieve this is to position the receive coil at right angles to B0 T1 relaxation time - T1 relaxation describes what happens in the Z direction - The time required for the interaction between nuclear - while T2 relaxation describes what happens in the X-Y spins and the tissue lattice to return to normal following plane. RF excitation. Also called spin-lattice or longitudinal relaxation time, (ms) - T1-weighted scans are a standard basic scan, differentiating fat from water - with water darker and fat brighter,use a gradient echo (GRE) sequence, with short TE and short TR - In the brain T1-weighted scans provide good gray matter/white matter contrast - T1-weighted images highlight fat deposition. - A T1 reducing gadolinium contrast agent is also commonly used - T1 of diseased or damage tissue is longer than that for corresponding healthy tissue - T1W images, tissue with short T1 appears bright, tissue with long T1 appears dark - T2 relaxation is the process by which the transverse components of magnetization (Mxy) decay or dephase - T2 relaxation is considered to follow first order kinetics, resulting in a simple exponential decay (like a radio-isotope) with time constant T2. - T2 is the time required for the transverse magnetization to fall to approximately 37% of its initial value - Just like T1 relaxation, T2 relaxation does not happen at once. Again, it depends on how the Hydrogen proton is bound in its molecule and that again is different for each tissue. - T1 relaxation is the process by which the net magnetization (M) grows/returns to its initial maximum value (Mo) parallel to Bo - T1 is the time constant for regrowth of longitudinal magnetization (Mz) - Synonyms: Spin-lattice relaxation, thermal relaxation, longitudinal relaxation - Requires energy transfer from spins to environment ("lattice") - Energy transfer must be stimulated - Stimulation by transverse field fluctuating near the Larmor frequency - Tightly bound protons will release their energy much REMEMBER THIS: quicker to their surroundings than protons which are - T1 and T2 relaxation are two independent processes, bound loosely which happen simultaneously. - T1 happens along the Z-axis; T2 happens in the X-Y plane. - T2 is much quicker than T1 - When both relaxation processes are finished the net magnetization vector is aligned with the main magnetic field (B0) again and the protons are spinning Out-Of-Phase; the situation before we transmitted the 90º RFpulse T2 relaxation time - The time required for the interaction between nuclear spins and adjacent nuclear spins to return to normal following RF excitation. Also called spin-spin or transverse relaxation time. (ms) - T2-weighted scans are another basic type. - Like the T1-weighted scan, fat is differentiated from water - but in this case fat shows darker, and water lighter. - Pulse sequence -long TE and long TR. TOS 3.1 Explain the magnetic resonance imaging examination in terms of: 3.1.1 protocol 3.1.2 preparation 3.1.3 use of contrast agent 3.1.4 limitations of examination. RF PULSE SEQUENCES - is the manner in which the RF is pulse for transmission into the patient - strength, order, duration, and repetition of RF pulses and magnetic gradient used to generate an image Four types of sequence: - Saturation Recovery (SR) – one of the earliest sequence but rarely used today(900-900-900) - Inversion recovery (IR) - provide superior anatomic detail, but takes too much time (1800-900-1800-900-1800) - Spin echo (SE) - it is the most often used (900-1800- 900-1800) - also called Carr-Purcell-MeiboomGill(CPMG) - Gradient Refocused echo (GRE) - employs less than 90 degree pulse and the basis for fast imaging FLASH- fast low angle shot FISP-fast imaging with steady state precession GRASS- gradient recalled acquisition in the steady state The time from the start one sequence to the start of the next sequence is the repetition time(TR) The time between the initial 90 degree RF pulse and the spin echo signal is called time-to-echo (TE) After a 90 degree RF pulse, the signal received from the patient is a. A free induction decay b. A gradient echo c. A spin echo d. Zero a. A free induction decay - The free induction decay (FID) is produced by the precessing spins that are initially in phase but rapidly dephase. - The resulting RF signal is at first of high intensity and rapidly falls to zero. The term pulse sequence refers to a. Gradient magnetic field pulses and RF pulses b. Only gradient magnetic field pulses c. Only RF pulses d. RF pulses and static magnetic field pulses WW II “WATER IS WHITER IN T2” * * Might not hold true for some sequences PRACTICAL POINTS: - T1 scans are better for anatomical detail due to good spatial resolution (tumor extent etc) - T2 scans are better for pathology, since T2 highlights fluid and many pathologies - Gadolinium can ONLY be imaged by T1 - PD is great for imaging the joints and cartilages Why do pathologies appear bright on T2? STIR - Accomplishes essentially the same thing as a T2 fat - A pathologic cell is Edematous. (Increased water content) saturated sequence: the fat becomes dark. - T2W sequences can image WATER accurately so - Most useful when we are looking at bones. pathologic structures in the body light up on T2W - The bone marrow (mostly fat) becomes dark, so most sequences. pathology (usually bright) will light up in contrast. Structures that are bright on T1 - Fat - SUBACUTE hemorrhage - Proteinaceous substances - Melanin - Paramagnetic agents – Gadolinium chelates Structures that are dark on T1 and T2 - Air - Flowing blood (signal void or flow void) - Cortical bone - Ligaments - Tendons MRI SEQUENCES - MR images can appear identical to one another despite difference in sequence. - Sometimes it will be VERY DIFFICULT to distinguish between pulse sequences unless you look at the TR-TE DWI values - Used in the brain to look for acute stroke. - Use in other parts of the body is still experimental. FAT SAT FLAIR Relaxation centers are regions of - This is similar to T2 except the signal from cerebrospinal a. Increased magnetization fluid (CSF) has been suppressed b. Increased proton density - We no longer confuse lesions that are T2- bright from c. Reduced T1 relaxation CSF which is also T2-bright. d. Reduced T2 relaxation - These are the best sequences for a quick look for - Relaxation centers -Contrast agents increase the pathology. magnetic field slightly and change the proton relaxation times, T1 and T2. - MRI contrast agents reduce T1 and T2 relaxation times. MRI - Three basic orthogonal planes used in MRI – axial, sagittal and coronal - Unlike CT, MRI has the ability to image biological functions not just electron density MRI COILS - Next to the part of the body being imaged is the coil. - There are coils made for the head, shoulders, knees, and GRE other body parts. - Very sensitive for blood in the brain. - The coil will emit a radiofrequency that makes a MRI - Also used for cartilage in joints to look for hemosiderin possible. deposits - Without the coil, the body part cannot be imaged properly. TOS 3.2 Illustrate the basic magnetic resonance imaging anatomy. CLASSIFICATION OF MRI CONTRAST AGENTS BY ROUTE OF ADMINISTRATION Via Ingestion - Soluble metal ion(ferric ammonium citrate) - Soluble metal ion chelates(gadolinium DTPA) - Insoluble particulates(perfluoroodylbromide clay, barium, and iron oxide) Via Inhalation - Oxygen Via Intravenous Injection - Metal ion chelates(gadolinium DTPA) - Nitroxide stable free radicals - Paramagnetic contrast agents presently available are 4.1 Assess the biological effects involving application of administered IV and distribute into the blood and magnetic resonance imaging as to magnetic field and extracellular space radiofrequency field effects. - They considered nonspecific agents-not taken by particular organ, tissue, or lesion type Biological Hazard - The elimination half-life is generally in the range of 1-2 - Although MRI is biologically safe, certain physical hours precautions must be taken near the magnet. - NSF-Nephrogenic Systemic Fibrosis - Personnel must remember that the magnetic fields involved are strong and are capable of exerting significant forces on magnetic materials. - One of the greatest potential hazards around a magnet is When use of an MRI contrast agent causes tissue to appear the missile effect brighter, which of the following has occurred? a. Negative contrast b. Positive contrast c. Relaxation contrast d. Proton density contrast MRI contrast agents generally increase signal intensity from the affected tissue. This is termed positive contrast enhancement  The main potential for biological response from RF is Static Magnetic Field a. Carcinogenesis b. Induction of current - Membrane permeability- the ability of a molecule to c. Suppression of relaxation time penetrate a membrane and be transported to the other d. Tissue heating side, because of high electric charge distribution on some membrane, changes in membrane permeability may occur in high magnetic fields - Enzyme kinetics - Nerve conduction - Biopotentials - Cardiac pacemaker- patient with cardiac pacemaker should not be allowed within the 0.5 mT isometric fringe field - Detrimental biological effects from exposure to static The human responses reported during imaging with a 4 Tesla magnetic fields are not evident below 10T system are presumed to be due to - Static fields, however ,have been known to augment the a. Ferromagnetic projectiles T-wave amplitude on ECGs b. The radiofrequency field c. The static magnetic field Gradient Magnetic Field d. Transient magnetic fields - Superimposed on a strong magnetic field are the - When patients and volunteers have been imaged with time-varying gradient magnetic fields these 4 T magnet systems, some responses have - When a time-varying magnetic field interact with the been reported. stationary electrons of certain tissues an electric current - These include metallic taste, twitching, some density measured in amperes per square disequilibrium, and phosphene induction. centimeter(A/cm2 ) can be induced Presumably, the transient magnetic fields caused - Visual phosphenes these responses. - Bone healing - Cardiac fibrillation MRI Safety Protocols - Time varying magnetic field induce currents in the - Important to prevent accidents and injuries in the MRI patient can induce mild cutaneous sensations, environment involuntary muscle contractions and cardiac - Before entering gantry – Remove ALL metal objects, arrhythmias devices and equipments. - Activation and deactivation current through the gradient - Staff should be fully trained in MRI safety coils during image acquisition creates a significant amount of acoustic noise.(LORENTZ) 1. Apply MR safety practices with respect to MR screening 2. Ensure that the patient screening forms are completed Radiofrequency Exposure 3. Demonstrate an understanding of the magnetic properties of - The principal result of the interaction between an RF field foreign objects and tissue is heat 4. Determine whether foreign objects in/on the patient’s body - Heating is expressed as the specific absorption rate constitute a contraindication to MR (SAR) measured in watts per kilogram(W/kg) 5. Determine exceptions to contraindications during the screening - The normal resting state is approximately 1.5 W/kg process - Looped wire on patient’s skin can heat up and cause burns - Prevent any electrically conductive material (i.e. cable of surface coil) form a "conductive loop" with itself or with the patient. - Tissue or clothing could potentially be ignited by uninsulated cables. - Coupling of a transmitting coil to a receive coil may cause severe thermal injury. 6. Determine patient’s pregnancy status and take appropriate - The SAR is highest with pulse sequences that require action many 180° RF pulses, such as fast spin echo (FSE). 7. Screen MR personnel and support staff - It is prudent to exclude women in the first trimester of pregnancy from the inner controlled area. - All unauthorized persons must be medically screened and warned of potential hazards (projectile effect, malfunctioning of some devices in the presence of a magnetic field), before entering the controlled area. MR CONTROLLED AREA 15. Demonstrate an understanding of potential RF bio-effects and - A volume totally enclosed and of such a size to contain specific absorption rate (SAR) the 0.5 mT (5 Gauss) magnetic field contour. Access 16. Utilize safe practice in RF coil and equipment cable placement should be restricted and suitable signs should be displayed at all entrances INNER MR CONTROLLED AREA - It may be convenient to define an INNER MR CONTROLLED AREA within the confines of the MR CONTROLLED AREA containing the 3 mT (30 Gauss) magnetic field contour. Where there is only one area (i.e. no INNER MR CONTROLLED AREA) all references in 17. Apply emergency procedures for incidents and accidents in the these guidelines to INNER MR CONTROLLED AREA will MR environment apply to the whole of the MR CONTROLLED AREA 18. Implement emergency procedure for fire - A field strength of 3 mT was chosen for the INNER MR 19. Implement emergency procedure for cardiac arrest CONTROLLED AREA to avoid the projectile hazard 20. Implement emergency procedure for anaphylactic reaction 21. Implement emergency procedure for a projectile in the magnetic field 22. Implement emergency procedure for changes in patient medical status 23. Implement emergency procedure for quench If a quench occurs and helium escapes, the principal effect on the MRI technologist will be a. Donald Duck’s voice b. Lowered body temperature c. Skin rash d. Superficial burn - There are safety concerns in the event of an unplanned quench. One may first hear a hissing noise caused by the release of helium, nitrogen, or both. Everyone should be immediately evacuated because both gases can displace oxygen and cause asphyxiation. 8. Assess and remove any ancillary contraindicated objects from - Then, as you are dying, your voice will sound like the patient, MR personnel and Support Staff that of Donald Duck 9. Identify the safety considerations related to the presence of ferromagnetic equipment/accessories (MR compatible or MR safe) in the magnetic field 24. Implement emergency procedure for pre-existing medical conditions 25. Educate emergency personnel about MR procedures and safety 10. Apply safety guidelines with respect to MR bioeffects The principal hazard to patient and personnel in an MRI 11. Provide hearing protection to the patient and attending facility is the biological effect of individuals a. Ferromagnetic projectiles 12. Knowledgeable of MR safety standard regulations for b. The main magnetic field radiofrequency (RF), static magnetic fields and gradient magnetic c. The RF field fields imposed by the government d. The transient magnetic field 13. Demonstrate an understanding of potential bio-effects of the static magnetic fields 14. Demonstrate an understanding of potential bio-effects of the time varying (gradient) magnetic fields  Unit of dose associated with MRI Principal Mechanism of Interaction of the three MRI energy fields Three Magnetic states of matter TOS 4.2 Distinguish the contraindications of magnetic resonance application and safety measures. MRI Patient Considerations - Heart pacemaker - Metallic foreign body (metal sliver) in the eye, - Aneurysm clip in the brain - Patients with severe claustrophobia - Patients who have had metallic devices placed in their back (such as pedicle screws or anterior interbody cages) can have an MRI scan, but the resolution of the scan is often severely hampered by the metal device and the spine is not well imaged.