MRI THE BASICS PDF

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Basics of MRI. Professor Sir Michael Brady FRS FREng. Department of Engineering Science. Oxford University. Michaelmas The Basics of MRI is an introductory, open-access, hypertext book on the physics of magnetic resonance imaging. Magnetic resonance imaging (MRI) is an imaging technique based upon the principles of nuclear magnetic resonance (NMR) [28]. Fortunately, the narrowband response is not a. Magnetic resonance imaging (MRI) is a spectroscopic imaging technique . http ://xumodaperma.ml Retrieved: 7/7/ ▫ Buckwalter, Ken.


Mri The Basics Pdf

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such as TI, T2, and T2*, as well as mepeti- tion time, echo time, and tissue contrast enhancement. Although it is not particu- lamly easy to read, the authors have. coordinate system and the relationship between a complex number, its angle θ, and its real and imaginary components. Page 6 of Ovid: MRI: The Basics. MRI: The Basics. Concise, readable, and engaging, this 4th edition offers an excellent introduction to the physics behind MR imaging. Clinically relevant.

It is therefore a very sensitive tool for detecting soft tissue and Metal prostheses obscure anatomy due to marrow pathology. Some centres use a similar susceptibility artifact.

Fast spin echo technique of fat suppressed T2 sequences. Fat techniques however reduce the artifact and suppression may be used to confirm the fatty should be employed if there has been previous nature of a lipoma Fig. All sequences however will show artifact to a lesser or greater extent due to metallic foreign structures.

Gadolinium is a paramagnetic contrast agent that causes tissue enhancement in vascular structures when administered intravenously. Enhancement is best evaluated on T1 sequences and fat saturation may also be used to optimize the enhancement especially when the abnormality is situated within fatty tissue, which would also be bright on T1. Fluid tissue.

Intra-articular dilute gadolinium is also appears bright as with other T2 sequences. It is used in detecting labral tears, meniscal tears in particularly good at imaging ligaments and postoperative knees, ligamentous injuries, and articular cartilage, particularly fibrocartilage osteochondral defects.

Figure 20 [35] such as the menisci and labrum of the hip and glenoid.

The images can be acquired in extremely thin 3D volumes making it very useful in assessing small structures such as the ligaments of the wrist and reconstruction is possible in various planes. The sequence is however degraded significantly if adjacent tissues have widely differing magnetic properties such as metal and soft tissue resulting in susceptibility artifact. Both T2 and PD sequences can be acquired at the same time, reducing imaging time and therefore MR Angiography techniques: Three primary decreasing motion artifact.

However, on fast techniques use are — time of flight TOF , spin echo T2 sequences fat remains quite phace contrast PC and contrast enhanced MR bright and lesions within bone marrow, which angiography. As short TR values are In addition, on fast spin echo PD image used, and the slices are oriented almost degradation may result in meniscal pathology perpendicular to the direction of blood flow , not being seen.

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The signal radiofrequency pulse has been applied by produced by these spins is low and results in varying the time to detect the signal TE— dark pixel values in the image. Blood flowing time to echo and the time to apply the next into the tissue contains fresh spins that have radiofrequency pulse TR— time to not yet experienced RF pulses. The signal repetition. Liquids have long T1 and T2 produced by these spins gives bright pixels in values whilst fat has short T1 and T2 values, the image.

Image is the reformatted using thus varying the TE and TR can weight the maximum intensity projection technique. For example, Vascular structures containing moving blood increasing the time to echo and time for pulse are thus visualized. On T1 any effect of bipolar gradients. Therefore a T1 shortening properties of a paramagnetic region of bone bruising would appear as a contrast agent e.

Rapid T1 weighted sequences are appear bright on a T2 sequence. Some tissues such as ligaments, Interpretation: Haemopoietic marrow seen in the times. Structures with STIR and fat suppressed T2 are the optimal little water or fat such as cortical bone, tendon sequences for looking for marrow pathology and ligaments remain dark in all types of and bone bruising in trauma whilst T1 sequences table 2.

Table 2 Muscle: Many MRI scanners have a specific cartilage sensitive sequence that may also be used. Fibrocartilage and hyaline cartilage return a dark and intermediate on all sequences respectively. When confronted with a series of MRI Synovium: The synovium is difficult to images, the first thing to do is identify known visualize unless it is abnormal where it areas of fluid, for example CSF, hydrated enhances post contrast administration but is intervertebral discs, the urinary bladder and indistinguishable from fluid on T2 and STIR renal pelvis.

If these areas return a high signal, sequence. Fat in these cases will high signal on then be dark and the images are usually STIR then compared with the T1 images pre-contrast to Glodolinium ascertain the degree, if any of enhancement.

MRI: The Basics, 3rd Edition

Scanners can be grouped roughly T1 by field strength. The strength of a magnet correlates with image Table 4 quality although very high field strengths may Body Sequence Plane paradoxically lead to increased artifact. These magnets usually examined this way. For evaluation of smaller generally operate at lower field strengths articular structures, such as the menisci of the Fig. Short bore magnets have also been knee or the rotator cuff, specialized surface developed that combine the high field strength coils are needed.

Several types of surface coils and anatomical accuracy of a tunnel magnet are available, including coils tailored for with the comfort of an open magnet. These coil to be performed in more anatomical positions serve as antennae placed close to the imaging Fig.

Larger coils have been developed with phased-array technology, providing the improved signal that is seen in smaller coils with an expanded coverage area.

These phased-array coils are commercially available for the knee, shoulder and torso and are compatible with most new MRI systems. Optimal coil selection is mandatory for high- quality imaging of joints or small parts.

The varying magnetic fields can induce electric current in biologic tissue especially the heart, nervous tissue and potentially retina. Since peripheral nerve stimulation represents only a sensation and not a deleterious effect, it can be used as a safe level of operation. This may be satisfactory when studying The radio frequency radiation stimulates the chest, abdomen, or pelvis, where a large oscillation of atoms and molecules that causes area is to be evaluated.

No one with in-situ devices like humans. All equipments entering scan room interpulse delay and the absorption properties including those that are moved in and of the patient. Absorption is essentially out should be verified to be MR safe.

SAR for a given pulse sequence for a given 4.

The ECG leads must be placed to patient, which is an overestimate of RF heating centre of magnet bore and not because most current models do not account touching the magnet bore wall. Position the patient away from any of healthy human.

So a rise in body wires and sides of bore. Any ferromagnetic substance can patients with uncompromised cardiovascular become projectile with potential to or thermoregulatory systems and RF heating in cause harm.

So pockets should be not considered a contraindication for use of emptied prior to entering scan room routine clinical MRI. FDA safety parameters to minimize chances of projectile. Where noise is an issue, Specific Absorption Whole body: Or 99 dB weighted rms Operational aspect sound pressure level The detailed operational procedures for any with hearing MRI system are dependent to the operation of protection the department and institution.

Security issues are very important for MRI. Safety Magnetic field strength of 1 to 4 T has The MRI environment is different from other sufficient power to lift a wheelchair off the imaging areas posing many issues associated ground and pull it to the magnet. So its with patient and occupational safety. Hence, important that the entrance to the magnet room the safety procedures are designed to protect be limited and under the control of appropriate everyone in scan room.

It is important to personnel. Access to individuals with provide personnel on going training and pacemaker should be restricted to areas with support with routine review of established fields greater than 5 g, as pacemakers may safety procedures and fire.

Metallic objects particularly the equipments that may The safety risks of patients and occupational leave the room for servicing or modification , risks can be minimized by taking care of should be tested with a strong, small magnet to following points: For There are many patient and personnel safety superconducting magnet systems, a structure issues that must be discussed and decided upon must be provided to allow for escape of large during the design and installation of MRI.

It may as to which devices, if present, would be important to consider hospital gas access constitute a contraindication for an MRI and a designated code area near MRI suite. Fullerton GD. Basic concepts for for both patient comfort and assurance of nuclear magnetic resonance imaging. Both respiratory and cardiac Magn Reson Imaging ;1: Nuclear magnetic resonance All alarm systems should be brought to one imaging. Volume 4, panel so that it can be located easily. Magnet Special Edition, January ;4: Young SW.

Nuclear magnetic authorized personnel.

Some warning signal can resonance imaging: Raven Press, Balter S. An introduction to the MR safe air packs can be made available for an physics of magnetic resonance unlikely event of a quench with the release of imaging.

Radiographics ; helium into magnet room, which has Magnetic resonance Its essential that apart from MRI personnel, the imaging signal concepts. American College of Radiology. Glossary of MR terms. Reston The design and implementations of VA: American College of Radiology, acquisitions protocols is highly important to NMR Fourier zeugmatography. J Magn Reson ; Hutchison J. NMR scanning: The RF shield is used to warp method. It is also needed to maintain a al.

Magnetic resonance imaging high degree of magnetic homogeneity because phase encoding: This can also be Acronyms common to MRI. SMRI by using additional magnetic coils to produce MR research guide, some small magnetic fields that increase or edition. Haacke EM. Magn Reson uniform field; or it can be passive by placing Imaging ;6: So an Motion actively shielded magnet may require minimal artifacts in brain and spine MR.

Physiol Chem Phys Physiol Meas ; A rapid gated Kressel HY, al. Change in human cardiac rhythum ed. Mgnetic resonance annual, New induced by a 2-T static magnetic York: Radiology ; Spatial Radiology anatomy in MR imaging. Radiology ; P: Food and Drug Administration.

Basics of MRI.pdf

Rapid NMR device: J report on petitions for MR Magn Reson ;67; Fed Reg Mansfield P. Multi-planar image ; J Phys E American College of Radiology Dixon WT. After termination of the radio transmission the nuclei will relax back with emission of radio waves NMR signal.

This basic phenomenon is illustrated in figure 4. T1 measures the rate as well as adjacent molecular movements. The of return of the nuclei to alignment with the first thing that occurs after RF transmission is static magnetic field B0 and is influenced by phasing of individual nuclear spins to create a the other surrounding atoms i.

This is similar to military lattice relaxation time. The phenomenon is hence called transverse relaxation measured by T2. In a chemical sense this is due to interactions that occur between adjacent nuclei and hence called spin relaxation. As all the nuclei are also simultaneously returning to alignment with main magnetic field B0 along z axis the longitudinal or z component of the magnetization vector Mz increases or grows with time.

It is useful to divide it into longitudinal Mz and transverse Mxy Free induction decay FID : The basic form components in reference to the static magnetic of NMR signal is a time oscillating decaying waveform called free induction decay field which is applied along z axis by detected in the xy or transverse plane. RF transmission being an electromagnetic wave carries with it a magnetic field B1 and hence can tip the nuclei out of equilibrium if it coincides with Larmor frequency.

The most commonly used imaging technique, 2 dimensional Fourier transform 2DFT uses 3 such gradient fields to localize a point in space, the particular application and relative timing the gradients will be turned on and off at different times of x, y and z gradients determines which axis x, y or z is being localized by selective application of slice selection, phase encoding and frequency encoding gradients. In this direction and helps in image acquisition , uniform situation image acquisition is radiofrequency RF unit for excitation and impossible!

MRI system and coordinates modified by additional field gradient in y- are shown in figure 8. As shown in figure 9, in uniform z axis field, all nuclei precess at Larmour frequency but on adding a field in y axis, nuclei in row 1 will precess at higher frequency than those in row 2,likewise for rows 2 and 3.

When the field is turned off, all the rows will again precess at the same frequency but nuclei in row 1 will be ahead of those in row 2 as row 1 was previously precessing at a higher frequency which in turn will be ahead of those in row 3 i.

This process is repeated many e. As it is applied at the time of signal reading, it is Spatial localization is commonly obtained by called readout gradient. We The MR signal is digitized and stored on the know that precessional frequency is directly acquisition workstation in k-space for related to magnetic field.

If a spatially varying subsequent Fourier reconstruction to form the magnetic field is superimposed on the main clinical image. Each point in k-space magnetic field, then precessional frequency represents a different spatial frequency in the object being imaged. The strength of the MR signal and thus the value of a data point in k- space indicates the degree to which the spatial frequency is represented in the object.

Lower Three dimensional Fourier imaging can also be spatial frequencies are located near the center performed, the third dimension added by of k-space and contain information related to exciting a larger slab or an entire sample, image contrast. Higher spatial frequencies are phase encoding the spatial information in two located at the periphery of k-space and contain directions and frequency encoding in third information related to image sharpness figure direction and using fast techniques.

Slice Pulse sequences: The recipe for MRI pulse sequences basically involves the application of RF pulses, the sequencing of gradient pulses in x, y and z direction and acquisition of the resultant MR signal. The entire pulse sequence is repeated following a delay interval TR. Flow artifacts occur because the nuclear spins in the flowing fluid accure phase in the presence of gradient which varies from view to view.

GMN is accomplished by variations in slice selection and frequency encoding gradients and essentially eliminates phase acccural and produces image in which vassel wall has high density. It eliminates high signal due to inflow and produces image in which vessel wall has low density.

Accordingly, techniques have been developed [12,16,17] to acquire MRI data in very short time periods. Hence, the repletion time can be sequence than with spin echo SE sequence in decreased, allowing faster acquisitions. The pulse sequence properties of tissues whereas TE and TR are of this technique is of gradient recalled echo properties of image acquisition under GRE or field echo FE is shown in Figure.

The TE and TR of image The principle of GRE imaging lies in the fact acquisition can thus be manipulated such that that the application of a gradient actually the difference between signals from tissues dephases the spin due to the difference in the which ultimately determines the image field strength, which is dependent upon contrast is weighted by spin density, T1 or T2 position. By reversing the direction of the relaxation times. Flow and motion compensation Another technique uses echoes to acquire Any movement of structures during MRI different lines of k-space data views , rather caused by fluid flow or physiological motion than to create multiple images.

Acquiring eight e. In totally obscure the image.

As the total time in general, useful image contrast and resolution phase encoding direction TR x no phase are obtained in shorter acquisition time when encoding steps is substantially greater than the compared to standard one view per TR sampling time in frequency encoding direction acquisition.

Often this require special hardware, artifacts usually appear as linear bands in although interleaved versions of EPI can be used on standard image hardware. This results in breathhold acquisition times for a single image of 1 to 20 seconds, depending upon number of interleaves and desired image resolution.

In general, the primary sources of artifacts are those affect the frequency or phase dispersion of the spin system. Recall that, fundamentally, the signal strength at a The chemical shift phenomenon can be utilized particular frequency is interpreted to reside at a to obtain images that suppress either fat or particular point in space.

Hence, anything that water signals. The technique first reported by changes the frequency or add signal at a Dikson et al [18] uses SE pulse sequence or particular frequency will result in a gradient echo images acquired at specific TE displacement or addition of intensity in values.

Due to differences in precessional direction of frequency encoding gradient. In an frequencies, the fat and water signals can be analogue fashion, something that changes the obtained when the two are either together or relative order of the phase of the spin in phase or opposite opposed.

Algebraic precession either during or between views combination of these images results in will produce artifacts in phase encoding suppression of one or the other signal. Figure direction. The sources of such artifacts are 15 wide ranging, such as data sampling problems, electromagnetic noise, patient motion, magnetic field inhomogeneities, magnetic susceptibility, computer malfunction and chemical shift [].

It is also important to understand that particular types of artifacts may also be associated with specific type of imaging procedures e. Chemical shift imaging techniques: The magnetic field actually experienced by a nucleus depends upon the molecular configuration of the chemical backbone to which the nucleus is attached as moving electrons around the atom bring about slight changes in the magnetic field truly experienced by the nucleus due to shielding effects.

Hence at a constant field strength, precessional Fat suppression can be accomplished using frequency of hydrogen nucleus in water will be STIR short tau inversion recovery pulse different from that in fat difference being sequence. Figure 14 minimal. Spectral presaturation fat sat is accomplished by applying RF pulse to saturate only fat based spins prior to application of the imaging pulse sequence.

Classic fat suppression or water excitation technique excites a narrow range of proton frequencies centered on water.

Fat protons that are not excited will not contribute to MR signal. Anatomical detail and spatial resolution are excellent. Fat, sub acute hemorrhage and proteinaceous fluid are bright. Fluid is however dark on T1. T1 is good for looking for meniscal pathology and marrow. The contrast is a function of the number of protons in the tissue figure This technique quality is miniscule but this is offset by the decreases the signal intensity from fat i.

It is therefore a very sensitive tool for detecting soft tissue and Metal prostheses obscure anatomy due to marrow pathology.

Some centres use a similar susceptibility artifact. Fast spin echo technique of fat suppressed T2 sequences. Fat techniques however reduce the artifact and suppression may be used to confirm the fatty should be employed if there has been previous nature of a lipoma Fig. All sequences however will show artifact to a lesser or greater extent due to metallic foreign structures. Enhancement is best evaluated on T1 sequences and fat saturation may also be used to optimize the enhancement especially when the abnormality is situated within fatty tissue, which would also be bright on T1.

Fluid tissue. Intra-articular dilute gadolinium is also appears bright as with other T2 sequences. It is used in detecting labral tears, meniscal tears in particularly good at imaging ligaments and postoperative knees, ligamentous injuries, and articular cartilage, particularly fibrocartilage osteochondral defects. Figure 20 [35] such as the menisci and labrum of the hip and glenoid.

The images can be acquired in extremely thin 3D volumes making it very useful in assessing small structures such as the ligaments of the wrist and reconstruction is possible in various planes.

The sequence is however degraded significantly if adjacent tissues have widely differing magnetic properties such as metal and soft tissue resulting in susceptibility artifact. Both T2 and PD sequences can be acquired at the same time, reducing imaging time and therefore MR Angiography techniques: Three primary decreasing motion artifact.

However, on fast techniques use are — time of flight TOF , spin echo T2 sequences fat remains quite phace contrast PC and contrast enhanced MR bright and lesions within bone marrow, which angiography. As short TR values are In addition, on fast spin echo PD image used, and the slices are oriented almost degradation may result in meniscal pathology perpendicular to the direction of blood flow , not being seen.

The degradation in image the net magnetization from stationary spins does not have time to recover fully. The signal radiofrequency pulse has been applied by produced by these spins is low and results in varying the time to detect the signal TE— dark pixel values in the image.

Blood flowing time to echo and the time to apply the next into the tissue contains fresh spins that have radiofrequency pulse TR— time to not yet experienced RF pulses.

The signal repetition. Liquids have long T1 and T2 produced by these spins gives bright pixels in values whilst fat has short T1 and T2 values, the image. Image is the reformatted using thus varying the TE and TR can weight the maximum intensity projection technique. For example, Vascular structures containing moving blood increasing the time to echo and time for pulse are thus visualized.

On T1 any effect of bipolar gradients. Therefore a T1 shortening properties of a paramagnetic region of bone bruising would appear as a contrast agent e. Rapid T1 weighted sequences are appear bright on a T2 sequence.

Collagen: Some tissues such as ligaments, Interpretation: tendons and fibrocartilage do not contain free hydrogen atoms water to contribute a signal The main sequences used in daily practice are and are therefore dark on T1 and T2 T1 and T2 sequences with STIR FAT sequences. Structures with STIR and fat suppressed T2 are the optimal little water or fat such as cortical bone, tendon sequences for looking for marrow pathology and ligaments remain dark in all types of and bone bruising in trauma whilst T1 sequences table 2.

Many MRI scanners have a specific cartilage sensitive sequence that may also be used. Fibrocartilage and hyaline cartilage return a dark and intermediate on all sequences respectively.

When confronted with a series of MRI Synovium: The synovium is difficult to images, the first thing to do is identify known visualize unless it is abnormal where it areas of fluid, for example CSF, hydrated enhances post contrast administration but is intervertebral discs, the urinary bladder and indistinguishable from fluid on T2 and STIR renal pelvis.

If these areas return a high signal, sequence. Fat in these cases will high signal on then be dark and the images are usually STIR then compared with the T1 images pre-contrast to Glodolinium ascertain the degree, if any of enhancement.

Scanners can be grouped roughly T1 by field strength. The strength of a magnet correlates with image Table 4 quality although very high field strengths may Body Sequence Plane paradoxically lead to increased artifact. These magnets usually examined this way. For evaluation of smaller generally operate at lower field strengths articular structures, such as the menisci of the Fig. Short bore magnets have also been knee or the rotator cuff, specialized surface developed that combine the high field strength coils are needed.

Several types of surface coils and anatomical accuracy of a tunnel magnet are available, including coils tailored for with the comfort of an open magnet. These coil to be performed in more anatomical positions serve as antennae placed close to the imaging Fig. Larger coils have been developed with phased-array technology, providing the improved signal that is seen in smaller coils with an expanded coverage area.

These phased-array coils are commercially available for the knee, shoulder and torso and are compatible with most new MRI systems.

Optimal coil selection is mandatory for high- quality imaging of joints or small parts. The varying magnetic fields can induce electric current in biologic tissue especially the heart, nervous tissue and potentially retina. Since peripheral nerve stimulation represents only a sensation and not a deleterious effect, it can be used as a safe level of operation.

This may be satisfactory when studying The radio frequency radiation stimulates the chest, abdomen, or pelvis, where a large oscillation of atoms and molecules that causes area is to be evaluated.Often this require special hardware, artifacts usually appear as linear bands in although interleaved versions of EPI can be used on standard image hardware. Click here to sign up.

Figure 14 minimal. Marve Felix. If the address matches an existing account you will receive an email with instructions to retrieve your username. In general, the primary sources of artifacts are those affect the frequency or phase dispersion of the spin system. Health and Basic principles of MRI. In: Kressel HY, al.

Basics of MRI.