MRI Physics

Dr. MD. Mofazzal Sharif


MRI


HISTORY• The phenomenon of Nuclear Magnetic Resonance (NMR)

was discovered in 1950s .• It was not until the 1970s when Lauterbur introduced the

concept of magnetic field gradients, that an image based on magnetic resonance could be produced.

• By the 1980s whole body magnets were being produced in England permitting the first in vivo images of human anatomy.

• Today the technique, known as MR imaging, is widespread and an estimated 20 million scans are performed worldwide each year.

• It provides images with excellent soft-tissue contrast which can be acquired in any imaging plane, and unlike CT it does not involve the use of ionizing radiation.


Basic NuclearMagnetic Resonance

•Atomic structure.•Motion within the atom.•MR active nucleus.•The Hydrogen nucleus.•Alignment.•Precission.•The Larmor Equation.•Resonance.•MR signal•NMR Signal.(The Free Induction Decay)• T1 Recovery.• T2 Relaxation.•Pulse timing parameter.








Normally, the direction that these tiny magnets (protons) point-in is randomly distributed.


Just as a compass aligns with the earth's magnetic field, a spinning proton placed near (or within) a large external magnetic field (called Bø) will align with the external field. At the atomic level, some of the protons align with the field and some actually align against the field.After canceling each other out the slight excess proton those will align in parallel with the external field is the net magnetic moment which give MR signal..









MRI (Image contrast)

•Contrast means defining the edge of an image.•It depends on signal emitted by the tissue.•Tissue with high NMV gives high signal.(Which appears bright)•Tissue with low NMV gives low signal.(Which appears black)•Generally the two extreme of contrast in MRI are water and fat.


Fat & Water• Fat is hydrogen linked to carbon &

consist of large molecules called lipid.• Water is hydrogen linked to oxygen which

tends to steal the electrons away from around hydrogen nucleus.This renders it more available to the effect of main magnetic field. So water has more molecular tumbling.

• In fat, electron cloud protects the nucleus from the effect of magnetic field. So water has less molecular tumbling.


Contrast mechanism

• Image obtained contrast by---

• T1 recovery.

• T2 decay.

• Proton density.(No. of proton / unit volume of tissue)


In Fat • Slow molecular tumbling

• Magnetic moments of nuclei are able to relax in transverse plane quickly,regain their longitudinal magnetization quickly by exchanging their energy to neighbor

• NMV realigns rapidly with Bo from the transverse plane

• So fat has short T1 recovery time & short T2 time(80ms.)


In water• Rapid molecular tumbling

• Magnetic moments of nuclei take long time to relax in transverse plane as they exchange their energy to neighbor in transverse plane slowly as well as regain their longitudinal magnetization slowly.

• NMV realigns slowly with Bo from the transverse plane

• So fat has long T1 recovery time & long T2 time(200ms.)


T1 contrast• T1 time of fat is shorter than water • NMV of fat realigns more rapidly than water • So the longitudinal component of fat is

larger than water in T1 time• After 90o RF pulse

• There will be more transverse magnetization of fat than water

• Therefore high signal of fat in T1 time

• Fat appears bright in T1 contrast image (and water due to low signal appears dark on a T1 contrast image)


T1


T2 contrast• T2 time of fat is shorter than water • The transverse component of fat decay

faster than water in T2 time• There will be less transverse

magnetization of fat than water • Therefore low signal of fat in T2 time

• Fat appears black in T2 contrast image (and water due to high signal appears bright on a T2 contrast image)


T2


Proton density contrastThis depends on the patient and area

being examined.• There is difference in signal intensity

between tissues with different PD

• Large PD will cause a large transverse component of magnetization

• High signal---Bright• Low signal ---Dark


Proton Density


Weighting

• To demonstrate either T1, PD or T2 contrast specific value of TR and TE are selected for a given pulse sequence.


T1 Weighting• T1 weighted image is one where the contrast depends predominantly on the

difference in the T1 time between fat and water. • The TR controls how each vector can recover before before it is excited by

next RF pulse, so to have a T1 weighting image TR must be short enough so that neither fat nor water has sufficient time to to fully return to Bo.

• The TR if is too long both fat and water returns to Bo and recover their longitudinal magnetization fully (no contrast difference).

• TR—Controls T1 weighting.• Short TR--- For T1 weighting image.


T2 weighting• T2 weighted image is one where the contrast depends

predominantly on the difference in the T2 time between fat and water.

• The TE controls the amount of T2 decay that is allowed to occur before the signal received.To have a T2 weighting image TE must be long enough to give both fat and water time to decay.

• The TE if is too short both fat and water has time to decay.(no contrast difference).

• TR —Controls T2 weighting.• Long TR--- For T2 weighting image


PD weightingProton density weighting is always present.• In order to achieve this the effect of T1 and

T2 must be diminished .• A long TR allows both fat and water fully

recover their longitudinal magnetization and diminishes T1 weighting.

• A short TE does not give fat or water time to decay and therefore diminishes T2 weighting.

• In any image the contrast due to PD with T1 and T2 mechanisms occur simultaneously and contribute to image contrast.


Typical value of TE and TR • For T1 weighting image:• To exaggerate T1 – TR is short• To diminish T2 -- TE is short

• For T2 weighting image:• To exaggerate T2 – TR is long• To diminish T1 -- TE is long

• For PD weighting image:• To diminish T1 – TR is long• To diminish T2 -- TE is short


But practically what is happened?

T2 decay faster due to –

1. Magnetic field inhomogenicity

2. T2 decay itself

This is called T2* decay which is the decay of the FID following following RF pulse.


Inhomogenicity• They are the areas within the within the magnetic

field that do not exactly match the external magnetic field strength.

• Some areas have a magnetic strength less than main magnetic field – processional frequency of the nuclei on that specific area decrease.

• Some areas have a magnetic strength more than main magnetic field – processional frequency of the nuclei on that specific area increase.

• This rate of dephasing due to inhomogenicity is an exponential process.


How this can be overcome?


The spin echo pulse sequence The spin echo pulse sequence

utilizes a 90o RF pulse to flip the NMV into transverse plane.

• The NMV precesses in the transverse plane inducing a voltage in the receiver coil.

• When the RF pulse is withdrawn a free induction decay produces T2* dephasing occurs immediately.

• A 180o pulse is then used to compensated for this dephasing.


• The 180o pulse is an RF pulse that has sufficient energy to move the NMV through 180o .

• The 180o RF pulse flips these individual magnetic moment through 180o.

• The magnetic moment that formed the trailing edge before the 180o pulse form the leading edge prior to 180o pulse.

• The direction of precession remains the same and trailing edge begins to catch up the leading edge.At specific time later two edges are superimposed.The magnetic moment are now momentarily in phase. The spin echo now contains T1 & T2 information as T2* dephasing has been reduced.


Timing Parameter in spin echo

• Long TR-2000 ms

• Short TR-250-700 ms

• Long TE-60 ms+

• Short TE-10-25 ms


Spine echo by using one 180o pulse• This pulse sequence can be used to produce T1weighted image if

short TR & TE are used.• A short TE ensures 180o RF pulse and subsequent echo occurs early

and so that only a little T2 decay has occurred. (This difference in T2 times of the tissue do not dominate the echo and the contrast)

• A short TR ensures that the fat and water vector have not fully recovered (So difference in the T1 time dominate echo and contrast) – T1 weighted image.


Spine echo by using two 180o pulses• This can be used to produce both a PD and

T2 weighted images.• The fast spin echo is generated early by

selecting a short TE (only a little T2 decay has occurred and so T2 difference between the tissue are minimized) – So PD image


Spine echo by using two 180o pulses• The second spin decay is generated much later by selecting long

TE(a significant amount of T2 decay has occurred now and difference in the T2 times of the tissue is maximized) – The TR selected is long so the T1 difference between the tissues are minimized – T2 weighted image





To Summarize

• Short TR short TE – T1

• Long TR long TE – T2

• Long TR short TE -- PD


The Gradient Echo Pulse Sequence• This can flip NMV through any angle not only 90o –So it uses a

variable RF excitation pulse.• Magnetic field gradient are generated by a coil of wire situated within

the bore of magnet.• When current passes through gradient coil it interacts with the main

magnetic field.• Gradient Echo pulse sequence use a gradient to re-phase the magnetic

moment.• The main advantage of gradient echo pulse sequence is it has a

shorter scanning time as there is no 1800 RF pulse.• Main disadvantage is there is no compensation for magnetic field

inhomogenecity.T2* effect are not eliminated, in gradient echo imaging T2 weighted image is called T2* weighted image.


Typical Value in gradient echo imaging

• Long TR – 100ms+

• Short TR– 30-60 ms

• Long TE – 15-25 ms

• Short TE – 5-10ms

• Long flip angle – 70o-110o

• Short flip angle – 5o- 20o

In Spin Echo:• Long TR-2000 ms• Short TR-250-700 ms• Long TE-60 ms+• Short TE-10-25 ms


Contrast values (Gradient echo)• PD weighted: Small flip angle (no T1), long TR

(no T1) and short TE (no T2*)

• T1 weighted: Large flip angle (70°), short TR (less than 50ms) and short TE

• T2* weighted: Small flip angle, some longer TR (100 ms) and long TE (20 ms)

In Spin Echo Sequence:Short TR short TE – T1 Long TR long TE – T2Long TR short TE -- PD


• Gradient-echo MRI is capable of detecting millimeter-size paramagnetic blood products in brain parenchyma, and has a greater sensitivity than spin-echo sequences. Microhaemorrhages appear larger than they are due to the “blooming” effect (due to distortion of the local magnetic field). Microhaemorrhages are visible for many years because haemosiderin is stored in macrophages. Causes of Microhaemorrhages other than trauma include: hypertensive vasculopathy; cerebral amyliod angiopathy; cerebral cavernous malformations; cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL); and cerebral infarcts.


• This 20 year-old female patient had a follow-up MRI after head injury 4 months previously. The axial gradient-echo (FLASH) sequence shows punctuate areas of signal loss at the grey-white interface in the frontal lobes.


Instrumentation

• The Magnet

• RF Coils

• Gradients Coils


The Magnet• The main component of the MR scanner is the magnet itself.

• Higher magnetic field strength magnet has shorter scanning time (Aligning time less) and better resolution. (Because there are more excess protons available to make the MRI signal as MFS increases and there is better SNR).

• Magnet may be permanent , resistive or superconductive. But superconductive magnet is best for imaging but superconductivity needs better cooling.

• To maintain this cooling the magnet is enclosed and cooled by a cryogen containing liquid helium (sometimes also nitrogen) which has to be topped-up (added) on a monthly basis. Imperfections in the superconductive windings means that the scanner will lose its magnetic field strength 5-10 G per year.

• Far more serious thing happens, when the magnet suddenly loses its superconductivity and begins to heat up causing the cryogens to boil and escape.


On the left is a picture of our 1.5 Tesla GE Sigma scanner.

On the right is a picture of a 1.5 Tesla system, a Philips intera. Also shown in this picture is the RF head coil on the patient bed.

Other types of whole-body scanner include open systems which use vertically orientated field designs to reduce claustrophobia or enable surgical procedures to be carried out.


RF Coils• This is used to transmit and receive RF pulse.• Smaller the coil better the resolution.• In order to optimize signal-to-noise ratio (SNR), the RF coils should cover only the

volume of interest. This is because the coil is sensitive to noise from the whole volume while the signal comes from the slice of interest.

• The most homogenous coils are of a 'birdcage' design. Examples of these include the head and body coils.

• The body coil is integrated into the scanner bore and cannot be seen by the patient.• Surface coils, as the name suggests, are used for imaging anatomy near to the coil.• Quadrate or circularly-polarized coils comprise two coils 90° apart to improve SNR

by a factor of 2½.


Gradient Coil

• This is separate coil used to modify the main magnetic field during scanning for short period of time without movement of the patient or the machine.


How image is obtained (by machine)?



isocentre

B0

0

B0+BB0-B 0+ 0-

Gradient in z-directionGz

B0


Slice Selection• This is done by using the gradient coil.• Protons alone the gradient field in the desired slice

have different frequencies and send RF signals only from these frequencies which create an image.

• The slice thickness can be altered by using different gradient strengths or RF bandwidths.

• This slice selection is done alone the Z-axis.• But to determine a point in a slice another two

gradients are used.1. Frequency encoding gradient.2. Phase encoding gradient.


Frequency Encoding• It sends RF pulse after slice selection

gradient.

• This is done along the Y-axis direction.

• Protons alone the Y-gradient field in the desired slice have different frequencies and send RF signals only from Y-axis which create an image.


Phase encoding• Phase encoding gradient sends RF pulse

after frequency encoding.

• This is done along the X-axis direction.

• Protons alone the X-gradient field in the desired slice have different frequencies and send RF signals only from X-axis which create an image.


• The signal in X- axis after FT is represented as lines. These are called K- space.


Fourier Transform (FT): By this process computer can analyze a mixture of signals that come out of a slice and determine the intensity of the component that have different frequencies phases.

• S1 has amplitude a and frequency f

• S2 has a/2 and 3f• S3 = S1 + S2• S3 is two sine waves

of different frequency and amplitude

• The FT is shown

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A

fS(t) = a0 + a1sin(w1t + f1) + a2sin(w2t + f2) + ...


Various MRI Sequences to produce image


Spin-echo sequence


• Here 180o is applied after 90o pulse and image is formed.


Gradient Echoes• A second type of echo important in MRI is the

gradient echo. • In contrast to the SE it is formed by applying a

gradient and then reversing the direction of this gradient.

• It does not require a 180° RF pulse meaning that one advantage is faster imaging time.

• However, the images are inherently T2* weighted as the decay due to B0 inhomogeneities is not recovered, and they are therefore prone to susceptibility artifacts.


Other Sequences• The majority of the many other sequences in common

use are variations of the above two. For instance a Multi-Spin Echo simply uses more than one 180o pulse to create separate echo images at increasingly longer echo times. The sequence can be used to measure T2 by fitting the signal decay at each echo time.


Fast Spin-Echo sequence


• A subtle but important difference in the Fast Spin-Echo sequence is that echoes are closely spaced so, the signal at each echo can be used to form a single image.

• Therefore in real time the total phase-encoding needed to be performed can be done much faster.

• The factor by which the sequence is speeded up compared to a normal SE sequence is given by the echo-train length (the number of echoes which are individually phase-encoded).

• The greater this number or the bigger the spacing between the echoes, then the poorer the quality of the final image.


EPI and k-Space


EPI and k-Space• One final sequence worth considering is Echo-Planar Imaging or EPI. To fully appreciate

the utility of EPI we must first consider k-space. • K-space is an array of numbers whose FT gives the MR image. Each row (or line) in k-

space corresponds to the echo data collected with each application of the phase-encoding gradient.

• The cells in k-space DO NOT equate one-to-one with the pixels in the image; in fact each cell contains information about every image pixel.

• Rows near to the centre of k-space correspond to low-order (small amplitude) phase encoding steps and are therefore related to the bulk of the image signal/contrast.

• The edges of k-space correspond to high-order gradient steps, where the image detail can be found. To fully image an object data in the whole of k-space must be collected.

• By acquiring only part of k-space (or fewer 'lines') the scan will be much faster but image quality will be compromised.


EPI and k-Space

Figure: (above) Examples of images obtained with full and partial k-space.

• The image in the middle was acquired with full k-space, while for the left hand image only the outer edges of k-space were collected and as a result only the edges or detail are present in the image. Conversely by acquiring only the central portion of k-space (right image) more of the signal is produced but the detail is missing.


Various MR Techniques


Contrast Enhanced MRI


• Although MR delivers excellent soft-tissue contrast sometimes there is a need to administer exogenous contrast usually an intravenous injection of some paramagnetic agent, most commonly Gd-DTPA.

• The effect of this agent is to shorten the relaxation time of local spins causing a decrease in signal on T2-weighted images and an increase on T1-weighted images.


• The example in the Figure shows brain images both before and after contrast allow disruptions in the blood-brain barrier to be investigated.

• The increased vascularity of tumors produces a preferential uptake of contrast agent and the technique can be used to better visualize these from surrounding normal tissue.

• Furthermore if MR scans are repeatedly acquired following the contrast injection, the dynamic nature of contrast uptake can be examined, which may improve the differentiation of benign and malignant disease.


Fat Suppression

• An important technique in MRI is fat suppression i.e. removing the high signal fat component from the image. There are many ways in which this can be achieved but each method relies on either the resonant frequency (chemical shift) or relaxation time differences between water and fat.

• In the Chemical selective saturation method a preparatory pulse sequence is acquired which utilizes a narrow bandwidth RF pulse to excite the fat peak alone. The fat magnetization is then deliberately dephased in the transverse plane leaving only the water available for subsequent detection.


• Another common method is the STIR sequence (Short TI Inversion Recovery). This sequence uses a 180° RF pulse to invert water and fat spins, then waits a given time (about 180 ms at 1.5 Tesla) for the more rapidly-recovering fat peak to reach the null point (i.e. the point at which it passes through the transverse plane). At this point a 90° 'inspection' pulse flips the magnetization into the transverse plane so that the fat peak is zero but the water peak, which still had a negative z component, is measured.

• The disadvantage of this technique is that the timing of the sequence has to be fixed, so the weighting in the final image cannot be altered.

• SPIR, or Spectral Presaturation with Inversion Recovery, is a combination of the two previous methods, only the fat is excited and then inverted as in the STIR method.

• The Dixon method involves acquiring images with fat and water in or out of phase and performing an image subtraction.


An example of fat suppression (using the first method) is given in the Figure for the breast. The bright fat signal in the left has been removed in the right image permitting a better visualization of breast parenchyma.

Figure: Example of an axial breast image pre and post fat suppression.


MR Angiography• One of the biggest growing areas for MRI is

angiography.

• In normal circumstances flow effects cause unwanted artifacts, but in MRA these phenomena are used advantageously to permit the non-invasive imaging of the vascular tree.

• Techniques can be generally divided into 'white' or 'black' blood methods depending on whether moving spins (blood) appear brighter or darker than stationary tissue.

• In high-velocity signal loss, blood which has moved in-between the 90° and 180° pulses will not produce a signal and appears darker than tissue which has experienced both pulses.


• A short TR is used so that spins in the imaging slice become quickly saturated (recover to a constant value) but 'fresh' spins flowing into this slice have their full magnetization available and therefore emit a high signal.

• This technique works best over thin sections and when blood flow is perpendicular to the imaging plane.

• Another increasingly used method is simply to take advantage of the high signal from i.v. contrast-agents. Although current clinical agents are extracellular, and quickly distribute into the extravascular space, accurate timing of the imaging sequence following the contrast injection can provide excellent results.

• The image in the Figure on the right is an example of what can be achieved.

• Other techniques include stepping or moving table MRA, where multiple table positions (called stations) are used to image peripheral arteries.


Functional MRI• Functional MRI is a technique for examining brain

activation which unlike PET (Positron Emission Tomography)and is non-invasive with relatively high spatial resolution.

• The most common method utilizes a technique called BOLD (Blood Oxygen Level Dependent) contrast. This is an example of endogenous contrast, making use of the inherent signal differences in blood oxygenation content.

• In the normal resting state, a high concentration of deoxyhaemoglobin attenuates the MR signal due to its paramagnetic nature. However, neuronal activity, in response to some task or stimulus, creates a local demand for oxygen supply which increases the fraction of oxyhaemoglobin causing a signal increase on T2 or T2*-weighted images.


• The patient is subjected to a series of rest and task intervals, during which MR images are repeatedly acquired. The signal changes during this time course are then examined on a pixel-by-pixel basis to test how well they correlate with the known stimulus pattern. Pixels that demonstrate a statistically significant correlation are highlighted in colour and overlayed onto a grayscale MRI image to create an activation map of the brain.

• The location and extent of activation is linked to the type of task or stimulus performed, for example a simple thumb-finger movement task will produce activation in the primary motor cortex.


In this case a patient was asked to perform a finger-thumb movement for 30 s, repeated three times and interspersed with 30 s periods of rest. Post-processing on a computer established which pixels in the brain had 'activated' during this task and these are displayed in orange. The plot on the right illustrates that the pattern of signal change in this region (shown in blue) closely followed the stimulus pattern (red). fMRI is widely used as a research tool for examining brain function. Clinically, it is finding application in the surgical or radiotherapy planning of patients with tumors to ensure that vital functions are preserved following these interventions.

Functional MRI

Functional MRI

Figure: Example of motor cortex activation in a patient study.


Diffusion-Weighted MRI


• Diffusion refers to the random motion of molecules along a concentration gradient.

• Diffusion-weighted MRI uses the motion of spins to produce signal changes.

• Here, Gradients with equal amplitude but opposite polarity are applied over a given interval. Stationary tissue will be dephased and rephased equally, whereas spins which have moved during the interval will suffer a net dephasing and signal loss.

• Signal attenuation will depend on the degree of diffusion and the strength and timing of the gradients, the latter expressed by the gradient factor or b-factor.

Dr. MD. Mofazzal SharifFigure: Example of white-matter fiber tracking in a normal subject.

Although a wide area of research, the major clinical use for DWI at the moment remains in stroke, where cell swelling caused by ischemia leads to changes which can be demonstrated with DW-MRI much sooner than with conventional MRI.


MR Spectroscopy


• MR Spectroscopy is a technique for displaying metabolic information from an image.

• It relies on the inherent differences in resonant frequency or the chemical shift that exists due to different chemical environments.

• MR signal is measured and a spectrum plotting amplitude against frequency is displayed. By using a standard reference the chemical species of each peak can be determined. For proton MRS, the reference compound is Tetramethylsilane (TMS).

• All chemical shifts are expressed as frequency differences from this compound giving a field-independent parts per million (ppm) scale.

• Using this standard gives water its characteristic peak at 4.7 ppm. Spectra of any 'MR visible' nucleus can be obtained (e.g. 31P, 17F, 13,C) so long as the RF coil is tuned to the specific resonant frequency.

• In proton MRS, an important consideration is the concentration differences between the metabolites of interest and the overwhelming fat and water peaks which need to be suppressed prior to acquisition.


• Since MRS relies on detecting frequency differences another method is needed to localize the signal. Most methods use the intersection of three slice-select RF pulse to excite a volume of interest (called a voxel).

• Multiple voxels can be acquired by using phase encoding in each of the desired dimensions. This technique, called Chemical shift imaging, is useful in isolating individual peaks and displaying the integrated area as a colour scale to produce a metabollic map.


Figure: Example of single voxel proton MRS in normal and

malignant brain tissue.


• The example in the Figure below illustrates the potential clinical use of MRS. The spectrum on the left was acquired in normal healthy brain tissue and displays the characteristic high N-Acetyl-Aspartate peak (NAA). On the right is a spectrum taken from a slightly enlarged but otherwise normal looking part of the Medulla, which did not show any enhancement with Gadolinium. In this case the NAA peak is absent indicating loss of viable tissue, and the chorine peak is elevated, which is indicative of the high cell proliferation in tumors


Artifacts


Gibbs Ringing or Truncation Artifact

• This arises due to the finite nature of sampling.• The artifact is prominent at the interface

between high and low signal boundaries and results in a 'ringing' or a number of discrete lines adjacent to the high signal edge.


Phase-wrap or 'Aliasing'• Aliasing can occur in either the phase or frequency

direction but is mainly a concern in the phase direction.

• This effect occurs whenever there is an object or patient anatomy outside the selected field-of-view but within the sensitive volume of the coil.


Motion Artifacts (Ghosting)• Ghosting describes discrete or diffuse signal throughout

both the object and the background. It can occur due to instabilities within the system (e.g. the gradients) but a common cause is patient motion. When movement occurs the effect is mainly seen in the phase direction.

• Motion causes anatomy to appear in a different part of the scanner such that the phase differences are no longer consistent. Periodic motion e.g. respiratory or cardiac motion can be 'gated' to the acquisition so that the phase encoding is performed at the 'same' part of the cycle.

• Modern scanners are now so fast that 'breath-hold' scans are replacing respiratory compensation. Non-periodic motion e.g. coughing, cannot easily be remedied and patient co-operation remains the best method of reducing these artifacts


Motion Artifacts (Ghosting)


Chemical Shift • This artifact arises due to the inherent differences

in the resonant frequency of the two main components of an MR image: fat and water.

• It is only seen in the frequency direction.


Susceptibility• The susceptibility of a material is the tendency for it to

become magnetized when placed in a magnetic field. • Materials with large differences in susceptibility create

local disturbances in the magnetic field resulting in non-linear changes of resonant frequency, which in turn creates image distortion and signal changes.

• The problem is severe in the case of ferromagnetic materials but can also occur at air-tissue boundaries.

• This example was acquired in a patient who had permanent dental work. It did not create any problems for the patient but the huge differences in susceptibility caused major distortions and signal void in the final image.


Susceptibility


Other Artefacts

• An RF or zipper artifact is caused by a breakdown in the integrity of the RF-shielding in the scan room. Interference from an RF source causes a line or band in the image, the position and width of which is determined by the frequencies in the source.


• A Criss-cross or Herringbone artifact occurs when there is an error in data reconstruction. In this example in the breast two window levels have been used to display the artifact clearly.


A DC-offset leads to the central point artifact, a bright spot at the centre of the image.


Safety• Although MRI is completely safe, it is instructive to

consider how the scanner interacts with the patient. • To put this section into historical context, in 1980 there

were concerns about using field strengths as little as 0.35 T but within 6 years this 'safe' limit had moved up to 2.0 T.

• Similarly, gradient performances were limited to 3 T/s in the mid-1980s whereas today MRI is routinely performed with gradients exceeding 50 T/s.

• What follows is a summary of each particular safety issue associated with MRI is intended to be educational and certainly should not be misconstrued: MRI is entirely safe


Static Field Effects & Safety• The most obvious safety implication is the strength of the

magnetic field produced by the scanner. There are three forces associated with exposure to this field: a translational force acting on ferromagnetic objects which are brought close to the scanner (projectile effect), the torque on patient devices/implants, and forces on moving charges within the body, most often observed as a superposition of ECG signal.

• The extension of the magnetic field beyond the scanner is called the fringe field. All modern scanners incorporate additional coil windings which restrict the field outside of the imaging volume. It is mandatory to restrict public access within the 5 Gauss line, the strength at which the magnetic field interferes with pacemakers.


Gradient Effects & Safety• These come under the term 'dB/dt' effects referring

to the rate of change in field strength due to gradient switching.

• The faster the gradients can be turned on and off, the quicker the MR image can be acquired.

• At 60 T/s peripheral nerve stimulation can occur, which although harmless may be painful. Cardiac stimulation occurs well above this threshold.

• Manufacturers now employ other methods of increasing imaging speed (so called 'parallel imaging') in which some gradient encoding is replaced.


RF Heating Effects & safety• The repetitive use of RF pulses deposits energy which in

turn causes heating in the patient. • This is expressed in terms of SAR (specific absorption rate

in W/kg) and is monitored by the scanner computer. • For fields up to 3.0 Tesla, the value of SAR is proportional

to the square of the field but at high fields the body becomes increasingly conductive necessitating the use increased RF power.

• Minor patient burns have resulted from use of high SAR scans plus some other contributory effect, e.g. adverse patient or coil-lead positioning, but this is still a rare event.


Noise & Safety• The scans themselves can be quite noisy. • The Lorentz forces acting on the gradient coils due to

current passing through them in the presence of the main field causes them to vibrate.

• These mechanical vibrations are transmitted through to the patient as acoustic noise. As a consequence patients must wear earplugs or head phones while being scanned.

• Again, this effect (actually the force on the gradients) increases at higher field and manufactures are using techniques to combat this including lining the scanner bore or attaching the gradient coils to the scan room floor thereby limiting the degree of vibration.


Claustrophobia & Safety• Depending on the mode of entry into the scanner

(e.g. head first or feet first) various sites have reported that between 1 % and 10 % of patients experience some degree of claustrophobia which in the extreme cases results in their refusal to proceed with the scan.

• Fortunately, modern technology means that scanners are becoming wider and shorter drastically reducing this problem for the patient.

• In addition, bore lighting, ventilation as well as the playing of music all help to reduce this problem to a minimum


Bioeffects and Safety• There are no known or expected harmful effects on humans

using field strengths up to 10 Tesla. • At 4 Tesla some unpleasant effects have been anecdotally

reported including vertigo, flashing lights in the eyes and a metallic taste in the mouth.

• Currently pregnant women are normally excluded from MRI scans during the first trimester although there is no direct evidence to support this restriction.

• The most invasive MR scans involve the injection of contrast agents (e.g. Gd-DTPA). This is a colorless liquid that is administered i.v. and has an excellent safety record. Minor reactions like warm sensation at the site of injection or back pain are infrequent and more extreme reactions are very rare.

MRI Physics

Documents

send rf signals

small flip

90o rf pulse

contrast depends

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t1 weighting

shorter scanning

t1 contrast