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MRI Physics

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Nami cml

on 22 September 2013

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Transcript of MRI Physics

TURBO SPIN ECHO (TSE) MRI SAFETY IMAGE WEIGHTING Short Inversion Recovery (STIR) Fluid Attenuation Inversion Recovery (FLAIR) GRADIENT ECHO (GE) ECHOPLANAR IMAGING BALANCED / FAST GRADIENT ECHO STEADY STATE FREE PRECESSION (SSFP) Spins are rephased by a gradient Spins are rephased by a 180-degree rephasing pulse
Gold standard (Good image quality)
very versatile
what you set is what you get (i.e. the contrast is truly based on the T1 and T2 relaxation times of the tissue)
true T2 weighting sensitive to pathology
scan times relatively long T1 Fat = bright ; Water = dark
Short TE and TR
Demonstrates anatomy because of a high SNR
Shows pathology if contrast is given T2 Scan times are much shorter than conventional spin echo As the scan time is a function of the TR, NEX and the number of phase encoding
Decreasing the TR and NEX = affect image quality and SNR. UNDESIRABLE
reducing the number of phase encoding = reduces spatial resolution. DISADVANTAGE.
In fast spin echo, the scan time is reduced by performing more than one phase encoding step and subsequently filling more than one line of K-space per TR.
This is achieved by using several 180-degree rephasing pulses to produce a train of echoes or echo train. at each rephasing, an echo is produced and a different phase encoding step is performed.
the number of 180-degree rephasing pulses performed per TR corresponds to the number of echoes produced and the number of lines of K space filled
Longer the factor = shorter the scan time.
the resultant image has more of a mixture of weighting b/c there are more data collected at the wrong TE. This is not important in T2 weighted scans, as the proton density data are offset somewhat by the heavily T2 weighted data.
in T1 and PD weighting, larger the turbo factors place too much T2 weighting in the image and hence shorter turbo factor must be used. (scan time saving is not as great as with T2 weighting)
the 180-degree RF pulses take time to perform and so fewer slices are available for a given TR USES:
It has largely replaced spin echo esp. for T2-weighting in CNS/Pelvis/MSK MRI. Dis: Image blurring may occur in FSE images at the edges of tissues with different T2 decay values. Fat bright on T2 weighted images. Some flow and motion affect increased
Adv: Artifacts fro metal implants is significantly reduced when using FSE because the repeated 180-degree RF pulses compensate for field inhomogenety. High resolution matrices and multiple NEX can be used. Image quality improved. Increased T2 formation. It is usually used to suppress the signal from certain tissues in conjunction with long TEs and T2 weighting
SO it gives very good SNR as TR is long and excellent T1 contrast. A pulse sequence that begins with a 180-degree inverting pulse. This inverts the NMV through 180-degree into full saturation. when the inverting pulse is removed, the NMV begins to relax back to the B0. A 90-degree excitation pulse is then applied at a time from 180-degree inverting pulse (TI-Time to inversion) The resultant FID is then rephased by a 180-degree pulse to produce a spin echo time TE. But long scan time unless used in conjunction with FSE. So instead of being used to produce T1 weighted images, fast inversion recovery is usually used to suppress signal from certain tissues in conjunction with T2 weighting so that water and pathology return a high signal. Two main sequences are: It uses a TI that corresponds to the time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat.
It is important to note that STIR should not be used in conjunction with contrast enhancement, which shortens the T1 times of enhancing tissues, making them bright. The T1 times of these structures are shortened so that they approach the T1 times of fat; therefore, enhancing tissue may also be nulled.
MSK: because normal bone, which contains fatty marrow, is suppressed and lesions within the bone such as bruising and tumours are seen more clearly. Selecting a TI corresponding to the time of recovery of CSF from 180-degree to the transverse plane nulls the signal from CSF: there is no longitudinal magnetization present in CSF. when the 90-degree excitation pulse is applied, because there is no longitudinal component of CSF there is no transverse component after excitation and the signal from CSF is nulled.
FLAIR is used to suppress the high CSF signal in T2 weighted images so that pathology adjacent to CSF is seen more clearly. It is used in brain and spine imaging to see periventricular and cord lesions more clearly.
It is especially useful in visualising multiple sclerosis plaques, acute sub-arachnoid hemorrhage and meningitis
Another modification is selecting a TI time that corresponds to the null point of white matter: Lesions within normal white matter appear much brighter by comparison, such as periventricular leukomalacia and for congenital gray/white matter abnormalities. CONTRAST As gadolinium reduces the T1 relaxation time to enhancing tissue so that it is similar to fat, enhancing tissue may appear brighter than when gadolinium is not given.

T1 weighted imaging with fat saturation are acquired after the administration of contrast and should be performed in the coronal and sagittal planes. It uses variable flip angles so that TR and therefore scan time can be reduced without producing saturation. T2* and PD weighting, which are normally associated with long TRs and scan times, can therefore be acquired using short TRs because the sequence begins with a flip angle less than 90-degree.
A gradient, rather than a 180-degree rephasing RF pulse is used to rephase the FID.
The frequency encoding gradient is used for this purpose because it is quicker to apply than a 180-degree pulse and therefore the minimum TE can be reduced.
However, the gradient does not compensate for magnetic field inhomogeneities. USES:
They can be used for single-slice or volume breath-hold acquisitions in the abdomen, and for dynamic contrast enhancement.
they are very sensitive to flow as gradient rephasing is not slice selective, so flowing nuclei always give a signal, as long as they have been previously excited (angiographic type image). It is defined as the stable condition that does not change over time. In MRI, energy is given to hydrogen during excitation and the amount of energy applied is indicated by the flip angle. Energy is lost by hydrogen through spin lattice energy transfer and the amount of energy lost is determined by TR. Therefore, by selecting a certain combination of TR and flip angle, we can insure that the overall energy of hydrogen remains constant as the energy 'in' as determined by the flip angle equals the energy 'out' as determined by the TR. There are critical values of flip angle and TR to maintain the steady state. As RF has a low frequency and hence low energy, for most values of flip angle very short TRs are required to achieve the steady state. in fact the TRs required are shorter than the T1 and T2 relaxation times of the tissues. There is therefore no time for transverse magnetization to decay before the pulse sequences is repeated. generally, flip angles of 30/45-degree in conjunction with a TR less than 50ms achieve the steady state. In the steady state, there is co-existence of both longitudinaland transverse magnetization. In particular, the transverse componenet of magnetization does not have time to decay during the pulse sequence and builds up over successive TRs (residual transverse magnetization). IT affects image contrast as it induces a voltage in the receiver coil. it affects image contrast as it results in tissues with long T2 times (water) appearing bright on the image. As the TR is so short, magnetization in tissue does not have time to reach its T1 and T2 times before the next excitation pulse is applied. Therefore in the SSP image contrast is not due to differences in T1 and T2 times of tissues but rather to the ratio of T1 to T2.
In tissues where T1 and T2 times are similar, the signal intensity if high. In the human body, fat and water have this parity (fat: very short T1 and T2 times; water: very long T1 and T2 times) and therefore return high signal intensity in SSP.
Other tissues such as muscle return a lower signal intensity because they do not have a similar T1 and T2 decay time. Most gradient echo sequences use the SS as the shortest TR and scan time is achieved. Gradient echo sequences are classified according to whether the residual transverse magnetization is in phase (coherent) or out of phase (incoherent) Coherent gradient echo Incoherent gradient echo (Spoiled) The steady state is created when the TR is shorter than the relaxation times of tissues and the energy 'in' as determined by the flip angle equals the energy 'out' during the TR period Residual magnetization therefore builds up in the transverse plane and is rephased by subsequent RF pulses to produce stimulated echoes. The resultant image contrast is due to the ratio of T1 to T2 in a particular tissue and whether the FID or the stimulated echo are sampled. These sequences use a variable flip angle excitation pulse followed by gradient to produce a gradient echo. They keep the residual magnetization coherent by a process known as rewinding, which is achieved by reversing the slope of the phase encoding gradient after readout. This results in the residual magnetization rephasing, so that it is in phase at the beginning of the next repetition. The rewinder gradient rephase all transverse magnetization regardless of when it was created. Therefore the resultant echo contains information from the FID and the stimulated echo. These sequences can therefore be used to achieve T1 or T2 * weighted image, although traditionally they are used in conjunction with a long TE to produce T2* weighting.
It usually produces rapid images that are T2* weighted.
As water is bright they are often said to give an angiographic, myelopgraphic effect. They can be used to determine whether a vessel is patent, or whether an area contains fluid.
They can be acquired slice by slice, or in 3D volume acquisition.
As TR is short, slices can be acquired in a single breath hold. Disadvantages:
Reduced SNR in 2D acquisitions
Magnetic susceptibility increases
Loud gradient noise The sequences begin with a variable flip angle excitation pulse and use gradient rephasing to produce a gradient echo. The steady state is maintained so the residual transverse magnetization is left over from previous repetition. These sequences dephase or spoil this magnetization so that its effect on image contrast is minimal.
Only transverse magnetization from the previous excitation is used, enabling T1 contrast to dominate.
Two ways: RF or Gradient spoiling. USES:
As the stimulated echo that contains mainly T2*and T2 information is spoiled, RF spoiled pulse sequences produce T1 or PD weighted images, although fluid may have a rather high signal due to gradient rephasing.
They can be used for 2D and volume acquisitions, and as the TR is short, 2D acquisitions can be used to acquire T1 weighted breath-hold images. FR spoiled sequences demonstrate good T1 anatomy and pathology after gadolinium. SPATIAL PRE-SATURATION Spatial pre-saturation pulses nullify the signal from flowing nuclei so that the effects of entry slice and time of flight phenomena are minimized. Spatial pre-saturation delivers a 90-degree RF pulse to a volume of tissue outside the FOV. A flowing nucleus within the volume receive this 90-degree pulse. When it then enters the slice stack, it receives an excitation pulse and is saturated. If it is fully saturated to 180-degree, it has no transverse component of magnetization and produces a signal void. To be effective, pre-saturation pulse should be placed between the flow and the imaging stack so that signal from flowing nuclei entering the FOV is nullified.
In sagittal and axial imaging, pre-saturation pulses are usually placed above and below FOV so that arterial flow from above and venous flow from below are saturated.
Right and left pre-saturation pulses are sometimes useful in coronal imaging (esp. in the chest), to saturate flow from the subclavian vessels. Spatial pre-saturation pulses can be brought into the FOV itself. This permits artefact-producing areas (such as aorta) to be pre-saturated so that phase mismapping can be reduced.
Pre-saturation pulses are only useful if they are applied to tissue.
The use of pre-saturation pulses may also decrease the number of slices available and should therefore be used appropriately.
Pre-saturation pulses are also only effective if the flowing nucleus receives the 90-degree pre-saturation pulse. FAT SATURATION To saturate fat signal, a 90-degree pre-saturation pulse must be applied at the precessional frequency of fat to the whole FOV. The excitation RF pulse is then applied to the slices and the magnetic moments of the fat nuclei are flipped into saturation. If they are flipped to 180-degree, they do not have a component of transverse magnetization and produce a signal void. The water nuclei, however, are excited, rephased and produce a signal. Using fat saturation has increased the CNR between the lesion and normal tissue as fatty componenets in the base of the skull have been nulled. FUNCTIONAL MRI DIFFUSION WEIGHTED IMAGING This motion is restricted by boundaries such as ligaments, membranes and macromolecules. Diffusion is also restricted in pathology. Diffusion: the movement of molecules in the extra-cellular space due to random thermal motion. (DWI) The net displacement of molecules diffusing across an area of tissue per second is called apparent diffusion coefficient (ADC). In areas of restricted diffusion the ADC is low. A sequence can be sensitized to this motion by applying two gradients on either side of 180-RF pulse (a similar way to phase contrast MRA in that stationary spins will acquire no net phase change after the gradients have been applied). Moving spins, however, will acquire this phase change and result in a signal loss. In diffusion imaging, normal tissue that exhibits a high ADC has lower signal intensity than abnormal that has a low ADC as the molecules within it are free to move, while diffusion becomes restricted when pathology is present. MAGNETIC RESONANCE ANGIOGRAPHY Usually very fast types of spin are used because we need to reduce other types of motion such as flow, so that motion from diffusion is measured. There are two types of DW images:
Diffusion or trace images
Those where damaged tissue has restricted diffusion (low ADC) is brighter than normal tissues where diffusion is free (high ADC). This is because spins inestricted tissue are refocused as they stay in the same place during the application of both gradients. However, in normal tissue where diffusion is random, refocusing is not complete and signals cancel.
ADC maps
Are acquired via post-processing by calculating the ADC for each voxel of tissue and allocating a signal intensity according to its value. Therefore restricted tissue (low ADC) is darer than free diffusing areas that have a high ADC. This is useful when T2 shine through is a problem. USES:
Brain after infarction:
Differentiate malignant from benign lesions, from tumour from edema and infarction
Image neonatal brains: may out myelination patterns in pre-term infants to assist in our understanding of this process and how hypoxic events cause certain types of brain damage. In early stroke, soon after the onset of ischemia but before infarct or permanent tissue damage, cells swell and absorb water from the extrqa-cellular space. Since cells are full of large molecules and membranes, diffusion is restricted and the ADC of the tissue is reduced. These areas appear bright on trance images and these changes can be seen within minutes of infarction as opposed to hours or days using conventional MRI techniques.
Diffusion MRI can show irreversible and reversible ischemia lesions, so has a potential to discriminate salvageable tissues from irreversibly damaged tissues before a therapeutic intervention. However timing is important: DWI can only visualize fresh lesions as water diffusion is decreased several days after stroke onset. MRA CLAUSTROHPOBIC EXPERIENCE:
Techniques such as a mirror or facecloth, panic alarm and regular communication help to gain maximum compliance.
The insertion of a cannula is best performed outside the room before the procedure. This saves time and unnecessary stress during the procedure. Dynamic studies should have extension tubing attached to avoid unnecessary movement during the procedure. b value is a diffusion gradient.
The first scan may have a b value = 0, meaning there is no diffusion gradient and a T2 weighted image results.
The b value is proportional to the conspicuity of an infarcted lesion BUT is also inversely proportional to SNR.
The second scan has a b value commonly around 1000. This is considered the ideal balance of lesion detection and SNR.
The third image is called an apparent diffusion coefficient (ADC) map. This is calculated by the machine based on the b-values used in the previous 2 images
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