Imaging Flashcards
What is Spin Warp Imaging?
- ‘regular’ MRI
- An MR signal is generated after each pair of
90°-180° degree RF-pulses. Slice-select
gradients are turned on simultaneously with
each RF-pulse so that only a single slice is
stimulated - Frequency-encoding is performed using a
de-phase lobe between the 90°- and 180°-
pulses and a readout lobe after the 180°-pulse.
*The de-phase lobe imparts a frequency-
dependent phase shift to protons along this
axis as a function of their spatial position
within the gradient.
The phases of these spins are inverted by the 180°-pulse then re-phased into an echo by the readout lobe. - The unique feature of the spin-warp sequence
is that a variable-amplitude phase-encoding
gradient is applied during signal evolution,
typically between the 90°- and 180°-pulses.
Additional MR signals are collected for this
slice during the next TR interval with the same
frequency-encoding gradient but with a
different phase-encoding gradient. - The phase-encode gradient provides a method
for differentiating signals according to spatial
location along this direction - With each successive application of the phase-
encode gradient, the digitized MR signal is
used to fill another row of k-space. For routine
MR imaging, this process is typically repeated
on the order 256 times. Once all the rows have
been filled with data, Fourier transform
methods can be used to reconstruct the image
Describe Inversion Recovery Imaging
- Inversion recovery sequences are a variant of
Spin Echo sequences. They are used to null the
signal from certain tissues, e.g. fat in a STIR
and fluid in a FLAIR, by first applying a 180° RF
pulse and then starting the cycle - This flips the Mz through 180° to a negative
value. As the Mz recovers, at some point it
reaches zero before becoming positive again. If
we apply our 90° RF pulse when the Mz is 0, at
time TI (time to inversion), there is no
magnetisation to create a Mxy signal. We have,
in effect, nulled that signal - The TI (time from initial inverting 180° pulse to
the subsequent 90° pulse) is altered based
upon the material that we want to null the
signal from
* As fat and fluid have different T1s and will
reach Mz of zero at different times, we can
select which tissue to null by selecting
when to start the 90° RF pulse
What is STIR imaging? Can this be used with contrast?
STIR = Short Tau Inversion Recovery
- Fat signal nulled by selecting short TI (130 ms,
1.5 T) - a fat suppression technique with an inversion
time, where the signal of fat is zero - Inversion recovery imaging allows
homogeneous and global fat suppression - Since STIR sequences use short inversion
recovery time, they cannot be used with
gadolinium injection because tissues that take
up gadolinium will exhibit T1 shortening and
may inadvertently be nulled
What is FLAIR imaging? Can this be used with contrast?
FLAIR = Fluid Attenuated Inversion Recover
- A special inversion recovery sequence with a
long inversion time (2500ms, 1.5T), removing signal from
the cerebrospinal fluid in the resulting images - Useful for brain imaging: Brain tissue on FLAIR
images appears similar to T2 weighted images
with grey matter brighter than white matter
but CSF is dark instead of bright - The FLAIR sequence is part of almost all
protocols for imaging the brain, particularly
useful in the detection of subtle changes at the
periphery of the hemispheres and in the
periventricular region close to CSF - Yes. FLAIR is able to show lower concentration gadolinium compared to T1-weighted post Gadolinium imaging
What is Echo Planer Imaging (EPI)?
- EPI relies on a RECALLED ECHO TRAIN formed
by repeated reversing of the readout gradient - Using EPI, whole of k-space can be covered in a
SINGLE SHOT- Echo train must be shorter than T2*,
otherwise signal would vanish before all of
k-space is covered - Must have very homogenous B0
- Echo train must be shorter than T2*,
- Subsequent positive and negative frequency-
encoding lobes swept k-space from left-to-right
and right-to-left respectively. Meanwhile, the
blipped low-amplitude phase-encoding
gradient pulses produced a step-wise increase
along the ky-axis. - Advantage: Minimizing the effects of patient motion
- Can do SINGLE-SHOT or MULTI-SHOT EPI
- The number of k-space lines (echoes) collected
in a single shot is called the “Echo Train Length
(ETL)
Describe briefly MULTI-SLICE IMAGING
- Collect multiple PARALLEL 2D SLICES
- While waiting for M to recover in first slice, we excite the second slice, etc, etc
- Parallel slices are independent of each other so exciting one does not affect the others
Describe what is meant by a SPIRAL SCAN, and what the advantages/disadvantages of this technique might be
- Main problem with EPI is that VERY FAST gradient switching is required
One answer is to cover all of k-space with a SPIRAL trajectory, thus avoiding SHARP EDGES
-ADVANTAGE-
- Scan starts at ORIGIN of k-space
-DISADVANTAGE-
- Problem with spiral scan is that data points DO NOT FALL on a RECTANGULAR GRID
Need to resample or interpolate
Describe what is meant by a PROPELLER/BLADE SCAN, and what the advantages/disadvantages of this technique might be
PROPELLER = Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction
- The basic idea was to sample k-space in a rotating fashion using a set of radially directed strips or “blades”
- Each blade is composed of multiple parallel phase-encoded lines that can be collected using fast spin echo or gradient echo methods. In common practice, 8-32 blade lines are acquired in a single shot. The blades are then rotated by a small angle (10°−20°) at which time a second set of data are acquired. The process continues until imaging data from the entire k-space circle has been collected
-ADVANTAGES-
- The center of k-space (which contains the highest signal amplitude and contributes most to image contrast) is oversampled, meaning that the signal-to-noise and contrast-to-noise will be high.
*Oversampling in this region also provides
redundancy of information, meaning that the
data for new each blade can be compared to the
data from previous blades for consistency. If the
patient moves between blades, the data for the
second blade can be corrected (or even
completely discarded) based on how anomalous
its central information appears.
-The degree of motion correction can be substantial, and we routinely use PROPELLER sequences for DWI, FLAIR and T2-weighted images on any patient we suspect will not hold still during the course of a scan. Due to its oversampling of k-space, susceptibility artefacts are also slightly reduced
-DISADVANTAGES-
- Although the centre of k-space is highly oversampled, the “corners” of k-space are not sampled at all.
-Complete coverage of the k-space circle (without gaps between the blades) requires a factor of 1.57 times as long as coverage using a Cartesian (rectangular) method
How would you adapt a regular 2D pulse sequence to collect 3D data?
- Collect a “thick slab” instead of a thin slice
- Weak selection gradients
- Introduce a second Phase encoding gradient along the Z, dividing the slab into a number of slices
What scan sequences would you expect to be in a routine MR Head scan?
- T1WI
- T2WI
- FLAIR
- DWI
- SWI
What is the clinical significance of performing T2WI?
- T2WI DONT pick up new lesions as well as T1WI, but are better at showing older, inactive lesions
- Important at tracking long-term disease progression
- T2WI differentiate anatomical structures based on T2 values of the surrounding tissue
- Typically Long TR/TE
What is the clinical significance of performing T1WI?
- T1WI highlight areas of enhanced fat content like recent haemorrhage
- In brain imaging, good at highlighting the white matter against the CSF as the former will appear bright while the latter dark
What is the clinical significance of performing a FLAIR sequence?
- Helpful in the differentiation of intracranial cystic lesions
- In the brain: As the CSF signal within the subarachnoid space is suppressed, pathologies within the subarachnoid space are typically visualized on the FLAIR sequence, including subarachnoid haemorrhage, meningitis, leptomeningeal carcinomatosis, and slow or occluded flow within the intracranial vasculature
What is the clinical significance of performing a DWI sequence?
- Diffusion weighted imaging (DWI) uses molecular diffusion as a contrast mechanism in MRI to identify areas where diffusion is more or less restricted than in normal tissue
- Ever since its inception, acute brain ischaemia has been the most successful application of DWI, and diffusion MRI today is the imaging modality of choice for stroke patients[4], where b-values of up to 1000 are used for standard neuroimaging applications. ADC values have also been shown to correlate with brain tumours, white matter diseases, paediatric brain development and ageing, as well as some cancers and bowel disorders
Can you describe how a DWI sequence functions, and how it compensates for shine through from the inherent T2 weighting?
- In DWI, diffusion refers to the probabilistic, random process by which water molecules move gradually over time. This random motion, known as Brownian motion, is due to thermal energy of the molecules. In isotropic free diffusion, the movement of water molecules is completely unrestricted, and the molecules can move equally in all directions
- In biological tissue, the rate of diffusion of water molecules is characterised using the apparent diffusion coefficient (ADC) rather than the true diffusion coefficient. The use of the ADC reflects the uncertainties in the measurement method due to the indirect measurement of diffusion, which is an average diffusivity over all tissue microenvironments within the voxel and contains contributions from other sources of tissue motion
- Based on a standard spin-echo pulse sequence, the DWI sequence consists of a pair of 90◦ and 180◦ RF pulses with additional diffusion-weighing gradients on either side of the 180◦ pulse, which control the sensitivity to diffusion
- The most common readout method for DWI is a single-shot echo-planar imaging (SS-EPI) acquisition. This sequence is chosen due to its speed, which freezes bulk motion that would otherwise obscure the diffusion contrast
- The sensitivity to diffusion is controlled by the b-value, which is proportional to the gradient strength^2, the gradient separation, and the gradient duration. Where any of the latter three parameters can be varied to change the strength of the diffusion weighting (a high b value increases the sensitivity to diffusion)
- To overcome some of the uncertainties associated with diffusion-weighted images, ADC maps are created. By obtaining diffusion weighted images at two or more b-values, calculating the ADC value for each voxel, and displaying these as a map of grey-scale values corresponding to the strength of isotropic diffusion, the diffusion can be separated from the relaxation effects such as T2 shine-through
