What is Perfusion? A special technique for evaluation of microscopic blood flow in capillaries and venules. Perfusion imaging is much faster than Diffusion. Both are used for the evaluation of stroke. Perfusion has other applications. Perfusion shows ischemic penumbra (healthy tissue that surrounds ischemic tissue) and Diffusion does not.
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What is Perfusion?
A special technique for evaluation of microscopic
blood flow in capillaries and venules.
Perfusion imaging is much faster than Diffusion.
Both are used for the evaluation of stroke.
Perfusion has other applications.
Perfusion shows ischemic penumbra (healthy tissue
that surrounds ischemic tissue) and Diffusion does
not.
What is Perfusion? (cont.)
Two Categories:
•Monitoring tissue signal changes using an
exogenous (injectable) MR relaxation
contrast agent. (Gadolinium)
•Monitoring tissue signal changes using an
endogenous contrast agent, which is an
inherent MR tissue contrast mechanism.
(Deoxyhemoglobin)
MR Perfusion Mapping
Visualizing signal changes during the
vascular transit of an injected MR contrast
agent bolus through the use of high speed
T2*-sensitive MR imaging.
The T2* FID-EPI is the most common
imaging sequence used in Perfusion
imaging.
MR Perfusion Timing (tracking)
What is the typical transmit time of a bolus
through the human brain?
15-20 seconds.
Therefore, the greatest challenge of
exogenous perfusion is acquiring the image
at the first pass of the bolus.
Timing is everything!
Peak Time
Factors that determine peak time:
•Heart Rate.
•Form of the bolus (speed).
•Concentration of contrast (amount and
osmolality).
Bolus Tracking Perfusion
This is used to differentiate between
normal and abnormal tissue.
Important factors used:
•Transit Time
•Blood Flow
•Blood Volume
Cardiac Perfusion
Parallel Imaging
Multichannel Coil Technology
(basics)
Radiological “Wish List” for MR (and perhaps,
other modalities as well):
Higher spatial resolution
Decreased acquisition time
Higher signal to noise ratio (SNR)
More images per patient for more diagnostic
information
Minimize SAR (problem with high-field MR)
Some factors to help meet these needs:
1) Protocol/Pulse-sequence optimization
2) Faster image reconstruction hardware
– but also –
3) Single-element coil: increasing SNR requires increased acquisition time
4) Receiving coil element size: decrease increased SNR per volume, but smaller tissue volume
Higher SNR achieved with multichannel technology allows
greater flexibility in sequence parameter selection.
If SNR is higher than needed, we can afford to lose a little SNR
to gain:
An increase in spatial resolution
A reduction in acquisition time (e.g., minimize motion-
induced artifacts, increase number of images per exam).
Multichannel Coil (cont.)
Advancements in multi-element/multichannel
technology (to 32 elements and beyond) will
continue to play a role in the development of
imaging techniques with higher spatial resolution,
faster scan times, and increased diagnostic quality.
Multichannel Coil (cont.)
18
Advancements in multi-element/multichannel coils:
New 96-channel head coil (Wald, MGH)
High-field imaging with 8-channel coil
Parallel Imaging
No image from a single surface coil element is optimally sensitive over the
whole area. However, an image reconstructed from all coil elements leads
to an increased SNR over a standard acquisition, because each region of
the image is reasonably sampled by more than one element.
If SNR is higher than needed, one can use the technique of parallel imaging
to increase acquisition speed.
How?
We can decrease sampling of data by each element receiver.
Also, reduced sampling less RF excitations per unit time lower SAR.
Decrease sampling of data = decreased k-space sampling
Parallel Imaging
Rather than fill all of k-space, parallel imaging acquires a
fraction of k-space to save time. Because the anatomy is
sampled by multiple coil elements, we can reconstruct the
missing information.
Less samples leads to decreased SNR.
Parallel Imaging
How fast can we go?
If we have M coil elements covering the FOV, we can skip up to M-1 lines
for each line in k-space we sample. The number of lines “skipped”:
acceleration factor (R). This can be fractional as well:
# of phase-encodes to cover k-space
R = ––––––––––––––––––––––––– # of phase-encodes used in acquisition
Names for acceleration factors: iPAT factor (Siemens)
SENSE factor (Philips)
ASSET factor (GE)
Parallel Imaging
Increasing acceleration leads to decreasing SNR. However, the benefits
may be greater than saving time as well.
For EPI images, which are greatly affected by susceptibility differences,
parallel imaging can improve geometric distortion and/or image voids.
Why?
Because the gradients are switching so quickly for an EPI image, one can
accrue errors that lead to distortion. These are alleviated using parallel
imaging, where the sequence requires less lines in k-space to be read out.
Parallel Imaging Example of Parallel Acceleration on the GE 3T:
R=1 R=2.0 R=2.8 R=3.2 R=4.0
SNR vs. Acceleration
Short-axis cardiac images – 32-channel coil – 1.5 T magnet
Reeder SB et al. MRM 54:748, 2005
Reconstructing an Image
Step 1:
The MR signal is detected by RF coils.
Step 2:
The resulting data set is digitized and arranged into mathematical construct called “k-space”.
Step 3:
Subsequent processing of this data set – the Fast Fourier Transformation (FFT) – yields the final MR image.
SMASH
SMASH (SiMultaneous Acquisition of Spatial Harmonics) is “k-space based” because the reconstruction algorithm operates on partial k-spaces (one from each coil), before image generation by the FFT.
1. Two (or more) k-space acquisitions with two (or more) coils. Each coil fills one k-space with a reduced number of lines (e.g., for an acceleration factor of 2,only every 2nd line is acquired).
2. “Artificial” lines are calculated to fill the gaps in k-space (matrix inversion with information from the coil sensitivity profiles). This is achieved via the SMASH reconstruction algorithm.
SMASH
Parallel MR Imaging with iPAT
More than Just Common SENSE
Daniel S.Grosu,MD,MBA
Siemens Medical Solutions USA,Inc.
Parallel Imaging (k-space Example)
The following slide shows fast spin echo T2 weighted sagittal
scans of the lumbar spine, without (A) and with (B) parallel
imaging.
In (B), every second Fourier line has been skipped
(acceleration factor of 2). Scan time is thus reduced by a
factor of two (comparing B to A).
SENSE
SENSE (SENSitivity Encoding) [2] is “image based” because the reconstruction algorithm operates on partial images (from each coil) that have been generated by the FFT.
1. The first step is identical to the first step in SMASH.
2. Each k-space (with a reduced number of lines) is subjected to a conventional FFT at this stage.
3. This results in two (or more) aliased images with rectangular FoVs.
SENSE-based techniques do not work well with “pre-aliased” images. If the original field of view (before using parallel acquisition) is smaller than the object and is already aliased, a wraparound artifact will be present.
SENSE
Parallel MR Imaging with iPAT
More than Just Common SENSE
Daniel S.Grosu,MD,MBA
Siemens Medical Solutions USA,Inc.
Parallel Imaging
(Image Based Reconstruction)
The following slide shows fast spin echo T2-
weighted sagittal scan of the lumbar spine , without
(A) and with (B) parallel imaging.
In (B) every second Fourier line (parallel imaging
with an IPAT factor of 2). Thus the scan time for (B)
is half that of (A). Note that there are residual wrap
around artifacts (arrow, B), a major drawback to the
2. mSENSE; Modified SENSE (Siemens) SENSE based technique.
Parallel Imaging (Drawbacks)
K-space based reconstruction: The ability to construct effective sensitivities from the spatial sensitivities for each coil element depends on the sensitivity profile. This, in turn, depends on the coil element design; therefore, coil design is more critical with this technique.
Image-based reconstruction: If an aliasing artifact would be present in the chosen FOV for a non-parallel image sequence, then this aliasing will cause reconstruction problems if parallel imaging is attempted.
1) The Physics of Clinical MR, for Neuroradiology, Taught
Through Images
AUTHORS: VAL M. RUNGE1 MD, WOLFGANG R. NITZ2 PHD,
STUART H. SCHMEETS2 BS, RT, WILLIAM H. FAULKNER, JR.3 BS, RT, NILESH K. DESAI1 MD
References:
2) The Physics of Clinical MR, Focusing on the Abdomen,
Oxyhemoglobin – diamagnetic (electrons from oxygen shields iron)
Deoxyhemoglobin – paramagnetic
Blood Oxygen Level Dependent
Imaging (BOLD) -Theory
Protons near paramagnetic tissue (i.e., more deoxygenated blood) experience a quicker dephasing after a spin excitation. Increase in T2* rate reduction in local signal.
So, for pulse sequences sensitive to T2* contrast, deoxyhemoglobin appears dark, and oxyhemoglobin appears bright.