Perfusion MRI
Perfusion
Perfusion is a
fundamental biological function that refers to the delivery of oxygen and
nutrients to tissue by means of blood flow.
Perfusion normally
refers to the delivery of blood at the level of capillaries and measures in ml/100gm/min.
Perfusion is closely
related to delivery of oxygen and other nutrients to the tissue.
Two main perfusion MRI
approaches have been developed:
A. Those using
exogenous contrast agent
B. Those without using
contrast agent
The first group of
technique include Dynamic Susceptibility Contrast (DSC-MRI) and Dynamic
Contrast Enhanced (DCE-MRI).
The second group
relates to Arterial Spin Labeling (ASL).
DSC-MRI Principle
DSC-MRI is one of the exogenous contrast based method and
relies on IV injection of paramagnetic contrast agent, Gd-DTPA.
This technique utilizes
very rapid imaging to capture the first pass of contrast agent, k/a Bolus
Tracking MRI.
After the bolus of
contrast agent is injected, hemodynamic signals of DSC-MRI depends on T2 or T2*
relaxation time and transiently decrease because of increasing susceptibility
effect.
DSC-MRI is based on
susceptibility changes after injecting contrast agent.
The contrast agent is a
paramagnetic material, which distorts the magnetic field and reduces T2 around
the vessel because of an increased susceptibility effect.
Two compartments must
be considered: intravascular (IV) and extravascular(EV).
When the tracer remains
IV, the compartmentalization of contrast agent creates strong, microscopic
susceptibility gradients which extend beyond the vessel size.
The contrast agent
modifies the blood T2/T2* relaxation rates (R2/R2*).
The changes of T2/T2*
(∆R2*) is the subtraction of R2* b/w post and pre-contrast injection.
DSC-MRI Parameter
Here, a bolus of
Gd-based contrast agent administered as a short venous injection of 10-15 sec,
by the time it reaches the ROI (brain), creating a signal dip of about 10-20
sec or longer.
To faithfully record
the tracer conc during this passage, images must be acquired at a rate much
faster than the time it takes the bolus to pass through the tissue (i.e. MTT),
which is usually of order of several sec.
Adequate coverage
(whole brain) with T2*WI at a time resolution TR<2sec needs rapid imaging
seq like EPI.
TE is chosen long
enough to produce sufficient CNR due to susceptibility effects, but not long
enough to dephase all signal during the max contrast agent conc.
A relatively large FA
is used, although not long enough to introduce unwanted T1 contamination.
DSC-MRI Signal Dynamics
Serial images are
acquired before, during and after injecting the contrast agent.
While passing through
the microvasculature, a bolus of contrast agent produces decrease in SI.
The time course images
can be divided into 3 stages: the baseline, the first passage of bolus and the
recirculation period.
DCE-MRI
This is the other
exogenous contrast based method.
After the bolus of
contrast agent is injected, hemodynamic signal of DCE-MRI depend on T1
relaxation time, and increase because of T1 shortening effect associated
with paramagnetic contrast agent.
DCE-MRI uses rapid and
repeated T1W images to measure the change in signal induced by paramagnetic
tracer in the tissue with change in time.
In this method,
contrast agent is also IV injected to generate bolus.
Shortening of T1
relaxation rate caused by contrast agent is the mechanism of tissue enhancement
(so called T1 or relaxivity based method).
This is usually scanned
with T1W in 2D or 3D.
GE methods (FLASH)
T1W high resolution isotropic volume acquisition are used.
For quantitative DCE, a
pre-contrast T1 value for each voxel is often performed.
These values are used
to calculate the post injection T1 values for each voxel.
DCE-MRI Parameter
DCE-MRI is scanned with
very short TR and TE to generate T1W images.
FA is also small
due to short TR.
With current MRI
technique, the volume scan time is usually b/w 5-10sec.
For breast and large
FOV, it may extend upto 20sec.
So, it may require
compromise in terms of spatial resolution, temporal resolution and coverage.
DCE-MRI Signal Dynamics
The time of enhancement
is related to the changes, which depend on the physiological parameters of
microvasculature in the lesion and on the volume fractions of the various
tissue compartments.
For a bolus inj, there
is always an initial rise in its conc in the plasma and possibly some leakage
into the interstitium for the duration of the inj.
Afterwards, the plasma
conc continuously decreases because of diffusion into the body and clearance
through the kidneys to the urine.
The dynamic acquisition
pattern is similar to that of DSC-MRI.
Images are acquired
before, during and after injection of contrast agent.
Most benign and
malignant lesions show signal enhancement in first few min after bolus
administration.
The normal tissue may
also show enhancement.
ASL
ASL gives absolute
values of perfusion of tissue by blood.
This technique utilizes
arterial water as an endogenous diffusible tracer which is usually achieved by
magnetically labeling incoming blood.
ASL is completely
non-invasive, using no injection of contrast agent or ionizing radiation and is
repeatable for studying normal or abnormal physiology and its variation with
time.
ASL required the
subtraction of two images, one in which the incoming blood has been
magnetically labeled (k/a label or tag image) and the other in which no
labeling has occurred (k/a control or reference image).
The signal diff is the
ASL signal and which removes the static tissue signal.
ASL signal is approx
<1% which makes SNR.
Types of labeling methods
Continuous ASL (PASL)
Pulsed ASL (PASL)
Velocity Selective ASL
(VSASL)
Vascular Territory
Imaging (VTI)
CASL
In CASL, the
magnetization of the arterial blood flowing through a major artery is
continously labeled (usually by inversion) using RF pulses.
It is based on steady
state approach.
A continous RF field is
applied for a few sec along with a field gradient.
PASL
PASL involves a
relatively short RF pulse, which results in labeling (usually inversion) of the
blood in a large region adj to the imaging volume.
1. Signal alternating
with alternating radio frequency (STAR)
2. Flow alternating
inversion recovery (FAIR)
3. Proximal inversion
with control for off-resonance effects (PICORE)
4. Transfer insensitive
labeling technique (TILT)
VSASL
VSASL does not label
the blood based on its spatial location (as in CASL and PASL) but rather does
so based on its velocity, e.g. using binomial pulses to selectively label blood flowing below a certain velocity.
This labeling is not
spatially selective, but can be made sensitive to different blood velocities.
ASL Parameter
ASL contrast is related
to the preparation module (blood labeling), but does not rely on T2/T2*/T1
contrast of the acquisition module.
Short TE
to maximize SNR
Long TR to
allow the labeling blood to reach an imaging plane.
Large FA
is used.
Long scan duration due
to NSA around 40.
ASL Signal Dynamics
In routine PASL and
CASL, the signal at a single TI or post labeling delay of 1.5-1.8 sec is often
used.
Two labeling times (Quantitative
Imaging of Perfusion using a Single Subtraction- QUIPSS I/II) are used to
minimize the definition error of bolus width.
The ASL signal curve
also shows 3 phases.
Summary
DSC-MRI is good for
quick measurement of transit time, whole brain coverage and fast scan time.
DCE-MRI is good for
measurement of BV, perfusion permeability and capillary permeability and for
reducing image artifacts.
ASL is good for BF
measurement, repeatable studies and provide unique opportunity to provide CBF
information without inj of contrast agent.
However, bolus methods
with inj of contrast agent provide better sensitivity with higher spatial
resolution.
References
Perfusion Magnetic
Resonance Imaging: A Comprehensive Update on Principles and Techniques, Korean
Journal of Radiology, July 2014.
Perfusion MRI:
Technical Aspects, 2009 Lecture.
Perfusion Imaging by
Monil Shah.
Dynamic Contrast
Enhanced MRI by Sungheon Gene Kim, PhD, NYUSOM/Radiology/CBI.
MR Perfusion Imaging
Techniques and Applications, Chen Lin, PhD, Indiana University School of
Medicine & Clarian Health Partners.
ASL Basics I,
University of Navarra, Pamplona, Spain.
Alternatives to BOLD
for fMRI, Harvard-MIT Division of Health Sciences and Technology.
Perfusion MRI: The Five
Most Frequently Asked Technical Questions, AJR Am J Roentgenol, 2013.
Contrast-Enhanced MR
Perfusion Imaging, University of Wisconsin, Madison, RSNA Chicago, 2008.
Advanced MRI Techniques
(and Applications), University of California, Los Angeles, January 2012.
Noncontrast Perfusion
MR Imaging with Arterial Spin Labeling (ASL) of cerebrovascular disease,
European Society of Radiology, 2011.
Magnetic Resonance
Imaging of Perfusion, D. Le Bihan, 1990.
