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VISIONS 11 . 07 MR ANGIOGRAPHY
Fresh Blood Imaging
Mitsue Miyasaki, Fridtjof Roder
Abstract
In recent years MR angiography techniques have
steadily created new possibilities in patient studies.
The lack of radiation exposure and the fact that
these new techniques are non-invasive, or at most
minimally invasive, are just two of the obvious
benefits. And in particular, within the last few years
contrast-enhanced MR angiography has become an
ever more preferred tool. This requires state-of-theart MR technology which must be able to provide
fast gradient echo sequences.
Fig. 1: 3D TOF
angiography of the
carotid arteries
with binomial water
excitation pulse
(WET1), flow compensation, multi-slab
acquisition, and SORS
(MTC) pulse.
All images
generated with
Toshiba Vantage 1.5 T
40
New perspectives
in MR angiography
Intimately related to this was the development of
fast gradient hardware. This was accompanied by
the development of fast spin echo techniques, such
as FASE (Fast Advanced Spin Echo) and SuperFASE
by Toshiba. When employed in fresh blood imaging
(FBI) mode, these techniques may also be used to image vessels without the need for contrast media.
With FBI and additional advances in this field it has
become possible to image almost all vascular regions
of the human body, while at the same time differentiating arteries and veins.
Fig. 2: 3D PC
angiography for imaging
of the cranial veins.
Flow encoding in all
three spatial directions.
MR angiography
techniques
There are four fundamental imaging techniques
in MR angiography: time of flight (TOF), phase contrast (PC), contrast enhanced (CE) and fresh blood
imaging (FBI). Each is based on a different fundamental principle of enhancing the contrast with the
surrounding tissue. Below each technique, their benefits and drawbacks as well as their typical applications are explained. Subsequently, the technique of
flow-spoiled FBI will be discussed in somewhat more
detail.
TOF MR angiography primarily uses the saturation of stationary tissue and the inflow of unsaturated signals into the field of view (FOV). Over the
years, various optimizations have been introduced
into TOF sequences in order to account for the local
flow conditions and the rising clinical demands
placed on image quality, for example:
- presaturation slabs and concomitant presaturation slabs for suppression of venous (and optionally arterial) signals
- 3D techniques for improved resolution, particularly along the slice direction
- MTC and SORS-MTC for improved suppression of
stationary tissue
- 3D multi-slab and high-frequency pulses with
locally variable flip angles (ramped RF) for decreasing the saturation effect within the vessels
- flow compensated sequences for averting artifacts induced by rapid blood flow
- fat suppression and water excitation sequences
for averting the superposition of fatty tissue.
Today, this technique is primarily employed in
MR angiography of the cranium since there it offers
the best image quality, in particular due to the high
resolution possible. In TOF angiography resolution
may be set freely over a wide range, with concomitant increases in the measurement period. TOF angiography is well suited for medium and high flow
situations. For instance, it is used in carotid imaging
(Fig. 1) when contrast agents cannot be employed or
are not desired.
PC-MR angiography is based on the signal phase
shift between non-stationary and stationary tissue.
In normal imaging this effect manifests itself as flow
or motion artifacts. The phase shift is proportional to
the flow rate and depends on the gradients switched.
In order to determine the phase shift two measurements must be performed – one for reference and
one flow encoded measurement. This leaves the
problem that the flow rate vector will only be detected along one direction. Thus, in somewhat more
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VISIONS 11 . 07 MR ANGIOGRAPHY
Fig. 3: Contrast-enhanced
MR angiography
of the carotid arteries
42
complex vascular conditons three flow-encoded
measurements must be performed. This will prolong
the measurement period. One application for PC-MR
angiography is cranial venography (Fig. 2).
PC-MR angiography offers the major benefit of
supplying quantitative flow data, somewhat akin to
color flow Doppler ultrasound. Therefore, this technique is used, for instance, in flow rate measurements around the cardiac valves or in the computation and visualization of rate vectors near stenoses.
Over the last ten years, contrast-enhanced MR
angiography (CEMRA) has been the technique with
the most widespread application. CEMRA primarily
depends on the T1 effect of the contrast agent employed. It is used quite often in subtraction mode,
similar to DSA. This ensures better suppression of the
background signal. It is mostly used in visualization
of the arterial signal. CEMRA requires the passage of
a contrast bolus which should not yet have arrived
in the venous bed. This usually leaves a time frame
of about 20 seconds for imaging. For the cranium
this time frame is shortened to about 10 seconds. For
most intracranial vessels this does not yield a satisfactory resolution.
Typical applications are the carotid arteries (Fig.
3), aorta, peripheral arteries, abdominal arteries, and
arterial axis of the pelvis and leg.
One application for contrastenhanced MR angiography of
the head are the modern dynamic techniques. They primarily study the dynamcis of the
contrast agent, e.g. in order to
assess arteriovenous malformations (AVM). Parallel imaging
(SPEEDER) and so-called keyhole
techniques (DRKS - differential
rate k-space sampling) offer the
opportunity of achieving a temporal resolution of one to two
seconds. This permits 3D visualization of the contrast agent
flowing into the malformation
and thus allows planning of subsequent diagnostic and/or therapeutic steps.
Fresh Blood Imaging (FBI)
In the Fresh Blood Imaging (FBI) technique the
fundamental contrast mechanism is the T2 relaxation period of the blood, which is longer than that
of the surrounding tissues. In principle, the vessels
may be imaged with FASE (Fast Advanced Spin Echo)
or SuperFASE- (FASE with very short echo spacing).
As in the MRCP technique, this is done by strongly
weighting T2. Toshiba has gathered experience in
these non-contrast enhanced techniques, such as
SPEED, from the very beginning.
Contrary to depicting purely stationary signals,
vessels display flow phenomena which are due either
to the TOF effects discussed in the MR angiography
techniques above, or to phase shifts of the blood
flow.
TOF effecst pertain to blood flowing from one
point to another. In standard fast spin echo (FSE) sequences these TOF effects may result in blood flowing out of the FOV volume or slice during imaging.
In particular when dealing with T2 weighted 2D multi-slice sequences this will make blood flow appear
dark. This effect of blood flowing out of the FOV volume is counteracted by employing a 3D imaging
technique and by keeping the course of the main
S.I.
Diastole
Systole
S.I.
Diastole
Systole
Spoiler
gradients
Arteries
Veins
Arteries
Veins
Fig. 4a: Signal intensity difference between
peripheral arteries and veins during systole
and diastole for an FBI sequence,
with the vessels aligned along the readout
(frequency encoding) gradient.
Fig. 4b: Signal intensity difference between
peripheral arteries and veins during systole and
diastole for an FBI sequence with the vessels aligned
along the readout (frequency encoding) gradient
and additional use of spoiler gradients.
vessel within the FOV plane. In other words, contrary
to TOF angiography the slices should not be aligned
orthogonal to the vessel.
On the other hand, in some FBI modifications the
blood flow from one point to the other is used on
purpose in order to separate the arterial and venous
images. For instance, this will be the case when using t-slip pulses which will be discussed later on.
In standard imaging phase shifting by non-stationary objects most often results in artifacts, e.g.
the characteristic respiratory artifacts or flow artifacts. In phase shift (PS) sequences the effect uses
specific phase modifications for flow measurement,
or it is employed as phase contrast (PC) angiography
in vascular imaging. The effect of blanking the signals from non-stationary objects is used on purpose
in flow-spoiled FBI.
chest) the phase encoding direction is along the direction of the main flow.
In the more peripheral regions of the body, such
as the lower legs, the difference between the diastolic and systolic flow is insufficient for differentiating between arteries and veins.
Only if the direction of readout and phase encoding is changed will the difference in signal
intensity between the systolic and diastolic image
suffice (Fig. 4a). Additional spoiler gradients along
the direction of readout will increase the difference
in arterial signal intensity (Fig. 4b). For this, additional gradient pulses along the direction of the
main flow are added to the gradient switching pattern (Fig. 5) in order to ensure maximum dephasing
of the flow signal.
The necessity for differential systolic and diastolic
measurements requires definition of the systolic and
diastolic phases. To this end, a 2D test sequence
(ECG prep scan) is acquired which automatically
tests various cardiac phases. A 2D SuperFASE sequence is executed which works with various gating
delays relative to the R-wave of the ECG obtained
from the patient. Subsequently, the images thus obtained are assessed according to maximum and minimum visibility of the arteries. This may be done on
the original images or on subtracted images, which
in the latter case would enhance contrast.
Flow-spoiled FBI
In areas of rapid arterial flow FBI images obtained under diastolic gating will depict bright veins
as well as arteries. On the other hand, imaging with
systolic gating will yield bright veins and dark arteries. Thus, differentiation between arterial and venous images is fairly easy by subtraction techniques.
The FBI sequence employs an ECG-gated half fourier
single-shot fast spin echo sequence with short echo
spacing (SuperFASE). In high flow areas (e.g. in the
90
HF & Signal
180
Time
Fig. 5: Gradient
pattern for flowspoiled FBI.
HF = high frequency
signal
Readout-Gradient
SUPERFASE
With spoiler
gradient
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VISIONS 11 . 07 MR ANGIOGRAPHY
Fig. 6: Flow-spoiled FBI
sequence of the upper
and lower leg in a healthy
female volunteer.
Original data without
subtraction. Venous
signal on the left. Arterial
and venous signal on the
right. Fig. 7 depicts the
subtraction and thus the
arterial signal.
Region
Healthy
Strength
Flow gradient
Stenosis
Strength
Flow gradient
Iliac
Femoral
Crural
Pedal
Hand
-10
0
+10
+30
+25
-5
+5
+15
+35
+25
Table 1: Flow-spoiler pulse optimization
as a function of ROI and patient studied
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Additional optimization becomes necessary because of the spoiler gradient. The flow conditions in
the vessels studied not only depend on their
anatomical site but also on the individual patient.
This variation may require individual optimization of
spoiler gradient strength. Here, too, the first step is
a 2D test sequence (flow prep scan) with gating delays already optimized. The strength of the flowspoiler gradient is changed automatically during the
flow prep scan. In this case, the image with optimum
contrast is chosen as well, after which the actual 3D
SUperFASE sequence with optimized gating delay
and optimized spoiler gradient is performed.
In many cases, spoiler gradient optimization may
not be necessary since there are empirical values for
many regions which are quite satisfactory in the
majority of patients (Table 1). One rule of thumb is
that the spoiler gradient must be increased the more
peripheral the region to be studied and the slower
the flow rates to be expected.
Fig. 7: Subtraction of the systolic
and diastolic images will result in good
demonstration of the arteries.
The same volunteer as in Fig. 6,
here after manual stitching of the various
vascular levels.
Once gating delay and flow-spoiler gradient have
been optimized, the 3D study of the region of interest may be performed. Initially, this will yield a venous image and then an image with all vessels of the
ROI (Fig. 6). Subsequent subtraction of the systolic
and diastolic images will result in a very good quality arterial image of the vessels of interest (Fig. 7).
For ease of visualization the images of the different
vascular levels, which may have been studied separately, may then be stitched together. This may
either be done manually (as in this case) or automatically (stitching software).
Discussion and perspectives
In principle, fresh blood imaging techniques are
well suited for non-contrast enhanced arterial
and/or venous imaging in all areas of the body.
Compared with phase contrast (phase shift) techniques they do no provide direct data on flow and
therefore cannot be used directly for flow measurements. Data on the flow may be obtained indirectly
from the strength of the spoiler gradient but has not
been used to date for quantitation of flow.
Presently, TOF angiography of the cranium has
been the most mature technique in arterial angiography. Because TOF angiography does not require contrast media it is not necessary to extend
the FBI technique to the cranium, in particular since presently
fresh blood imaging has not yet
been optimized for signal suppression of the cerebrospinal
fluid.
Compared with contrast-enhanced MR angiography (CEMRA), the principal characteristic is
its lack of contrast media. Except
for a small number of cases where
no contrast agent should be administered, the financial aspects
are important. This primarily pertains to two issues: the contrast
agent itself and the time sequence.
The financial aspect of contrast agent administration depends on the local situation but increasingly will be roped in by the reimbursement discussion.
Presently, CE angiography is faster than fresh
blood imaging because the latter requires patient
repositioning for each vascular level, particularly
when dealing with ilio-femoro-crural angiography.
In addition, there is much less experience worldwide
with FBI sequences. Since some steps of this study
have not been fully automated yet, e.g. ECG prep
scan and flow prep scan, they still require manual intervention by the user. However, in the near future
this technique will be fully automatic and therefore
will make handling of FBI studies even easier.
T-slip prepulses, i.e. prepulses which may be varied in time and space, will offer new opportunities
for imaging of even more complex flow situations
such as the portal vein.
Literature
1 Miyazaki M, Ichinose N, Sugiura S, et al., A novel MR angiography
technique: swap phase encode extended data (SPEED) acquisition
using half-Fourier RARE. J Magn Reson Imaging 1998; 8:505–507.
2 Miyazaki M, Sugiura S, Tateishi F, Wada, F, Kassai Y, Abe H., Non-contrast-enhanced MR angiography using 3D ECGsynchronized
half Fourier. fast spin echo. J Magn Reson Imaging 2000;
12:776–783
Fridtjof Roder
Toshiba Medical
Systems GmbH
Germany
Mitsue Miyasaki, PhD
Toshiba Medical
Systems USA
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