cardiovascular magnetic resonance imaging at 3.0 tesla
TRANSCRIPT
Cardiovascular Magnetic ResonanceImaging at 3.0 Tesla
M. Gutberlet1, P. Freyhardt1, B. Spors1, K. Schwinge1, M. Grothoff1, R. Noeske2, T. Niendorf3, R. Felix1
1Department of Diagnostic Radiology and Nuclear Medicine, Charite, Campus Virchow Klinikum, University Medicine Berlin, Berlin,Germany2GE Healthcare Technologies, Berlin, Germany3Applied Science Laboratory (ASL), GE Healthcare Technologies, Boston, MA, USA
Correspondence to:Matthias Gutberlet, MDDepartment of Diagnostic Radiology and Nuclear Medicine, Charite, Campus Virchow Klinikum, University Medicine Berlin, AugustenburgerPlatz 1, 13353 Berlin, GermanyTel: +49 30 450557001; Fax: +49 30 450557901; E-mail: [email protected]
Key words: Cardiovascular MRI, 3 Tesla, 3.0 Tesla, parallel imaging, tagging, viability.
Abstract
Cardiovascular MR imaging often requires high temporal
and spatial resolution, especially in order to acquire data
about cardiac function. Furthermore, the current results at
1.5 T for coronary artery imaging or plaque imaging are
still not satisfying even with the use of the latest technology.
Therefore, cardiac imaging inherently demands high sig-
nal-to-noise (SNR) and contrast-to-noise ratios (CNR) and
hence may benefit from higher magnetic field strengths.
However, higher magnetic field strengths do not inevitably
improve the image quality for all cardiac imaging techni-
ques as compared with their 1.5 T counterparts. At higher
magnetic field strengths one has to cope with increased
field inhomogeneities, longer T1, shorter T2* relaxation
times and radiofrequency power deposition constraints,
which require further methodological developments. Initial
studies using 3.0 T whole-body scanners for cardiac ima-
ging revealed that optimized steady-state free precession or
spin-echo sequences meet the expected SNR increase at
3.0 T but showed different results for CNR. These results
are especially encouraging for cardiac tissue characteriza-
tion at 3 T together with the evolving parallel imaging
techniques. This review focuses on the feasibility of cardiac
MR imaging at high magnetic field strengths. The pros
and cons of cardiac imaging at 3.0 T vs. 1.5 T are
examined and technical solutions are discussed.
Introduction
Limitations in cardiac MR imaging are very often caused
by temporal and/or spatial resolution constraints. There-
fore, a high signal-to-noise (SNR) and/or contrast-to-noise
ratio (CNR) is mandatory (1). One of the main determi-
nants of SNR is the static magnetic field strength (B0) so
that whole-body MRI at 3.0 T may help to overcome
some of these limitations. Higher static magnetic field
strengths (B0) give higher SNR; but the radiofrequency
(RF) field (B1) of both transmit and receive coils is much
more dependent on the sample, i.e. patient (1). In addition,
RF power limitations due to specific absorption rate (SAR)
constraints have to be taken into account (2) for human
studies. Increased susceptibility related B0 field inhomo-
geneities may result in additional artefacts. There are only
few cardiovascular MR (CVMR) studies in which the SNR
has been compared at different field strengths (1, 3) while
the number of high-field 3.0 T whole-body MR systems is
constantly increasing. The majority of studies have been
reported for research platforms (1, 4), but initial studies
recently described the use of 3.0 T in cardiac imaging
using commercially available systems, together with com-
mercial hardware and software (5, 6). All studies demon-
strated a significant SNR increase (3, 4, 6–10) but also
reported some image quality problems associated with B1field inhomogeneities in different regions of the heart (3, 5).
A pronounced sensitivity to susceptibility artefacts, which
might result in a SNR decrease and an image quality
degradation were also reported for 3.0 T CVMR studies
(3, 5). The aim of this review is to give a short overview of
the potential benefits, current results and remaining tech-
nical challenges for cardiovascular MR imaging at 3.0
Tesla.
Potential benefits of 3.0 T in cardiac imaging
Cardiac MRI has been proven to be the gold standard for
the depiction of cardiac anatomy under several circum-
stances and also for the evaluation of cardiac volumes and
mass, especially for the right ventricle or in pathological
geometries of the left ventricle (11, 12). 18F-FDG PET is
considered to be the gold standard for myocardial viability
assessment from a nuclear medicine point of view. With
the widespread use of myocardial viability assessment using
the ‘delayed contrast enhanced’ techniques (13) where
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several studies suggested advantages over nuclear medicine
techniques (14, 15), MRI becomes more and more the gold
standard for viability assessment. Several encouraging re-
sults were reported for the analysis of myocardial perfusion
using MRI (16). This approach is not been considered as
being clinical routine and hence has to prove its reliability
in larger clinical trials, including direct comparison with
established nuclear medicine techniques like technetium
SPECT or PET (17). Clinical routine coronary MR angi-
ography remains technically challenging, which renders
CT a superior competitor, which is mainly due to the
improvements in the spatial resolution and temporal
resolution of multidetector computed tomography
(MDCT) (18). The even more challenging attempt of MR
coronary plaque imaging remains very difficult with the
current 1.5 T systems. All these applications demand faster
imaging, if possible almost ‘real-time’, and a higher spatial
resolution. It has been demonstrated that high magnetic
field strengths improve the baseline SNR in cardiac ima-
ging (3, 6) as compared with the conventional 1.5 T ap-
proach. This merit of high-field imaging can be translated
into an enhancement in the spatial and temporal resolu-
tion. It has been also predicted that high-field strengths
promise to reduce the noise amplification in parallel
imaging (Fig. 1a and b). The need for reducing the RF
power deposition at high magnetic fields accentuates the
complementary advantage of parallel imaging (19).
Thus high-field strengths together with tailored imaging
strategies offer the potential to overcome physiological
motion and SNR constraints of current cardiovascular MR
imaging approaches (3, 6, 19).
Problems and potential solutions
Cardiac imaging at 3.0 T is different from imaging at
1.5 T due to SAR limitations, increased susceptibility
artefacts (Fig. 2), differences in tissue relaxation (Fig. 6),
and RF homogeneity issues (Fig. 7) (20).
SAR limitations
State of the art routine cardiac imaging procedures for the
assessment of ventricular function andwall motion are based
on steady-state free precession (SSFP) imaging techniques (3,
19, 21). These sequences offer a high intrinsic CNR between
myocardial muscle (Figs 1 and 2) and blood within the
ventricles at a high SNR, but need a short TR, which is
achieved by using themaximumamplitude of theRF fieldB1
c d
a b
j Fig. 1. SSFP 4 chamber view (4 cv) at 3.0 T without (a)/with (b) the use of parallel imaging (ASSET) – acceleration factorR ¼ 2. The acquisition time (a) was 14 s and (b) 7 s. The following imaging parameters were used for the SSFP (FIESTA)sequence: TR ¼ 3.4 ms (minimum), TE ¼ 1.5 ms, slice thickness ¼ 8 mm, field of view ¼ 350 mm, PFOV ¼ 1.0,matrix ¼ 2242, flip angle ¼ 30�, views per segment ¼ 12, retrospective gating, 50 phases per RR interval, TA ¼ 16heartbeats. (c, d) Short axis fast spin-echo ‘black blood’ images which demonstrate the high image quality obtained at 3.0 T.A double inversion recovery sequence (a) and a triple inversion recovery sequence (b) were used together with an eightchannel cardiac phased array coil.
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IMAGING DECISIONS n 3/2004
(19), which keep the excitation pulse duration as short as
possible. Schar et al. (19) showed SAR constraints at 3.0 T.
The SAR limit of 4 W/kg (2) has been already exceeded at
lowB1 field strengths. In consequence the repetition timeTR
was increased. This approach can result in suboptimal image
quality at 3.0 T due to the increased sensitivity for B0 in-
homogeneities and off-resonance effects (3, 19) (Fig. 2A).
One solution to keep TR values as short as at 1.5 T is the
reduction of the flip angle (Fig. 2), which has already been
described by Schar et al. (19) with a reduction from 54� at1.5 T to 42� at 3.0 T, by Hinton et al. from 60� at 1.5 T to
36� at 3.0 T (6) and by Gutberlet et al. (3) from 45� to 30�(Fig. 2) depending on the different scanners used for com-
parison. Fortunately, it could be shown that the optimal
CNR of cardiac muscle and blood within the ventricles at
3.0 T is achieved also at lower flip angles (19) as compared
with 1.5 T. Another possibility to keep TR values as short as
at 1.5 T without exceeding SAR limits is been offered by the
use of parallel imaging techniques, which reduces the
number of applied RF pulses (3). The application of variable
flip angle and hyperechoes is another technical approach to
offset SNR limitations (22). Busse demonstrated a 75%
reduction in the RF power deposition using fast spin echo
sequences operated at high-field strength together with a
modulated angle refocusing train method to reduce power
and prolong relaxation, and a smooth transition between the
high and low flip angle regimes (23), without impairing the
image quality.
Susceptibility artefacts and RF homogeneity issues
Shorter T �2 values which can be attributed to larger static
magnetic field (B0) inhomogeneities within the left ventricle
(LV), which are mainly caused by the heart–lung interface
(24) and mostly occur close to the posterior coronary vein
(Fig. 2) of the LV (3, 4), make fast imaging of the heart
more challenging (4, 19, 25) at 3.0 T. The most common
artefacts are off-resonance artefacts, which manifest itself
by ‘dark band’ coherence patterns. These artefacts (Fig. 2)
can be minimized by reducing the repetition time (TR), or
by improving the B0 homogeneity using sophisticated
shimming algorithms (3, 19). B1 field inhomogeneities in
spin-echo sequences at 3.0 T tend to deteriorate image
quality depending on patients shape, especially at the right
ventricle (3). These ‘shading artefacts’ occur at the free wall
of the right ventricle, close to the diaphragm (Fig. 7). These
artefacts are already known from 1.5 T (5, 26), but seem to
be pronounced at 3.0 T due to higher B1 field inhomo-
geneities. This renders the application of spin-echo-based
imaging techniques in some cases unsuitable for the
assessment of the free wall of the right ventricle (Fig. 7).
This is in alignment with the observation that the SNR
obtained for the LV (especially the septum) can show lower
values as compared with other regions (5). The application
of adiabatic RF-pulses holds the promise to overcome
B1-inhomogeneity related image artefacts obtained for
spin-echo-based cardiac imaging at 3 T (3).
Differences in tissue relaxation
While SSFP imaging sequences need a reduction in the flip
angle at 3.0 T due to SAR limitations, ‘black blood’ ima-
ging techniques require an increase in the preparation time
(TI) to null signal contributions from blood. The increase
in TI can be as large as 30%, due to longer T1 relaxation
times published by Noeske et al. (3, 4). This finding has
a b
j Fig. 2. Two chamber, short-axis view obtained with 2D SSFP (FIESTA) at 3.0 T. The volunteer data show a susceptibilityartefact at the inferiolateral wall (white arrow) of the left ventricle (LV). Due to a TR prolongation (a) the artefact is morepronounce for higher flip angles [(a) flip angle ¼ 30�; (b) flip angle ¼ 45�].
C A R D I A C I M A G I N G A T 3 . 0 T n 2 5
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implications not only for native imaging but also for con-
trast-enhanced techniques like ‘delayed enhancement’,
which has been used for the assessment of myocardial
viability. A higher CNR has been obtained for contrast-
enhanced cerebral MRI due to lengthening in T1. In
analogy to this observation it is to be expected that car-
diovascular ‘delayed enhancement’ studies might require
approximately half of the dose of Gd-DTPA at 3.0 T to
achieve the same image quality derived at 1.5 T (Fig. 6).
Furthermore, the change in the contrast sensitivity might
also result in different inversion times to ‘null’ the myo-
cardium at high-field strengths.
Current studies
Cardiac anatomy
Several cardiac imaging studies consistently demonstrated
an SNR increase at 3.0 T versus 1.5 T (1, 3, 4, 6, 19). The
SNR increase has a large variability ranging from 20% (6)
to 160% (3) depending on the RF coil configuration (6) and
imaging sequence type (3, 19, 20) used. This affords image
quality improvements and facilitates a higher spatial and
temporal resolution (6, 19, 20), which may improve the
depiction of smaller anatomical structures like valves or
trabeculae of the right ventricular musculature (6).
Greenman et al. (5) and others (3) also found a decrease in
image quality at specific regions of the heart depending on
patients shape, i.e. at the septum (5) or the free wall of the
right ventricle (3) when using spin-echo imaging tech-
niques. This effect can be attributed to B1 field inhomo-
geneities at 3.0 T (Fig. 7).
Cardiac function
Various studies suggested that functional imaging tech-
niques, like SSFP sequences (3, 6, 7, 19) or myocardial
tagging (8) (Fig. 3) benefit from the application of higher
magnetic field strengths due to the SNR increase, which
in return allows higher temporal and spatial resolution.
The SNR increase also fosters the use of higher accel-
eration factors in parallel imaging (Fig. 1) as compared
with 1.5 T. As the SNR of a parallel imaging study is
always reduced as compared with an unaccelerated study
obtained using the same coil array and identical imaging
parameter. The scaling of SNR maybe expressed as
follows (27):
SNRaccelerated ¼ SNRunaccelerated=gffiffiffiffi
Rp
j Fig. 3. Short axis view cardiac images obtained with a tagging sequence at 1.5 T (a, c) and 3.0 T (b, d) at end-systole (a,b) and end-diastole (c, d) using the same volunteer. At 1.5 T the tags are hardly visible at end-diastole (c) while the tags arevery well preserved at 3.0 T (d).
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IMAGING DECISIONS n 3/2004
where R represents the acceleration factor chosen and g the
so-called geometry factor, which is among other factors
depending on the coil design used. The SNR increase
obtained for SSFP sequences at 3.0 T overcompensates the
signal loss due to the use of parallel imaging with an
acceleration factor of R ¼ 2 (19).
Myocardial perfusion, viability contrast enhancement
To date there are no studies available concerning the use of
3.0 T magnetic field strengths to assess myocardial perfu-
sion, viability or other post-contrast studies. Brain studies
at 3.0 T revealed (22) that the prolongation of T1 relaxa-
tion times halves the amount of contrast agent, which is
needed to achieve the same CNR and image quality at
3.0 T as compared with 1.5 T (Fig. 6). Furthermore, for
delayed enhancement examinations different inversion
times have to be used to ‘null’ the myocardial signal.
Myocardial perfusion deficits can also be detected without
the need of contrast agent applications. This approach
exploits the blood oxygenation level-dependent (BOLD)
contrast, which has been successfully applied in the heart
and for the assessment of the endothelial function at 1.5 T
(28, 29). BOLD imaging at 3.0 T keeps the promise to
benefit from the higher SNR and the increased sensitivity
for microscopic susceptibility gradients.
Coronary artery imaging
The first preliminary report, on in vivo coronary artery MR
imaging at 3.0 T was published in 2002 by Stuber et al.
(10). For this human study free-breathing 3D Navigator
techniques where applied. Although this study was not
j Fig. 5. Coronary artery images obtained at 3.0 T using an eight-channel-phased array surface coil together with a 3D-fatsat SSFP (FIESTA) MR-sequence (30) showing the RCA (a), the left main (LM) and its separation into LAD and CX (bluebox), its magnification (b) and the resulting 3D-MIP (c) in a volunteer. Images (d, e) illustrate the RCA at its origin (d) and atthe magnification of the area of the ‘crux cordis’ (e) as well as the resulting 3D-MIP (f) reconstruction of the same volunteer.
j Fig. 4. Right coronary artery image acquired with a 2Dspiral imaging technique at 3.0 T displaying the origin andthe proximal segment of the RCA. The image also demon-strates distal branches at the ‘crux cordis’ (white arrow). Ao,ascending aorta; RA, right atrium; LA, left atrium.
C A R D I A C I M A G I N G A T 3 . 0 T n 2 7
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aiming at a direct comparison between 1.5 and 3.0 T
higher mean SNR and a slightly higher visible vessel length
has been demonstrated at 3.0 T. A recently published
study by Nayak et al. used a ‘real-time’ approach to image
coronaries at 3.0 T (20), including encouraging results with
regard to the observed SNR and CNR improvements
(Fig. 4). Robust breath-held contrast-enhanced magnetic
resonance angiography (MRA) at 3.0 T can be completed
in two to three breath-holds covering the main branches of
the coronary arterial systems. Niendorf et al. (30) used fat-
suppressed ECG gated 3D-SSFP (FIESTA) sequences at
3.0 T combined with an eight-channel cardiac-phased
array prototype coil.
Short breath-hold periods were achieved by turning
the higher SNR/CNR at 3.0 T into shorter acquisition
times due to the application of parallel imaging without
impairing the image quality (Fig. 5). It has been also
reported that the SNR improvement afforded by a 3.0 T
field strength coupled with the enhanced CNR between
the blood pool and the myocardium may provide
benefits for clinical coronary MR angiography (30). As
larger acceleration factors are explored with high-chan-
nel MR systems, the benefits of high-field strengths for
coronary artery imaging will become even more pro-
nounced through an increase in the volume coverage,
which may permit the visualization of the entire
Gd-DTPAGd-DTPA0.2 mmol / kg / BW0.1 mmol / kg / BWa b
3.0 T 1.5 T
j Fig. 6. Delayed contrast-enhancement images of the same patient with transmural septal infarction obtained at 3.0 T (a)vs. 1.5 T (b). The delineation of the scar tissue is possible at a high quality for both field strengths. Due to a prolongation of T1
relaxation at 3.0 T, half the amount of Gd-DTPA (Magnevist�; Schering AG, Berlin, Germany) is necessary to achieve thesame image quality at 3.0 T.
j Fig. 7. Images derived from a triple inversion recovery MR sequence using a transverse orientation in a volunteer at 1.5 T(top row) and 3.0 T (bottom row). The following parameters were applied: TR ¼ 2 RR intervals, TE ¼ 42.0 ms, slicethickness ¼ 8 mm, field of view (fov) ¼ (350 · 350)mm2, PFOV ¼ 0.75, TI ¼ 150 ms, matrix ¼ 2562, ETL ¼ 32, receiverbandwidth ¼ ±62.5 kHz, TA ¼ 16 heartbeats. The images show a significantly higher SNR at the myocardium of the leftventricle at 3.0 T as compared with 1.5 T. Higher B1 field inhomogeneity (RF profile) in 3.0 T images on the other hand,causing ‘shading’ artefacts at the free wall of the right ventricle, can also be observed. The signal intensity at the septum islower than at the lateral wall of the LV (3, 5).
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coronary tree within a single breath-hold acquisition.
The access to higher accelerations would also (i) further
reduce RF deposition constraints at high-field strengths
and (ii) would serve to enhance the immunity to
physiological motion by using shorter acquisition win-
dows. This offers the potential to integrate breath-held
coronary MRA into clinical cardiac examinations for the
non-invasive detection of coronary artery anomalies or
luminal stenoses in the proximal parts of the coronary
arteries at high magnetic field strengths.
Conclusion
Clinical cardiac MR imaging is very well established at
1.5 T and has been shown to be feasible in volunteers at
3.0 T using commercial whole-body scanners (1, 3, 6–8,
10, 19, 20, 31). The higher magnetic field strength requires
methodological developments and imaging protocol
adjustments due to SAR limits (3, 6, 19), T1 relaxation time
prolongation, T2* shortening as compared with 1.5 T
applications. All applied imaging techniques revealed a
significant SNR increase. In the majority of cases a CNR
increase was also obtained, which primarily improved the
image quality at 3.0 T as compared with 1.5 T. Both
effects allowed a higher spatial and temporal resolution,
which is especially beneficial in conjunction with the use of
parallel imaging. However, increasing susceptibility arte-
facts in SSFP sequences as well as artefacts due to B1-field
inhomogeneities depending on patients shape (3, 5) were
observed, which may also result in a degradation in the
image quality derived from cardiac imaging at 3.0 T.
The first group of artefacts can be compensated by
advanced shimming (3, 19), while it is more challenging to
address the B1-field inhomogeneities (3). First reports on
coronary artery imaging in humans at 3.0 T hold the
promise to integrate coronary magnetic resonance angi-
ography (CMRA) into clinical cardiac examinations for the
non-invasive detection of coronary artery disease (10, 20,
30). Comprehensive studies dealing with the use of cardiac
MRI for the detection of perfusion deficits and the
assessment of myocardial viability have not been published
yet. It is to be expected that these approaches will also
profit from the higher field strength. In summary, further
volunteer and patient studies are required to prove the
potential clinical advantages and extra diagnostic value of
cardiac MRI at 3.0 T over current 1.5 T applications.
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