The majority of existing clinical magnetic resonance (MR) systems operate in the range of 1.0T to 1.5T. The desire to perform human studies at higher field strengths is related to the higher signal-to-noise ratio, contrast-to-noise ratio, and spectral resolution. Challenges in operating at higher fields include increase in field inhomogeneity and radiofrequency energy deposition. Moreover, stronger and faster magnetic field gradients are desired at higher fields, raising concerns about eddy-current stimulation and acoustic noise. This paper will discuss operation at high field, innovations addressing the challenges and clinical and potential economic benefits.
is currently a 4th-year Radiology Resident at the University of
California, San Francisco, CA. He attended the Medical Scientist
Training Program at the University of Minnesota, Minneapolis, MN
and completed his PhD in Biomedical Engineering in 1997 and his
MD in 1998. Dr. Le will remain at UCSF for a Neuroradiology
Fellowship following completion of his residency program.
The development of magnetic resonance imaging (MRI) represents
one of the greatest achievements in medical imaging. No other
modality in the field has progressed as rapidly in its first 30
During this brief period since its first introduction in 1973,
innovation in superconducting technology further accelerated
development of MR technology, allowing imaging at field strengths
of >=1T. The desire to perform human studies at higher field is
related to the higher signal-to-noise ratio (SNR),
contrast-to-noise ratio (CNR), and spectral resolution. The
majority of existing clinical MR systems operate in the range of
1.0T to 1.5T. Although whole-body higher field (>=3T) systems
were devised more than a decade ago,
these systems have been available only to highly specialized
research institutions. Physicists and engineers had to be present
at installed sites to provide daily operational support. Initial
high-field experiences were promising but less than satisfactory
due to the lack of understanding about the high-field technology
and specialized tools designed to operate at higher frequencies.
Since then, advancements in hardware and software engineering have
made high-field human MR imaging a clinical reality. At present,
all major MR manufacturers have received clearance from the Food
and Drug Administration (FDA) for their whole-body 3T systems.
Around the globe, >50 clinical 3T systems have been installed,
with >100 additional 3T systems being purchased or built.
Manufacturers also are actively seeking FDA approval for whole-body
4T systems. In addition, the FDA has deemed magnets up to 8T safe
for adult humans and up to 4T for neonates >=30 days. This paper
will review progress in clinical high-field MRI. In particular,
advantages and challenges of operating at high field will be
discussed. Hardware and software innovations addressing the
challenges at high field will also be reviewed. Finally, clinical
and potential economic benefits of high-field MRI will be
Benefits at high field
SNR varies linearly with field strength (B
); therefore, increasing from 1.5T to 3T can theoretically double
the SNR. The higher SNR can be exploited to improve spatial
resolution by imaging with thinner slices and/or higher in-plane
resolution. The higher SNR can also be utilized to reduce imaging
time. In addition, improving the temporal resolution has the added
benefit of reducing gross and physiologic subject motion
Higher SNR also benefits spectroscopic imaging. Clinical utility
of spectroscopic imaging has been hindered by small concentrations
of the targeted nuclei and long measurement times. An improvement
in SNR translates to an increase in peak amplitude to background
noise ratio for any given chemical species. Alternatively, the
improved SNR can be exploited again to reduce acquisition time,
making spectroscopic imaging more clinically practical.
Spectroscopy and chemical shift
Chemically different protons in an organic molecule do not
experience the same magnetic field. The local chemical environment
can alter the resonance frequency of the nucleus, resulting in a
shift in the nuclear resonance frequency. The magnitude of the
shift for a given proton depends upon the strength of the applied
magnetic field (B
). For a given proton, the shift (in Hz) doubles from 1.5T to 3T.
Therefore, the increase in chemical shift at high fields improves
the spectral resolution of chemical shift imaging.
In imaging nuclei other than hydrogen, such as sodium and
phosphorous, the inherently low concentrations of these nuclei in
biological tissue necessitate long imaging times at lower field
strengths. At higher field strengths, spectroscopic imaging of
these nuclei becomes more clinically feasible.
The longitudinal or spin-lattice relaxation time (T1) increases
at higher field strength.
The prolonged T1 at high field has been successfully employed for
better imaging using the time-of-flight (TOF) technique.
The longer T1 of background tissue allows for a smaller flip angle,
resulting in reduced pulsation artifacts and reduced power
deposition. While the T1 relaxation of gadolinium (Gd) decreases
slightly with field increases, the contrast-to-background ratio
actually increases due to the relatively more dramatic increase in
T1 of biological tissues. Although the static magnetic field (B
) also affects the spin-spin relaxation time (T2), the T2s in
biological tissues are typically unchanged or only slightly
decreased with increases in field strength. Magnetic susceptibility
also increases with field strength, resulting in more intravoxel
dephasing and hence T2* signal losses. Susceptibility artifacts can
be problematic, especially at airsoft-tissue interface and near
surgical metallic devices. However, the increase in susceptibility
can also improve detection of hemorrhage and mineralization, and
boost the sensitivity of blood-oxygenation-leveldependent (BOLD)
contrast functional imaging.
Challenges at high field
) field homogeneity
Modern magnets are based on multiple solenoidal coils connected
with >=2 correction coils.
The design requires that the static magnetic field is homogeneous
over a diameter-sensitive volume (DSV).
A homogeneity requirement of ¾0.5 ppm over 40 cm DSV is typical for
modern whole-body MR systems. The magnet may be shielded to reduce
stray fields and siting costs. To maintain an equivalent static
field homogeneity for a particular diameter, a higher field magnet
requires a longer axis of the magnet and/or tighter and more coil
windings. The end result is increasing both the size and weight of
the magnet. For instance, a 4T magnet can weigh up to 20 tons with
cryogens and requires up to 150 tons of steel shielding to contain
the 5-gauss line to within a couple of meters of the magnet.
To restrict the total length of high-field magnets to the same
length as current clinical 1.5T scanners, while fulfilling the
above homogeneity requirements is a major production challenge,
requiring innovative magnet design.
) field homogeneity
Problems related to B
or radiofrequency (RF) homogeneity represent another challenge at
Because the Larmour frequency is directly proportional to B
, increasing in the main static field also increases the proton
precession frequency. At higher frequencies, the RF penetrance
through biological tissue decreases. In addition, the increase in
dielectric constants of biological tissue at high field interferes
with the wavelength of RF in the tissue.
The end effects are phase and amplitude modulations of the RF
inside the tissue, resulting in unpredictable off-resonance image
Numerous studies have shown that dissipation of RF energy in the
human body can result in tissue heating.
The International Electrotechnical Commission (IEC) requires that
the specific absorption rate (SAR) not exceed 8 watts (W) per kg of
tissue for any 5-minute period or 4 W/kg for a whole body averaged
over 15 minutes.
The SAR is a measure of energy deposited by an RF field in a given
mass of tissue. The average and peak SAR are roughly proportional
to the square of the main magnetic field and the RF flip angle
SAR * B
* D [Equation 1]
is the radius of the tissue (assuming a spherical object) and
is the duty cycle of the pulse sequence. Therefore, for a given
body part, the SAR roughly quadruples at 3T compared with 1.5T. To
keep SAR within federal regulations, some of the early clinical
applications at high field were limited to imaging with a small
field-of-view (FOV) or with pulse sequence compromises, such as
longer TR, longer RF pulse duration with lower amplitude, or lower
Gradient and safety
Stronger and faster switched or time-varying magnetic field
gradients are desired at higher field strength, motivated by
applications requiring high spatial and temporal resolution (eg,
functional, cardiac, and diffusion-weighted MRI). Especially at
higher fields, reactive coupling causes increased nonlinear
gradient performance and eddy currents, resulting in more image
distortion, ghosting, and conductor heating. Even more troublesome
at high field, mechanical coupling results in acoustic noise,
mechanical stress, motion artifacts, and motion-induced shimming
Safety concerns for fast and strong gradients include acoustic
noise and cardiac and peripheral nerve stimulation (PNS). The sound
pressure level (SPL) generated by high switching gradient speed can
reach extreme levels, in excess of 130 dBA at 3T.
Rapidly switched magnetic field gradients have been shown to induce
electric currents in skeletal and cardiac muscles and peripheral
Eddy-current stimulation is major obstacle to overcome for clinical
Hardware and software innovations
Magnet design and siting were initial major barriers to the
clinical acceptance of high-field MR scanners. For high-field MR to
be clinically acceptable, the size of the magnet must be kept
similar to current 1.5T systems. Prior to the late 1990s, actively
shielded magnets >2T were not available, due to lack of clinical
applications. However, as data supported the clinical benefits of
high-field MR, industry leaders became more willing to invest in
more compact magnet design. Actively shielded magnets at and above
3T field strengths were available by the turn of the new
millennium. Different proprietary methods were developed by
manufacturers to reduce magnet length while keeping the same
aperture. Literature regarding this topic is limited. Nevertheless,
the principal approach was to incorporate more active elements into
the magnet. One valuable modeling technique relied on simulated
annealing, which had been shown to be effective for compact
and, recently, for magnet
designs. As of mid-2003, most major MR manufacturers were selling
the latest generation 3T whole-body systems, for example, GE Signa
Excite (GE Medical Systems, Milwaukee, WI), Philips Intera (Philips
Medical Systems, Best, The Netherlands), and Siemens Magnetom Trio
(Siemens Medical Systems, Iselin, NJ), with a magnet length of 1.5
m and an aperture of 60 cm. These magnets are actively shielded and
weigh approximately 5 metric tons. The active shielding reduces the
fringe field, keeping the 5- gauss line to only 3 m from the
isocenter in the radial direction and about 5 m in the axial
direction. These new specifications permit siting to be compatible
with current clinical MR settings, allowing for easy upgrades and
Gradients and acoustics
Multiple strategies have been developed to reduce acoustic
noise. At the 2003 ISMRM meeting in Toronto, Canada, at least 8
abstracts addressed novel acoustic noise analysis and reduction.
Mechanical decoupling methods, using both passive and active
approaches, have been designed. Passive approaches include ear
protection using earplugs and earphones, and novel gradient bore
liners (such as cantilever
and vacuum-based bore liners
). Active approaches include antiphase noise reduction with
and acoustically shielded "quiet" gradient coils using novel
Radiofrequency coils and parallel imaging
Clinical high-field, whole-body MR imaging would have not been
practical without advances in specialized RF coil designs and
parallel imaging. The development of phased-array coils was a major
breakthrough in coil design.
The novel phased-array design provides increased SNR and reduced
image distortion. The introduction of multiple receiver coils
for simultaneous phased-array acquisition (eg, parallel imaging),
further accelerated the clinical acceptance of high-field
High field strength and parallel imaging have synergistic
effects. Parallel imaging has been applied to compensate for many
of the limitations at high field strength. For instance, reducing
the RF duty cycle with parallel imaging can minimize SAR (Equation
1). The shortened acquisition times and reduced FOV with parallel
imaging can also lessen the inhomogeneity problem at high field
strength. On the other hand, high field strength can also overcome
some of the constraints of parallel imaging. The improved speed of
parallel imaging is hampered by reduced SNR--the higher the
acceleration factor, the greater the SNR penalty--due to noise
amplification known as the g factor
and due to collecting fewer data points. The improved SNR at high
field strength permits greater acceleration factors and/or higher
resolution while maintaining image quality. Although not fully
understood, recent investigations have found that the synergistic
effect of combining high field strength and parallel imaging
results in improved SNR beyond that from increased spin
At the 2003 ISMRM meeting in Toronto, Canada, numerous abstracts
devoted to multichannel coils for parallel imaging applications at
high field strength were presented. These coils ranged from
to multichannel transmit/receive head and neurovascular coils.
Of particular interest is the development of a 64-channel planar
coil array for parallel imaging at 4.7T, allowing for image
acquisition in as little as a single echo.
Pulse sequence designs and processing strategies
Parallel imaging was not an overnight success. In reality, the
idea of parallel imaging was introduced more than a decade ago.
However, parallel imaging captivated little attention until recent
advances in pulse sequence designs and processing strategies such
as simultaneous acquisition of spatial harmonics (SMASH)
and sensitivity encoding (SENSE).
In both of these novel techniques, the k-space is sampled with a
reduced FOV smaller than the object being imaged. The reduced FOV
images from multiple receiver coils are combined with information
regarding the spatial sensitivity profiles of these coils to
reconstruct a nonaliased full FOV image.
Advances in pulse sequence design also alleviate problems
related to RF power deposition at high fields. The SAR concern at
high fields is more pronounced with spin-echobased sequences
utilizing multiple 180š pulses. Recent ingenious designs using
and multi-echo sequences with variable flip angle and optimized
signal behavior using smooth transitions between pseudo steady
allow acquiring high resolution T2-weighted images at high field
strength with reduced SAR while maintaining high SNR and tissue
Another novel technique involves replacing the standard excitation
and refocusing pulses with variable rate selective excitation
(VERSE) pulses. The VERSE technique modulates the standard RF pulse
envelope to reduce the peak B
and the slice select gradient to compensate for the change in
SAR reduction is achieved because SAR is proportional to the time
integral of B
. Successful implementation of the VERSE pulses on clinical 3T
scanners for SAR reduction and improved body coverage were reported
in several abstracts at the 2003 ISMRM meeting.
Advances in pulse sequence development also play a major role in
inhomogeneity at high fields. Developments of B
inhomogeneity-compensated RF pulses are in continuous evolution.
Adiabatic pulses were developed more than a decade ago to
compensate for variable RF power and off-resonance effects.
However, adiabatic pulses are long and have high power consumption.
New specialized short duration (1.3 msec) two-dimensional (2D)
and small-tip three-dimensional (3D) tailored RF
pulses with reduced B
inhomogeneity have been shown and implemented on clinical
high-field MR scanners.
Current state-of-the-art clinical high-field MR imagers are
equipped with special hardware capable of performing parallel
imaging with reduced SAR and inhomogeneity effects. The latest
generation MR imagers are preloaded with user-friendly software
with specialized pulse sequences and processing power. These
high-field, high-performance scanners are capable of producing
morphologic and functional images with exquisite details without
jeopardizing clinical safety. However, the drive to higher field
strengths can succeed only if the new technology can produce
improved patient care. Among the imaging subspecialties, brain
imaging has been an area of clear benefits. However, imaging of
other body parts (with FOV >40 cm) has also shown clinical
utility. In the following subsections, the clinical benefits of
high-field imaging will be illustrated.
Clinical improvements at high field strength have been shown
convincingly with detection of brain parenchymal abnormality in
multiple sclerosis (MS). In a study evaluating 15 patients with
clinically definite MS, fast spin-echo (FSE) images obtained at 4T
showed a mean of 45% more lesions compared with images obtained at
Images were obtained with similar SNR and acquisition time (6 min
35 sec at 1.5T versus 5 min 47 sec at 4T). Four radiologists, who
inspected the images independently, agreed that the higher spatial
resolution afforded at 4T enhanced the perception of small
perivascular lesions (Figure 1). In another study, 25 subjects with
MS were scanned at 1.5T and 3T by using FSE and T1-weighted spoiled
gradient-recalled acquisition in the steady state (SPGR) with and
without Gd contrast injections.
Relative to images obtained at 1.5T, 3T images showed a 21%
increase in the number of detected contrast-enhancing lesions.
Similarly, imaging at high field has also shown obvious benefits in
tumor evaluation. Image analysis from a recent study showed
2.5-fold higher CNR for the full standard dose (0.1 mmol/kg body
weight of Gd) at 3T compared with the full dose at 1.5T. Even more
striking, half-standard dose at 3T still yielded a 1.3-fold higher
Finally, the improved spatial resolution at high field also
provided improved detection of structural abnormality in epileptic
Improved SNR and CNR at high field in imaging the heart,
and the musculoskeletal system
have also been established. Clinical benefits in imaging these
organs at high field are being actively investigated.
Disease evaluation that could potentially profit from superior
angiographic techniques at high field include vasculitis, vascular
malformation, and atherosclerotic disease. The improved SNR and
increased T1 relaxation time at high field translate to superior
blood-to-background contrast in time-of-fight MRA.
By using a modulated magnetization transfer (MT) approach and an
altered phase encode order to keep the SAR below 3 W/kg over any
8-second time period,
3D-TOF MRA images were obtained at 3T with improved background
suppression (Figure 2). The improved SNR and/or reduced acquisition
time at high field will also benefit time-resolved
contrast-enhanced angiography, which relies on the rapid passage of
contrast through the arterial tree. Longer imaging time at 1.5T has
been a major limitation of this method, restricting the ability to
resolve the arterial and venous phases of contrast enhancement.
Similarly, longer acquisition time at 1.5T has also been a major
obstacle to the clinical utility of phase-contrast angiography
(PCA), which provides excellent visualization of vessels with high
background suppression. In conjunction with parallel imaging,
high-resolution isotropic whole-brain PCA can be achieved at 3T
within a clinically acceptable time (<10 minutes).
Another area of benefit is carotid bifurcation imaging using the
"dark blood" technique. Even with a 30% to 40% reduction in
acquisition time, turbo spin-echo images obtained at 3T clearly
resolved internal and external arterial walls that were difficult
to delineate from images obtained at 1.5T.
Besides neurovascular imaging, high resolution (0.6 * 0.6 * 3.0 mm)
coronary MRA with improved CNR and SNR has been reported by several
With continuing advancement of the moving table technique at 3T,
clinical high-field, whole-body MRA will probably be available in
the near future.
Perhaps the area in which high-field MR has the greatest impact
is functional MRI (fMRI) based on BOLD contrast. Because
susceptibility effects vary as the square of the field strength,
BOLD contrast imaging reaps additional advantage in addition to the
gain in SNR at high field. Even without correction for
physiological noise, which has been shown to be more detrimental at
higher field strengths,
stimulation of the primary and visual cortex results in a 36% and
44% increase in detected "activated" pixels at 3T compared with
those at 1.5T, respectively.
With the added benefit of physiological noise correction
and fast EPI acquisition, the spatial specificity of fMRI at 4T can
be improved by mapping the early hemodynamic response without the
need for multisubject averaging.
Even more striking, functional mapping exclusively from
microvasculature based upon Hahn spin-echo BOLD contrast increases
almost quadratically from 4T to 7T
(Figure 3). With regard to clinical application of fMRI, besides
being an excellent tool for presurgical mapping,
high-field fMRI has also been used to investigate psychiatric
and drug addictions.
Two widely used perfusion MR methods have proved successful:
injection of exogenous paramagnetic contrast agents for measuring
relative cerebral blood volume (rCBV) and relative cerebral blood
flow (rCBF); and arterial spin labeling (ASL) for measuring
cerebral blood flow (CBF). Paramagnetic contrast agents, such as
Gd-DTPA, do not cross an intact blood-brain barrier. As the
contrast agent crosses the capillary tree, it causes magnetic field
variation within a voxel, leading to spin dephasing and signal
loss. Gradient-echo (GE) and spin-echo (SE) echoplanar imaging
(EPI) sequences can be used. GE-EPI sequence is more sensitive to
the inhomogeneity produced by the contrast agent, but is less
specific to the presence of contrast agent in capillary vessels
when compared with the SE-EPI sequence. Arterial spin-labeling
(ASL) methods involve the inversion of magnetization in supplying
arteries and subsequent imaging. Inversion of magnetization,
usually by application of a spatially selective 180š RF pulse, can
be achieved with either pulsed (PASL) or continuous (CASL)
labeling. Images are acquired with and without flow weighting, and
the differences in signal intensity reflect perfusion changes.
Because the difference is small, multiple averages are needed for
the generation of perfusion maps based on established models.
Both exogenous and endogenous MR perfusion methods can benefit
from the improved SNR at higher field. Because the first pass of a
contrast bolus occurs rapidly, relatively few data points are
available for computation. The boost in SNR allows for a more
precise measurement, especially in disease processes, such as
stroke, in which there is already underlying hypoperfusion. In
addition, the faster acquisition speed allows for whole-brain
coverage that is not possible at 1.5T.
Moreover, lower doses of contrast agents can be used at high field
while maintaining the equivalent diagnostic quality. With regard to
ASL methods, the boost in SNR reduces acquisition time because
fewer averages are needed. Moreover, the longer T1 at higher fields
allows for a longer inversion time (TI) between tagging and imaging
to ensure the validity of the assumption that all the tagged blood
has perfused the slice before the acquisition. Perfusion images
obtained at 4T using PASL technique showed almost a two-fold
increase in SNR (in gray matter) and 2.4 times the CNR (between
gray and white matter) compared with those obtained at 1.5T.
Diffusion-weighted and diffusion tensor imaging
Diffusion-weighted imaging (DWI) exploits the variability of
random (Brownian) motion of water molecules in biological tissue.
The most commonly employed diffusion-encoding approach is pulsed
gradient spin-echo, using paired magnetic field gradient pulses to
measure water displacement --the more the displacement of water
molecules, the more signal loss in the spin-echo formation.
Diffusion of water in biological tissue, such as white matter, is
anisotropic, such that diffusion exhibits directional preference,
for example, along white matter tracts in the brain. By performing
diffusion encoding in multiple directions (>=6), white matter
fiber tracts can be reconstructed based upon sophisticated
While DWI has revolutionized the early detection of cerebral
ischemia, diffusion tensor imaging (DTI) is still an
investigational tool. Numerous studies at 1.5T and 3T have been
performed to assess the clinical usefulness of DWI and DTI in the
evaluation of white matter tracts in disease entities such as
traumatic brain injury,
However, DTI at 1.5T is limited by poor SNR, and multiple signal
averaging is required to have adequate SNR. The improved SNR at
high field translates to lower uncertainty associated with the
estimate of the principal eigenvector of the diffusion tensor, with
consequent improvement of fiber-tracking methods (Figure 4).
Diffusion-tensor imaging also benefits from parallel acquisition,
in which the reduced FOV minimizes the geometric distortion at high
field, allowing for ultra high-resolution DTI
(Figure 5) and 3D tractography of the brainstem.
Spectroscopic and chemical shift imaging
While the role of H
MRS in tumor imaging and metabolic diseases has been clearly
beneficial, even at 1.5T, high-field MR may improve evaluation of
other disease entities such as psychiatric and developmental
disorders. For instance, in an H
MRS study at 3T, increased NAA in the right frontal gray matter and
reduced NAA in left frontal gray matter relative to controls were
observed in children with autism.
Likewise, consistent and unobstructed quantification of glutamate
has been achieved at 3T but not at 1.5T.
Because glutamate plays a central role in neurotransmission, the
ability to obtain reliable measurements of glutamate may have
clinical benefits, especially in the field of psychiatry.
In imaging of other body parts, proton MRS at high field has
been found to be useful in the evaluation of chronic hepatitis.
Besides proton MRS, fast direct imaging of phosphocreatine in the
human myocardium is feasible at 4T
and may be of importance in the evaluation and management of
Imaging at high field strengths has proven to be superior,
providing exquisite morphologic and functional details that would
equate to enhanced patient care. However, the current environment
of cost-conscious medicine requires more than just better patient
outcome for the ultimate acceptance of high-field MR imaging. The
new technology must also prove to be cost effective, which can be
difficult to demonstrate. Several options to improve cost are
available, when considering the following scenarios. If an MR unit
is conducting >3000 examinations per year, and its average rate
of growth is equivalent to the national average of 15% per year,
then upgrading or adding a high-field magnet should be strongly
A clinical 3T MR scanner with strong and fast gradients and
parallel imaging capability costs approxinmately $600,000 to
$800,000 more than a 1.5T scanner with similar specifications.
Based on a report from
Decisions in Imaging Economics
, the typical fixed cost to operate a low-field magnet (¾1.5T) is
approximately $700,000/ year.
The total fixed cost for a high-field magnet should not be more
than $800,000/year because the main additional costs are likely
secondary to maintenance and service costs. The reduced acquisition
times will increase productivity while maintaining superior
quality. The fixed cost per 3000 examinations is approximately $233
($700,000/$3000). A 25% increase in throughput with a high-field
magnet leads to a reduced fixed cost per examination of $213
($800,000/$3750), or a total saving of $75,000 per year.
Unfortunately, even such an optimistic estimate still falls short
because a saving of $75,000 per year still requires roughly 10
years to compensate for the additional upfront cost of the
high-field magnet. Another approach to reduce cost would be to
perform contrast studies at half-standard dose, which can reduce
the cost per examination by approximately $50. Again,
hypothetically, if one-third of the MR studies require contrast,
then reducing the contrast dose by half can potentially reduce
operating costs by $50,000 per year (given a reduction of 1000
patients/year @ $50/patient). Thus, with these approaches to cost
reduction, the greater upfront cost of a high-field magnet can be
paid for within 5 to 6 years.
Another option, which is most ap-plicable to practices with
multiple MR scanners, is to consider organ-specific high-field MR
scanners. For example, a 3T head-only MR scanner costs nearly the
same as a whole-body 1.5T system. Academic centers and large
radiology practices that perform >3000 head MRIs per year can
triage these examinations to their dedicated head MR. Such an
approach will provide better patient care and will still be able to
reduce operating costs.
Recent advances in hardware and software engineering have
overcome some of the technical challenges of operating at high
field strength. Development of short-bore magnets with improved
homogeneity permits siting to be compatible with a current clinical
MR setting. Novel passive and active ap-proaches have been devised
to minimize the increased acoustic noise associated with stronger
and faster gradients. Innovations in phased-array coils and
multichannel receivers benefit high-field imaging by providing
increased SNR, reduced image distortion, and reduced RF power
deposition. Novel pulse sequence designs and postprocessing
strategies further propel parallel imaging and reduce SAR.
Whole-body high-field imaging can now be achieved within clinical
safety standards. Also reviewed in the paper, abundant data support
the clinical benefits of high-field imaging. With continuing
progress, imaging at 3T or 4T field strength will soon be the
standard of care in radiology practices.
The author would like to thank Drs. Pratik Mukherjee, Max
Wintermark, and Essa Yacoub for help with the preparation of this
manuscript. Helpful discussions with Drs. Xiaoping Hu and Gregory
Sorenson at the 2003 ISMRM Meeting in Toronto, Canada are also