Clinical 3T MRI: mastering the challenges.
Over the last several years, systems that operate at higher fields have become more prevalent, particularly at academic and research centers. An informal survey of the market in mid-2005 revealed that approximately 350 to 400 3T whole-body-capable MR systems are currently operational, with roughly 25% used primarily for research. On the other hand, market projections estimate that approximately 400 new 3T systems will be installed during the next 12 months, with approximately 80% of these planned for routine clinical whole-body applications. Fueling the shift in interest from 1.5T to 3T and from academic to clinical practice is the validation that very high-field MRI (3T) is feasible and, indeed, is now or is potentially superior to 1.5T for clinical indications throughout the body.
At our clinical practice site, we have been using 3T for whole-body imaging in the community setting for approximately 4 years; for the last 25 months, we have been using a third-generation, broadband 8- to 16-channel short-bore system. The easily recognized boost in image quality and consistency achieved through higher resolution and higher net signal-to-noise ratio (SNR) scanning has driven referrals from all subspecialty areas. Physicians who were engaged in the practice of neurology and neurosurgery were the first to become aware of the benefits of 3T MRI and have most readily directed cases to our higher-field system. However, specialists in orthopedics, vascular surgery, and oncology have rapidly followed suit and are seeking the higher overall quality available with 3T (Figure 1). (1,2)
3T MRI has inherent advantages over more commonly utilized 1.5T systems, as well as some challenges that must be met for higher-field MR to be clinically practical. Concerns about surface coil availability, radiofrequency (RF) deposition limits, and higher ambient noise must be addressed. Challenges with respect to system homogeneity, increased sensitivity to magnetic susceptibility, chemical shift effects, and reduced tissue contrast need to be overcome. Siting concerns differ from that of 1.5T systems, as the 3T magnets are much heavier, although the footprint and fringe field on state-of-the-art systems differ only slightly from that of 1.5T. The final obstacle to increased implementation of 3T into the clinical setting worldwide is clear demonstration of the incremental benefits of 3T over 1.5T with respect to image quality and efficiency (Figure 2).
3T MRI challenges
Specific absorption rate
Specific absorption rate (SAR) is a measure of energy deposited by a radiofrequency (RF) field in a given mass of tissue. The SAR is limited by the International Electrotechnical Commission (IEC) to not exceed 8 watts per kg (W/kg) of tissue for any 5 minute period or 4 W/kg for a whole body averaged over 15 minutes. (3) The dissipation of RF energy in the body can result in tissue heating. The doubling of the field from 1.5T to 3T leads to a quadrupling of SAR (Figure 3). Considerations related to SAR, therefore, inherently limit scanner performance by limiting the rate of RF energy deposition and cumulative deposition. This can manifest as a reduction in slices per repetition time (TR), longer scan times, and "cooling"' delays between acquisitions.
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Manipulations that are traditionally used to limit SAR (including reducing acquisition flip angle--eg, from 180[degrees] with fast spin-echo [FSE] and ~40[degrees] with gradient recalled-echo [GRE] sequences) could potentially affect image contrast. The reduction from a flip angle of 180[degrees] (to typically approximately 120[degrees]) has been well tolerated thus far without any noticeable deleterious effect on image contrast. Reducing the flip angle from what might be ideal for a contrast-enhanced 3-dimensional (3D) GRE MR angiography (MRA) study (approximately 40[degrees]) to what will deliver the shortest TR and echo time (TE) at the scanner (roughly 25[degrees]) is also well tolerated.
Duty cycle (D) can be defined for this purpose as the individual tasks (RF pulses) that play out during a given MR experiment (TR period). The RF energy deposited during a given scan is proportional to the intensity of the duty cycle. Longer TE acquisitions that are common with high-performance-gradient systems and fat-suppression techniques that are commonly used, particularly with musculoskeletal and body imaging, exacerbate the duty cycle load. Reducing D by using a slightly longer TR than the minimum necessary (in a sense, building in cooling time) is a very effective technique that comes at the expense of only slightly longer scan times (Figure 4).
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Parallel imaging (PI) is an another powerful method of reducing RF exposure as well as scan times by reducing the number of phase-encoding steps that are performed in a given scan. The typical trade-off in SNR (a parallel imaging factor of 2 reduces SNR by 40%) is balanced by the higher signal of 3T and facilitated by the improved higher SNR high-density 8- to 16-channel surface coils now available (Figure 5). Parallel imaging is practical only for applications in which there is ample SNR at a single scan repetition (1 number of excitations [NEX]) and is most useful for applications such as contrast-enhanced MRA and breath-hold body and cardiac imaging. As surface coil and scanning technology advances with further optimization for parallel imaging, this technique may become increasingly important at 3T. (4)
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Another technique for managing RF deposition load is to interleave SAR-intensive sequences with low-RF-deposition scans, eg, follow a long-echo train [ET], fat-suppressed FSE scan with a 2-dimensional (2D) gradient-echo acquisition before starting the next FSE scan (Figure 6). This technique has limited utility for most applications but has some value, particularly for body imaging on early-generation 3T systems.
Innovative methods of reducing SAR without compromising imaging are now or will soon be available. New short-bore magnet designs, widely present in the clinical setting, are inherently more SAR-efficient than are earlier-generation long-bore systems, as less of the body is exposed to the shorter transmitting body coil. Innovations in RF chain technology have also improved the efficiency of energy deposition with a resultant formidable net reduction in SAR.
Innovative pulse-sequence manipulations, such as applying magnetization transfer prepulses only at the center one third of k-space, can maintain improved tissue contrast while depositing considerably less RF energy. Advances in pulse-sequence design, such as reshaping RF and gradient waveforms (variable rate selective excitation [VERSE]), reduce peak RF power up to 40% to 60% compared with conventional techniques (Figure 7). This modification in pulse-sequence design, along with others from scanner manufacturers (eg, smooth transitions between pseudo steady states [TRAPS], hyperechoes) should also lead to RF limitations and slice acquisition efficiency equal to or slightly greater than those currently in place at 1.5T.
Removing the body coil from the transmission process by the use of transmit-receive (T/R) surface coils has a significant impact on RF deposition. The development and availability of an increasing number of local T/R surface coils will facilitate efficiency and encourage even higher-resolution scanning by reducing the obstacle that SAR limitation presents.
Sound pressure levels (SPLs) increase with field strength. The noise levels at 3T approach twice that of 1.5T and can be in excess of 130 dBA (5) (the IEC and U.S. Food and Drug Administration [FDA] limit permissible sound levels to 99 dBA). Higher-gradient performance comes at the cost of higher SPL as well. The inherent noise-dampening effect of magnet length and weight is also influential on ambient SPL, thus the shorter-bore systems sold today are inherently louder.
Methods of reducing SPL include passive approaches, such as the routine use of earplugs, as well as active noise cancellation via headphones. Reducing gradient performance for certain demanding applications (echoplanar imaging [EPI], balanced steady-state free-precession imaging) is another approach, but this, by nature, limits clinical efficacy. Some late-generation 3T systems are equipped with advances, such as acoustically shielded vacuum-based bore liners, that keep noise levels below limits without restricting gradient performance.
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Inhomogeneous RF distribution is caused by a variety of conductive and dielectric effects in tissue. While present at 1.5T, these effects are exacerbated at higher field strength and typically manifest as image nonuniformity. The use of high SNR surface coils may increase the conspicuity of these effects for body applications, which manifest as areas of shading or signal drop-off. (6)
The use of pads filled with a medium of high electric permittivity can significantly homogenize the B1-distribution in tissue with high conductivity (Figure 8). The same effects manifest as brightness in the center of the brain on cranial studies. It is a fortuitous result of the use of small-element, high-density head coils (which are brighter in the near field) that the resultant images of the brain are, in the end, highly uniform (Figure 9).
Susceptibility effects scale with field strength. This effect is exploited at 3T in improving the sensitivity of FSE imaging to the presence of hemorrhage and mineralization. (7) While susceptibility effects might be expected to be prohibitive and limiting for patients with implanted hardware, appropriately designed protocols leverage the combination of efficient coil designs and high bandwidth scanning, which keep artifact manageable and similar in severity to that of 1.5T (Figure 10). Protocol manipulations that are used to manage susceptibility artifacts include utilization of shorter TEs, reductions in voxel size, and utilization of higher receiver bandwidth than would be employed at 1.5T. The higher SNR that is afforded by 3T also permits compensation with parallel imaging in EPI and longer echo-trains with FSE acquisitions.
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Susceptibility contrast-based, first-pass, gadolinium-contrast-dependent perfusion imaging at 3T is plagued by signal loss in regions prone to susceptibility artifact, such as the frontal sinuses and skull base. Protocols that include minimum TE GRE or moderate TE SE acquisitions go a long way toward ameliorating these effects (Figure 11). Parallel imaging techniques reduce susceptibility artifact and distortion on single-shot EPI studies. (8)
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Chemical shift effects
Chemical shift effects also scale with field strength. This provides a boost in metabolite peak separation/resolution for spectroscopy and makes RF fat suppression more robust at 3T. An increase in chemical shift artifact at 3T could be a significant limiting factor in routine anatomic imaging. Fortunately, with appropriate scanning protocols, the SNR of 3T and late-generation multichannel coils are leveraged via the routine use of higher bandwidths (32 to 125 KHz) for spin echo (SE) and FSE imaging, keeping chemical shift effects in a range similar to that of 1.5 T (Figure 12).
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Tissue contrast issues
T1 relaxation times are prolonged at 3T with respect to 1.5T. This has been exploited to produce superior time-off-light (TOF) MRA studies. The same effect leads to reduced, and thus somewhat unsatisfactory, contrast resolution on traditional (short TR short TE) SE acquisitions. These considerations do not plague other methods for obtaining T1 contrast that are widely employed at 1.5T, such as RF spoiled gradient-recalled, often magnetization-prepared techniques, such as inversion recovery (IR) or magnetization transfer (MT) 3D spoiled gradient-echo (SPGR) (Figure 13). Inversion recovery techniques, that produce superior T1 contrast at 1.5T--such as phase-sensitive IR for the brain (Figure 14) and T1 fluid-attenuated inversion recovery (FLAIR) for the brain, spine, and musculoskeletal system--are equally well suited to higher-field imaging and can yield spectacular results (Figure 15).
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In the end, with parallel imaging techniques, T1 studies are faster and higher in resolution than those obtained at 1.5T. A routine shift to high bandwidth, moderate TE inversion recovery FSE (T1 FLAIR) from SE, has the additional benefit of reducing susceptibility artifact, a benefit in patients who have had surgery or who have metal implants, and chemical shift sensitivity (Figure 16).
While the relaxivity properties of gadolinium are not significantly different at 1.5 than they are at 3T, the longer T1 of tissues and higher net SNR at 3T contribute to an increase in conspicuity of enhancement (greater contrast-to-background ratio). Therefore, many sites utilize a lower dose of contrast (0.05 mmol/kg) for routine brain imaging purposes (Figure 17). In biological tissues, T2 values are unchanged or only slightly decreased with increases in field strength. Since T2* effects scale with field strength, 3T studies are thus more sensitive to deposition of blood products and tissue mineralization.
The greater signal intensity afforded at 3T is particularly enticing for diffusion-weighted imaging (DWI) needs. Signal-to-noise ratio can be marginal for routine clinical imaging purposes at 1.5T, and the quest for higher B values (>1000 s/[mm.sup.2]), thinner slices (<3 mm), and white matter anisotropy mapping (tensor imaging) further stresses the SNR equation (Figure 18). (9) DWI studies at high field are typically acquired using EPI techniques. These single-shot studies are inherently prone to susceptibility artifacts that can limit the evaluation of structures in close proximity to the bony skull base and air-filled paranasal sinuses. The artifact is exacerbated by the presence of metal (eg, from dentures, dental braces, or foreign bodies). As susceptibility scales with field strength, these artifacts are proportionally worse at higher field. Parallel imaging techniques are routinely applied on modern 3T systems equipped with optimized surface coils and broadband reconstruction hardware, effectively balancing these considerations by decreasing the echo-spacing (ES) and TE of the scan. This reduces susceptibility artifact and ameliorates signal loss due to T2 decay on these long ET acquisitions (Figure 19). As a result, multishot FSE DWI techniques (eg, periodically rotated overlapping parallel lines with enhanced reconstruction [PROPELLER]), which are inherently less susceptibility sensitive, have increased utility at 3T (Figure 20). (10)
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Perhaps the greatest impact of 3T in neuroimaging has been in the enhanced quality and consistency of blood-oxygenation-level-dependent contrast BOLD) functional MRI (fMRI). The greater susceptibility contrast sensitivity and higher SNR inherent to 3T scanning can produce up to a 40% increase in detected activation with BOLD imaging over 1.5T (Figure 21). (11) Improved contrast resolution enhances the success rate of these procedures for routine presurgical mapping of the eloquent cortex (eg, sensorimotor, language) and, coupled with scanner-integrated paradigm delivery, has facilitated community practice utilization to evaluate disorders such as dementia and other psychiatric disorders.
Time-of-flight neurological MRA
The longer T1 of background tissues can be exploited for superior inflow MR angiography (TOF MRA). Scanning techniques employ lower flip angles, managing RF deposition as well as reducing pulsation artifacts. The higher SNR provided by 3T with 8-channel surface coils encourages routine utilization of high imaging matrices (512 to 1024), producing studies that can rival the resolution of catheter angiography (12,13) (digital subtraction angiography; Figure 22). Optimized coils coupled with PI techniques maintain scan times similar to or shorter than those at 1.5T.
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Chemical shift doubles when moving from 1.5T to 3T, resulting in improved spectral resolution. This may allow routine evaluation of metabolites that may be obscured at 1.5T. (14) Along with the higher SNR of 3T, this may increase the efficacy of proton and multinuclear spectroscopy of many disorders (Figure 23).
Radiofrequency deposition limits the number of slices available per given time, encouraging multiple breath-hold acquisitions, particularly with 2D GRE. Optimized 3D GRE acquisitions benefit from the high SNR of 3T and high-density coils to produce thin-slice, functionally isotropic volumetric whole abdominal scans in a single breath-hold (Figure 24). Motion-resistant techniques with single-shot FSE and respiratory-triggered multishot FSE are also commonly used. Eight-channel phased-array surface coil designs that are optimized for PI ameliorate many SAR-based restrictions.
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Studies of the abdomen and pelvis are routinely accomplished with thinner slices and higher imaging matrices, comparable to those utilized with computed tomography, facilitating comparison and lesion characterization (Figures 17 and 18). (15) The higher SNR afforded by 3T may also facilitate applications such as spectroscopy and might obviate the need for endocavitary surface coils for advanced applications, such as prostate imaging (Figure 25).
Body vascular imaging
The higher SNR of 3T coupled with PI-compatible surface coils produce high-quality, higher spatial resolution, contrast-enhanced vascular studies with greater consistency than at 1.5T. (16) Lowering the flip angle reduces SAR and, as a result, keeps TE and TR in a range similar to that of 1.5T. The longer T1 values of background tissue serves to augment visualization of intravascular contrast, potentially allowing a reduction in the contrast dose administered (Figure 26). While full-body vascular coils are not yet available, the increasing importance of multistation time-resolved MRA techniques at the expense of so-called bolus-chasing reduces their significance (Figure 27). Dedicated high-density coil designs, such as a "boot" coil, should allow image quality that is unattainable at 1.5T.
Eight-channel phased-array coils are widely available for spine imaging. Practical considerations yield studies that are more consistent in quality at higher resolution and are roughly one third faster than those of 1.5T (17) (Figure 28). While susceptibility is a theoretical concern, long-ET, high-bandwidth acquisitions yield excellent image quality, even for patients with implanted metal hardware. Further testing is required before the safety of spine-implanted electronic devices can be fully established.
Joint imaging is responsible for upwards of 20% of the study volume of the typical clinical scanner and the quality of joint imaging is a major factor in determining the financial feasibility of higher-field MR. Until recently, coil availability has been limited and SAR concerns are prominent, as high duty cycle applications, such as fat suppression and long ET FSE, are common. The homogeneity of the latest-generation short-bore devices is critical, as joints are rarely scanned near isocenter, and fat suppression is so important to maintain contrast resolution (Figure 29).
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Fortunately, high-quality, high-SNR, phased-array surface coils are becoming available and generally provide studies that are recognizably superior to those from 1.5T systems in often significantly less time (Figure 30). Offering higher spatial resolution, 3T MR may yield additional useful information in the study of smaller body parts and cartilage than examinations obtained at 1.5T. Studies obtained with older and less capable coils (eg, quadrature) benefit from the SNR of 3T to be competitive with studies obtained with more advanced surface coils at 1.5T.
Receive-only coils require SAR-intensive body-coil transmission, limiting performance. The increasing availability of T/R-capable surface coils will significantly improve efficiency and encourage higher-resolution scanning techniques.
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The higher SNR of 3T also facilitates the utilization of smaller fields of view (FOV), thinner slices, and larger imaging matrices. The greater susceptibility sensitivity of 3T should make tissue mineralization easier to appreciate (Figure 31). Instrumented joints can be imaged with manageable artifact with high bandwidth, long ET FSE, and T1-weighted IR FSE techniques.
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Clinical practice impact
Fundamentally, 3T offers twice the signal of 1.5T. The overall power of 3T easily allows the creation of studies that are recognizably better, with higher resolution and greater patient-to-patient consistency, in the same or less time than those practical at 1.5T.
For neurologic applications, 3T images are higher resolution than 1.5T images, while still maintaining higher SNR. Scan times are similar to or shorter than those at 1.5T, depending on the type of scan and goals of the imager. Higher SNR and resolution contribute to better lesion definition and delineation (Figure 32).
The demands of small-part orthopedic imaging can exceed the capability of 1.5T to deliver in clinically feasible scan times. The combination of the inherent signal boost of 3T and dedicated surface coils produces a vast improvement in the delineation of small structures, such as the triangular fibro-cartilage complex and the intrinsic ligaments of the wrist (Figure 29). The greater image quality that 3T generates is met with great enthusiasm from orthopedic referrers.
The ability of 3T to deliver thinner slices at higher in-plane resolution in body imaging applications has also led to increased clinician preference for 3T. Near isotropic resolution acquisitions obtained at transient physiologic phases (eg, early-to-late arterial phase) allow diagnostic-quality multiplanar reformatting. Hepatic and delayed-phase images at the same slice thickness and nearly the same in-plane spatial resolution of multichannel CT has significantly facilitated abdominal and pelvic lesion detection and characterization. Our more accurate and definitive interpretations have encouraged a shift toward 3T as well as toward MR from other methods of lesion assessment (Figure 33).
Traditional MRA techniques at 1.5T suffer when compared with gold-standard conventional angiography both from lower spatial resolution and a lack of physiologic information. The combination of an 8-channel surface coil at 3T and novel time-resolved acquisition techniques have helped us overcome both of these hurdles. MR now routinely delivers what only a catheter study could before: high spatial resolution, multiphase vascular assessment during the full cycle of vascular transit with complete freedom from interference from venous overlay. The surgeon's response to these superior physiologic studies has been impressive and has been a big factor in further reducing the number of conventional angiographic studies that we are asked to perform.
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3T MRI is clearly ready to meet the needs of clinical practice today. Specific absorption rate limitations are minimized because of technical advances, and surface coils are available for all core applications. With appropriate adjustments to scanning protocols, one can master the challenges of scanning at 3T. Studies of the brain, spine, chest, abdomen, pelvis, vasculature, and extremities can be consistently higher in quality than those obtained at 1.5T.
The superior studies provided by 3T have great appeal to clinicians who are sophisticated about MRI technology in areas such as neurology, orthopedics, vascular surgery, and oncology and encourage a shift in referrals toward practices that invest in the scanners. The greater sensitivity of 3T to magnetic susceptibility offers unique benefits in functional neuroimaging, and available software and hardware packages enhance clinical setting feasibility, adding a source of new referrals. The greater overall signal of 3T can be manipulated to make scanning more comfortable and with less motion artifact, as scan times can be half as long. Spectacular anatomical delineation provided by high-definition scanning at true 1024 resolution can improve preoperative assessment and may improve sensitivity to smaller lesions.
With 3T MRI, practices have an advantage that is increasingly sought by high-field purchasers in a competitive market. Only cost considerations stand in the way of 3T systems eventually dominating the high-field market.
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* Signa HD and HDx 3.0T short-bore MR scanner (GE Healthcare, Waukesha, WI)
* ProHance and MultiHance (Bracco Diagnostics, Inc., Princeton, NJ)
Dr. Tanenbaum is the Section Chief of MRI, CT, and Neuroradiology, New Jersey Neuroscience Institute and Edison Imaging--JFK Medical Center, Edison, NJ; and Assistant Professor, Department of Neuroscience, Seton Hall University School of Graduate Medical Education, South Orange, NJ. He is also a member of the editorial board of this journal.
Although the figures are original, portions of the text of this article are substantially similar to one of the author's previous publications: Tanenbaum LN. Clinical 3T MR imaging: Mastering the challenges. Magn Reson Imaging Clin N Am. 2006;14(1):1-15. The text has been edited and adapted from this previous publication with permission from Elsevier, Inc.
Lawrence N. Tanenbaum, MD, FACR
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|Author:||Tanenbaum, Lawrence N.|
|Date:||Nov 1, 2006|
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