MR scanning began to make an impact in the clinical practice setting in the mid-1970s. The most common early systems operated at a field strength 0.6T, amid credible doubt that more powerful magnets would be feasible, particularly for imaging needs beyond the brain. Eventually, technological advances made high-field MR imaging practical, and systems operating at 1.5T have become the current clinical benchmark. Lower field strength systems sold today are open designs directed to large or claustrophobic patients. While a market once existed for closed systems operating at 1T, the decreasing cost differential with 1.5T has all but eliminated these systems from new purchase considerations.
Systems operating at higher fields have become more prevalent, particularly at research centers. An informal survey of the market as of mid-2004 counts about 125 operational 3T MR systems capable of whole-body imaging, with about 75% used primarily for research. In contrast, market projections suggest that of the 150 to 200 new 3T systems that will be installed over the next 12 months, about 80% are planned for routine clinical setting whole-body applications.
What was once considered very high field is now considered feasible and indeed potentially superior to 1.5T for clinical indications throughout the body, fueling this shift in interest from 1.5T to 3T and from research to clinical practice.
At Edison Imaging, we have been using 3T for whole-body imaging in the clinical community setting for about three years, moving to a broadband eight-channel short-bore system in July 2003. The boost in image quality and consistency achieved through higher resolution scanning with a higher signal-to-noise ratio drives our referrals in all subspecialty areas. Although physicians engaged in neurology and neurosurgery are most aware of the benefits of 3T MR and have most readily directed cases to our higher field system, specialists in orthopedics, vascular surgery, and oncology increasingly demand the higher image quality of 3T.
Some challenges had to be overcome for higher field MR to be clinically practical. We had to alleviate concerns over surface coil availability, radio-frequency deposition limits, higher ambient noise, system homogeneity, increased magnetic susceptibility, chemical shift effects, and reduced tissue contrast. The demonstration of the incremental benefits of 3T over 1.5T with respect to image quality and efficiency is driving increased penetration of 3T into the clinical setting worldwide.
SPECIFIC ABSORPTION RATE
Specific absorption rate is a measure of energy deposited by an RF field in a given mass of tissue. The International Electrotechnical Commission (IEC) does not allow SAR to exceed 8 watts per kg (W/kg) of tissue for any five-minute period or 4 W/kg for a whole body averaged over 15 minutes.1
Dissipation of RF energy in the body can result in tissue heating, and the doubling of field from 1.5 to 3T leads to a quadrupling of SAR (Table 1). SAR considerations therefore effectively limit scanner performance.
Manipulations traditionally used to limit SAR include reducing acquisition flip angle (e.g., from 180 degrees on fast spin-echo and about 40 degrees on gradient-recall echo), which could potentially affect image contrast. The longer echo train (ET) acquisitions common with high-performance gradient systems and the fat-suppression techniques often employed with musculoskeletal imaging exacerbate duty cycle load. Reducing duty cycle by using a longer TR than the minimum necessary-in a sense, building in cooling time-is an effective technique, but it comes at the expense of somewhat longer scan times (Table 2).
Parallel imaging is a powerful method of reducing RF exposure as well as scan times by reducing the number of phase-encoding steps that are performed. The typical trade-off in SNR (a parallel imaging factor of 2 reduces SNR by 40%) is balanced by improved surface coil design and the higher signal of 3T. Another technique to manage SAR concerns is interleaving SAR-intensive sequences with low-RF deposition scans. Following a fat-suppressed FSE scan with a gradient-echo acquisition before starting the next FSE scan is an example of this approach.
Innovative methods of reducing SAR without compromising image quality are already available, and new ones will emerge in the future. New 3T system designs now seen in the clinical setting are inherently more SAR-efficient than earlier systems. Clever pulse sequence manipulations, such as applying magnetization transfer (MT) prepulses only at the center third of k-space, can maintain improved tissue contrast while depositing considerably less RF energy. Reshaping RF and gradient waveforms and other advances in pulse sequence design can reduce peak RF power up to 40% compared with conventional techniques. The development and availability of more local transmit/receive surface coils will also reduce SAR deposition and further enhance efficiency.
Sound pressure levels (SPL) increase with field, and the noise levels at 3T approach twice that of 1.5T and can exceed 130 dBA, above the 99 dBA limit set by the IEC and FDA.2 Higher gradient performance comes at the cost of higher SPL as well. Because magnet weight also influences the gradient noise generated, 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. Reduced gradient performance for certain applications is another approach, but this by nature limits clinical efficacy. Some currently available 3T systems are equipped with acoustically shielded vacuum-based bore liners that keep noise levels below limits while maintaining full gradient performance.
TISSUE CONTRAST ISSUES
T1 relaxation times are prolonged at 3T, leading to reduced contrast resolution on traditional (short TR, short TE) spin-echo acquisitions. These considerations do not plague other methods for obtaining T1 contrast: RF spoiled gradient recalled (SPGR), magnetization prepared techniques such as inversion recovery (IR), or MT 3D-SPGR. IR techniques that produce superior T1 contrast at 1.5T-such as phase-sensitive IR for the brain (Figure 1) and T1 FLAIR for the brain, spine, and musculoskeletal system-are equally well suited to higher field imaging and can yield spectacular results.
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 ET FSE (T1 FLAIR) from SE has the additional benefit of reducing chemical shift effect sensitivity as well as susceptibility artifact, a benefit in patients who have had surgery or who have metal implants.
While the relaxivity of gadolinium at 3T is not significantly different from that at 1.5, the longer T1 of tissues at 3T contributes to an increase in conspicuity of enhancement (greater contrast-to-background ratio). Therefore, many sites use a lower dose of contrast (0.05 mmol/kg) for routine brain imaging purposes. T2 values in biologic tissues are unchanged or only slightly decreased with increases in field strength. T2* effects scale with field strength, and 3T studies are thus more sensitive to deposition of blood products and tissue mineralization. Conversely, susceptibility artifacts are proportionally more problematic at 3T.
The higher SNR afforded by 3T, augmented by the power of the latest generation phased-array coils, allows a variety of techniques to compensate for T2* effects, including the use of parallel imaging and higher bandwidth, longer ET FSE acquisitions.
The greater signal intensity afforded at 3T is particularly enticing for diffusion-weighted imaging needs. SNR can be marginal for routine clinical imaging purposes at 1.5T, and the quest for higher diffusion sensitivity, or b-values ( > 1000 sec/mm2), thinner slices (
DWI studies at high field are typically acquired using echo-planar imaging techniques. These single-shot studies are inherently prone to susceptibility artifact, which can limit evaluation of structures in close proximity to the bony skull base and air-filled paranasal sinuses. Since susceptibility effects scale 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 echo time (TE) of the scan. This reduces susceptibility artifact and ameliorates signal loss due to T2 decay on these long ET acquisitions. While slower to acquire, multishot FSE DWI techniques such as PROPELLER are essentially free of artifact and should become popular at 3T.
Perhaps the greatest neuroimaging impact of 3T is in enhancing the capability of functional MRI. The greater susceptibility contrast sensitivity and higher SNR inherent to 3T scanning can produce up to a 40% increase in detected activation with blood oxygen level-dependent imaging over 1.5T.3 Improved contrast resolution enhances the success rate of these procedures for routine presurgical mapping of eloquent cortex (e.g., sensorimotor, language), which may lead to use in community practice to evaluate disorders such as dementia and other psychiatric conditions (Figure 2).
Chemical shift doubles when moving from 1.5T to 3T, resulting in improved spectral resolution and allowing evaluation of metabolites that may be obscured at 1.5T. Enhanced shimming techniques and spatially selective saturation pulses mitigate susceptibility effects inherent to higher field, even in demanding areas such as the base of the brain. This feature, along with the higher SNR of 3T, may increase the efficacy of proton and multinuclear spectroscopy of many disorders.
The longer T1 of background tissues can be exploited for superior inflow MR angiography (time-of-flight MRA). Scanning techniques employ lower flip angles, reducing SAR deposition as well as pulsation artifacts. The higher SNR provided by 3T with eight-channel surface coils encourages routine utilization of high imaging matrices (512 to 1024 pixels), producing studies that can rival the resolution of catheter digital subtraction angiography (Figure 3). Optimized coils coupled with parallel imaging techniques maintain scan times similar to or shorter than those at 1.5T.
The higher SNR of 3T, coupled with parallel imaging-compatible surface coils, produces high-quality, perhaps higher resolution, contrast-enhanced vascular studies with greater consistency than at 1.5T. Lowering the flip angle reduces SAR and, thus, acquisition time. The longer T1 values of background tissues serve to augment visualization of intravascular contrast, potentially allowing the user to cut the contrast dose. 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.
SAR considerations reduce the slices available per given time, encouraging multiple breath-hold acquisitions in some circumstances. Motion-resistant techniques with single-shot FSE and respiratory-triggered multishot FSE are also commonly used. Eight-channel phased-array surface coil designs optimized for parallel imaging ameliorate many SAR-based limitations.
Studies of the abdomen and pelvis are routinely accomplished with thinner slices and higher imaging matrices, comparable to those used with CT, facilitating comparison and lesion characterization. Large fields-of-view, essential for imaging in the coronal plane, are possible with new scanner designs that preserve z-axis homogeneity. The higher SNR afforded by 3T may also facilitate applications such as spectroscopy and could obviate the need for endocavitary coils for advanced applications such as prostate imaging (Figure 4).
Eight-channel phased-array coils are widely available for spinal imaging. Practical considerations yield studies that are generally higher in resolution and somewhat faster than at 1.5T. While susceptibility is a theoretical concern, long ET, high-bandwidth acquisitions yield excellent image quality even for patients with implanted metal hardware.
Joint imaging is responsible for more than 20% of the study volume of the typical clinical scanner, and the quality of this imaging is a major factor in determining the financial feasibility of higher field MR. Coil availability has been limited until recently, 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 because joints are rarely scanned near isocenter and fat suppression is crucial for contrast resolution.
Fortunately, high-quality, high-SNR, often phased-array surface coils are becoming available, providing studies that are recognizably superior to those from 1.5T systems in somewhat less time. The higher SNR of 3T allows utilization of higher spatial resolution protocols with smaller FOV, thinner slices, and larger imaging matrices. As a result, 3T MR studies typically provide additional information in the study of smaller parts and cartilage than do exams obtained at 1.5T. The greater susceptibility sensitivity of 3T should make tissue mineralization easier to appreciate. Instrumented joints can be imaged with manageable artifact with high-bandwidth, long ET FSE and T1-weighted IR FSE techniques.
Skeletal studies obtained with older and less sophisticated coils benefit from the SNR of 3T to the point of being competitive with studies obtained with more advanced surface coils at 1.5T. Receive-only coils require SAR-intensive body coil transmission, limiting performance. The future availability of transmit/receive-capable surface coils should significantly augment efficiency and further extend quality.
CLINICAL PRACTICE IMPACT
Fundamentally, 3T offers twice the signal of 1.5T. The overall power of 3T easily allows creation of studies that are recognizably better, with higher resolution and greater patient-to-patient consistency, in the same time or less than those practical at 1.5T. Diagnostic confidence is typically higher when images are better.
For neurologic studies, our 3T images are higher resolution than at 1.5T while maintaining higher SNR. Scan times are similar to or shorter than those at 1.5T depending on the type of scan. SNR and resolution contribute to better lesion definition and delineation.
The demands of small-part orthopedic work 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 delineation of small structures such as the triangular fibrocartilage complex and the intrinsic ligaments of the wrist (Figure 5). The greater image quality we deliver at 3T has generated a large volume of skeletal cases from satisfied orthopedic referrers.
The ability of 3T to deliver thinner slices at higher in-plane resolution has also led to increased referrals at our institution. Near-isotropic resolution acquisitions obtained at transient physiologic phases (e.g, early-late arterial) 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 have significantly facilitated abdominal and pelvic lesion detection and characterization. Our more accurate and definitive interpretations have encouraged a shift to our facility as well as a move to MR from other methods of lesion assessment.
Traditional MRA techniques at 1.5T suffer when compared with gold standard conventional angiography from both lower spatial resolution and a lack of physiologic information. The combination of an eight-channel surface coil at 3T and novel time-resolved acquisition techniques has helped us overcome both of these hurdles. We can now routinely deliver vascular surgeons infrapopliteal MRA that provides what only a catheter study could do before: high spatial resolution, multiphase vascular assessment during the full cycle of vascular filling, and complete freedom from venous overlay interference. The response of surgeons to these superior physiologic studies has been impressive. It has been a big factor in further reducing the number of conventional angiographic studies that we are asked to perform (Figure 6).
Three-T provides superior studies that have great appeal to clinicians sophisticated about MR technology in neurology, orthopedics, vascular surgery, and oncology, encouraging a shift in referrals toward our practice. The greater sensitivity to magnetic susceptibility offers unique benefits in functional neuroimaging, and available software/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 reduce motion artifact with scan times cut by as much as half. Spectacular anatomic delineation provided by high-definition scanning at true 1024 x 1024 resolution can aid preoperative assessment and may improve sensitivity to smaller lesions.
Three-T MR imaging is clearly ready to meet the needs of clinical practice today. Studies of the brain, spine, chest, abdomen, pelvis, as well as the vasculature and extremities obtained at 3T
are consistently higher in quality than those obtained at 1.5T. They define excellence in our clinical practice. SAR considerations are low due to technical advances, and surface coils are available for all core applications. Three-T provides our practice with 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.
Dr. Tanenbaum is section chief of MRI, CT, and neuroradiology at the New Jersey Neuroscience Institute of Seton Hall University, Edison Imaging-JFK Medical Center in Edison, NJ, and president of the Clinical Magnetic Resonance Society. He serves as an educator and consultant for GE Medical Systems.
1. International Electrotechnical Commission. Medical electrical equipment-part 2: Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. IEC 2002:601-602,633.
2. Foster JR, Hall DA, Summerfield AQ, et al. Sound-level measurements and calculations of safe noise dosage during EPI at 3 T. J Magn Reson Imaging 2000;12:157-163.
3. Kruger G, Kastup A, Glover GH. Neuroimaging at 1.5T and 3.0T: Comparison of oxygenation-sensitive magnetic resonance imaging. Magn Reson Med 2001;45:595-604.