Musculoskeletal 3T imaging gains clinical acceptance

High-field MR imaging is rapidly gaining clinical acceptance as a preferred platform. Its impact on imaging of the musculoskeletal system has been dramatic, spurred in part by the increasing availability of 3T systems in clinical and academic settings and by ongoing research demonstrating numerous advantages over 1.5T in many areas, including neurological and vascular imaging.^1,2 Reduced concerns over surface coil availability, radiofrequency deposition, ambient noise, magnetic susceptibility, chemical shift, and altered tissue contrast are contributing, along with improved efficiency, to the increased importance of 3T scanners in the clinical setting.

A crucial contributor to signal-to-noise ratio performance is the RF coil. Surface coils used in 3T musculoskeletal imaging now rival those for 1.5T in availability and sophistication. High SNR eight-channel phased-array coils are widely available for critical shoulder, wrist, foot, and ankle applications.³ Phasedarray coils enable parallel imaging, which in some circumstances can be used to manage RF energy deposition and reduce scan time.^4,5

Factors other than SNR that affect image quality are also important at 3T. T1 relaxation time is increased at higher fields. The increase in T1 relaxation time of musculoskeletal tissues at 3T is 15% to 22% greater than at 1.5T.⁶ The most important consequence of the increase of T1 relaxation time in clinical imaging is the need to increase TR times in spin-echo imaging and to lower flip angles in gradient-echo sequences at 3T to maintain tissue contrast.^6-8 T2 relaxation decreases at 3T in comparison with 1.5T. This decrease is approximately 10% to 19% in typical musculoskeletal tissues but 37% in synovial fluid.6 The effect at higher fields means that similar contrast may be obtained at slightly shorter echo times at 3T versus 1.5T. More important, shorter T2 times allow the use of pulse sequences with a shorter TE.

CHALLENGES OF 3T

Three-T imaging also presents new challenges as well, however.

• Chemical shift artifact. Chemical shift artifact manifests as misregistration occurring in the frequency- encoded direction, which can be counteracted by doubling the receiver bandwidth (Figure 1). Misregistration can also be negated by using fat-suppression techniques, such as short-tau inversion recovery,⁹ water excitation,¹⁰ or spectral fat suppression.¹¹ IDEAL (based on Dixon) techniques¹² image fat and water separately, then fuse and correct for pixel misregistration, leaving tissue contrast and SNR intact (Figure 2).

• Fat suppression. The proportionately greater chemical shift effect at 3T yields increased separation in the lipid and water Larmor frequencies, allowing fat suppression with less suppression of the water peak (less SNR reduction)^13,14 and higher slice efficiency.⁷ A major challenge of spectral fat suppression in musculoskeletal imaging is susceptibility artifact in the postoperative joint or spine.

These effects are more severe at 3T than at 1.5T but minimized with higher bandwidth, longer echo train lengths, and smaller voxels. STIR imaging is less prone to failure of fat suppression and is preferable to RF techniques in the setting of implanted metal. IDEAL water-only (fatsuppressed) techniques are perhaps most resistant to failure, and these are applicable for gadolinium contrast-enhanced imaging (Figure 3).

• Magnetic susceptibility effects. Susceptibility artifacts are a result of local field gradients induced by bone/tissue and air/tissue interface and ferromagnetic effects, and they are up to four times greater at 3T.

Increasing bandwidth, using IDEAL/STIR techniques instead of spectral fat suppression, and avoiding gradient-echo sequences minimize these effects (Figure 3). Strategies to reduce susceptibility artifacts include managing the frequency- and phase-encoding gradients, decreasing voxel size, and using fast spin-echo sequences instead of conventional spin echo.^15,16 Aligning the frequency- encoding gradient parallel to a magnetic screw makes susceptibility less problematic (Figure 4).¹⁷ Reducing voxel size minimizes the volume affected by artifact. These techniques can reduce artifacts by up to 79% as well as increase the receiver bandwidth.¹⁸

SAFETY CONCERNS

Specific absorption rate, a measure of the RF energy deposited per unit of tissue, increases fourfold on a 3T MRI over a 1.5T system. Several techniques are available to decrease the RF energy exposure. By using a locally applied transmit and receive coil, only the imaging volume is exposed to the RF power, markedly decreasing the energy deposited in the whole body. One of the major risks in MRI comes from forces placed on ferromagnetic objects in a magnetic field.^19,20 The torque forces are dependent on many factors but scale directly with B0. Some implanted electronic devices are SAR-restricted and of limited compatibility at 3T. As a general rule, strict adherence to conservative precautions used at 1.5T is recommended at 3T.²¹

OPTIMIZING 3T PROTOCOLS

The increased SNR obtainable at 3T allows routine imaging at higher spatial resolution than at 1.5T.^1,10 As usually implemented, the in-plane resolution is often highest in the frequency-encoded direction and lower in the phaseencoded direction to minimize data acquisition time. Through-plane spatial resolution is markedly worse than in either of the in-plane directions.

Depending on the geometry and orientation of objects being imaged, through-plane dimension may or may not be the limiting factor in detection of pathology. For detecting large articular cartilage defects on a flat surface; e.g., the tibial plateau, the throughplane resolution may be of minimal importance for sagittal images.

For detecting and following changes in early erosions in rheumatoid arthritis, lesions typically spherical in shape and on the order of 1000 to 2000 microns in size, however, a 3- mm resolution in any dimension severely limits diagnostic accuracy. We have found increased sensitivity to inflammatory erosions and peripheral triangular fibrocartilage tears when spatial resolution is no greater than 1 mm in all planes. Consequently, the most appropriate way to increase spatial resolution using 2D imaging at 3T is to use the added SNR to decrease slice thickness and interslice skip, an approach that can lead to long scan times.⁷ On the other hand, we have found no diagnostic benefit to maintaining 3-mm slices and doubling the in-plane resolution.

Three-D Fourier transform (3D FT) imaging provides increased SNR and thin slices with no skip between slices, but it can be a time-consuming sequence unless the acquisition volume is small. One potential solution is to image the entire joint of interest using a traditional 2D FT protocol and then target smaller areas of pathology for high-resolution imaging.

STANDARD IMAGING AT 3T

Most clinical musculoskeletal studies have found 3T to be a distinct advantage. ^6-8,22-24 Anterior labral tears are well visualized, especially using proton- density fat-suppressed axial images (Figure 5). Superior labrum from anterior to posterior (SLAP) tears, which can be somewhat challenging to detect at lower field strengths, are well visualized in our experience. Anatomic variants, such as those associated with shoulder impingement, are displayed with increased clarity at high field. Routine knee imaging is also well performed at high field.²⁵ Meniscal tears are exquisitely detailed with increased resolution and contrast, and anterior cruciate ligament abnormalities are well demonstrated. Evaluation of the articular cartilage and subchondral plate is significantly improved at 3T (Figure 6). Advanced phased-array coils show great promise in ankle imaging at 3T. Achilles tears are well delineated. Tendinopathy, ossicles, and other joint and soft-tissue pathology commonly evaluated at 1.5T are well evaluated at 3T (Figure 7).

Much interest is focused on 3T imaging of the wrist, which allows detailed views of the anatomy of the triangular fibrocartilage complex using proton-density fat-suppressed sequences and high-resolution GRE 3D FT imaging. High-resolution proton-density fat-suppressed sequences are excellent at detecting triangular fibrocartilage disc tears (Figure 8). Characteristic MR findings of impingement syndromes, such as ulnar impaction, and inflammatory arthropathy, such as rheumatoid arthritis, are also displayed in detail at high field.

CLINICAL ADVANTAGES

The major advantage of 3T is the doubling of SNR. This advantage can be used to either decrease scanning time or improve spatial resolution. Higher resolution may improve diagnostic accuracy and increase the indications for musculoskeletal MR.^{1-3,7,8,22-25} The 3T has been shown to improve visualization of cartilage lesions made with ceramic scalpels in bovine articular cartilage. Three-T was significantly better than 1.5T, however, only with slice thicknesses of 2 mm or less.²⁴ No significant differences were seen using proton-density-weighted images at 3-mm slice thickness.²³

Another comparison of human cartilage imaging between 1.5T and 3T using the same voxel sizes found that the increased SNR/CNR at 3T produced a small increase in precision of thickness measurements. Decreasing the partition thickness from 1.5 to 1 mm (equivalent to slice thickness in 2D FT imaging) at 3T, however, produced an even greater improvement in precision, possibly resulting in the ability to detect smaller longitudinal changes in cartilage morphology over time.⁸ Studies strongly suggest that the greatest potential benefit of imaging articular cartilage at 3T is the use of the increased SNR to obtain small, more isotropic voxel sizes than can be currently obtained at 1.5T.²⁶

The increased sensitivity of highfield MRI to magnetic susceptibility changes between tissues may be beneficial in detecting some pathology. MR imaging with spoiled gradient-echo techniques at 4T using a partition thickness of 1.5 mm is more sensitive for detecting calcification in chondrocalcinosis involving articular cartilage and menisci than is 3D CT, radiography, or arthroscopy.²⁷ This effect can also be seen in calcific tendinitis and other regions of calcium deposition in the musculoskeletal system.

Another region of the musculoskeletal system where 3T’s increased resolution may significantly affect clinical imaging is in the wrist. FSE intermediate-weighted images were shown to be superior at 3T in imaging the interosseous ligaments, the triangular fibrocartilage complex, articular cartilage, and nerves.²²

The potential for 3T MRI to improve evaluation of the musculoskeletal system is becoming well established. Standard imaging protocols are performed with improved image quality, contrast, and resolution when compared with 1.5T imaging. State-of-the-art musculoskeletal MRI is moving to 3T, and its value will grow as new technologies are developed and implemented.