3T MR Imaging of the Musculoskeletal System (Part I): Considerations, Coils, and Challenges
Section snippets
Increased signal-to-noise ratio at 3T
The clear advantage of 3T scanners is the improved SNR. An in-depth understanding of how the SNR is obtained is necessary to know the appropriate application and limitations of such a signal. The signal that is gained at higher field strengths is related, predictably, to the equilibrium magnetization, voxel size, and resonance frequency as demonstrated in this equation [1], [2], [3]:
S = signal, ω0 = resonance frequency, M0 = equilibrium magnetization, V =
Increased spatial resolution at 3T
With the improved SNR at 3T, one can immediately take advantage of increasing the spatial resolution in several fashions [1], [2], [3]. To understand how to accomplish this, a discussion of the relationship between SNR and spatial resolution is helpful. SNR is proportional to slice thickness and in-plane resolution as follows:
or
K = constant, Vx = voxel size in x direction, Vy = voxel size in y direction, Vz = voxel size in z
Decreased scan time at 3T
The improved SNR at 3T may be exploited to improve spatial resolution, and to decrease scanning time [1], [2], [3] and increase patient throughput. This can be deduced from the following discussion. The SNR is proportional to the number of acquisitions (time) as follows:
or
Therefore, if the SNR at 1.5T is one half of the SNR at 3T, then to maintain the SNR of 1.5T, one could decrease the number of acquisitions to one fourth:
Coil selection
As rapidly as the manufacturers of MR imaging systems have improved and fine-tuned 3T MR systems, coil design and development have been unable to demonstrate the same rapid pace. A brief discussion of coil designs is needed to understand the latest in coil development and applications to 3T MR imaging. There are two major fundamental coil designs in regards to the RF pulses that are used in MR imaging: transmit-receive and receive-only. Transmit-receive coils have the ability to apply RF pulses
Specific absorption rate
SAR is the term that is used to describe the energy that is deposited in a patient per unit of mass or weight. SAR increases with the strength of the magnetic field (see later discussion), which is a potential limitation in the clinical applicability of high-field strength systems. The U.S. Food and Drug Administration and the International Electrotechnical Committee set SAR safety limits of an increase in core body temperature of no more than 1°C. To operate within this limit, several specific
Parallel imaging
Parallel imaging is a technique that uses spatial data that are derived from phased-array coil elements to construct a portion of k-space [12], [13], [14], [15], [16]. By using coil elements to supply k-space data, the burden of filling lines of k-space with individually acquired phase gradients is diminished, which decreases the duty cycle of the scan. The end result is a decrease in scan time and—equally important at 3T—a decrease in SAR because of the reduced number of RF pulses that is
Susceptibility artifact
Metallic hardware in orthopedic imaging notoriously introduces susceptibility artifact; the degree of artifact worsens at higher magnetic field strengths [17]. At 3T, a shortening of T2 and T2∗ occurs that results in greater signal loss and geometric distortion. In addition, specifically for orthopedic imaging, chemically selective fat-saturation techniques often are markedly inhomogeneous or fail altogether (Fig. 8).
With knee imaging, the most common metallic hardware are anterior cruciate
Chemical shift artifact
Chemical shift artifact is an issue that must be addressed on musculoskeletal MR images at all field strengths [24]. At 3T, the effects of chemical shift are more pronounced than they are at lower field strengths. An understanding of the factors that determine chemical shift, and techniques to ameliorate these artifacts will dispel any trepidation about imaging at 3T.
The precessional frequency difference between fat protons and water protons is doubled at 3T relative to 1.5T. For instance, at
Pulsation artifact
Pulsation or flow artifact worsens at higher field strengths [27], [28]. In the knee, the popliteal vessels may result in artifact that obscures the posterior horns of the menisci, especially on sagittal sequences (Fig. 11). Several techniques may be used to diminish pulsation artifact (see Fig. 11). Application of saturation bands superiorly, inferiorly, or in both locations may help to decrease pulsation; however, this may result in prolonged scan times and increased SAR.
In addition, choosing
Truncation artifact
Truncation artifact is a well-known artifact that occurs commonly at the interface between low- and high-signal structures [30], such as the vertebral bodies and cerebrospinal fluid (CSF) on a sagittal T2-weighted sequence. At 3T, because of the higher SNR of all tissues, this phenomenon also becomes apparent in extremity imaging. In the knee, truncation may occur at the interface of bone and joint fluid. To correct truncation, an increase in phase resolution is required.
Dielectric resonance (field focusing)
At 3T, the Larmor frequency is approximately 128 MHz. As a result, the wavelengths of RF pulses are decreased proportionately compared with 1.5T scanners. In addition, the dielectric constant of tissues increases at higher field strengths. A higher dielectric constant effectively results in slower electromagnetic waves with shorter wavelengths. As a result of the shorter wavelengths of the RF pulses and higher dielectric constant of tissue, there is an increased incidence of standing waves. The
T1 relaxation times
The effect of higher field strength on T1 relaxation times is a component of 3T imaging that requires consideration. It is well documented that the T1 relaxation time for most tissues increases with increasing field strength [33], [34], [35], [36]. As a result, the TR for T1-weighted sequences needs to be prolonged to optimize T1 differences between tissues. In addition, for proton density and T2-weighted images, TRs also need to be prolonged to eliminate T1 weighting.
A recent study
T2 decay times
Although not as dramatic as the effect on T1 relaxation, higher field strengths shorten the T2 decay times of various tissues [33], [34], [35], [36]. As a result, TE times need to be shortened on T1-, T2-, and proton density–weighted images to compensate for the decrease in T2 and T2∗. In addition, a theoretic increase in image blurring may occur with FSE sequences, because the lower-signal, longer TE echoes fill the peripheral lines of k-space. Susceptibility artifacts also may worsen as a
Summary
As 3T MR imaging systems become more widespread in the clinical realm, a full understanding of the opportunities for image improvement and the limitations in the applications of the signal gain is needed. It is clear that even with current coil technology, much of the gain in signal can be harnessed effectively; however, continued coil development is necessary to realize the full potential of 3T, especially with the wonderful synergy that can be achieved with the use of parallel imaging and
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