Anatomic and functional magnetic resonance imaging (MRI) has been shown to be a valuable tool in staging prostate cancer above and beyond what is possible with standard clinical tests such as digital rectal exams, transrectal ultrasound-guided biopsy, and serum prostate-specific antigen measures (1). Although anatomic MRI based primarily on T2-weighted contrast provides delineation of zonal anatomy, extracapsular extension, and regions of cancer, functional MRI studies have been shown to further increase sensitivity and specificity over anatomic imaging alone by improving the differentiation of cancer from benign processes. Many studies have demonstrated the utility of combining anatomic imaging with various combinations of functional studies in determining disease volume, targeting biopsy, treatment planning, and therapy monitoring (2–5).
As with many other applications, the impetus behind the use of higher magnetic field for prostate MRI is driven by the promise of improved disease characterization resulting from increased spectral dispersion, parallel imaging performance, and signal-to-noise ratio (SNR). When comparing 3 T with 1.5 T, increase in SNR alone has resulted in a diagnostically relevant increase in spatial resolution, which is crucial for the identification of extracapsular extension in anatomic T2-weighted images, and for decreasing partial voluming in spectroscopy studies, thus improving the identification and characterization of smaller volumes of disease (6, 7). Increasing field strength also results in increased spectral resolution in spectroscopy, increasing the potential to quantify individual metabolites (8) and to lead to more sensitive and specific biomarkers as demonstrated in high-resolution magic angle spinning spectroscopy spectra of prostate cancer tissue (9).
Increased temporal resolution directly benefits dynamic contrast-enhanced MRI (DCE-MRI), where higher temporal sampling has been shown to improve the determination of pharmacokinetic parameters used to characterize neovascularization (10). In addition to increasing SNR and thus spatial resolution, the increase in field strength can be used to improve temporal resolution by reducing the necessity to signal average and, even more significantly, by improved parallel imaging performance. It was shown by Wiesinger et al. (11) that the increased spatial-encoding capability of the receive B1 field (B) of each coil element at high field can be used to increase reduction factors (R) while maintaining a relatively low geometry factor (g-factor). Maintaining a low g-factor is critical as it inversely scales the resulting SNR for a given R based on the relation SNR ∝ 1/(g*√R).
To take full advantage of the promise of prostate MRI, coil configurations have been optimized based on different design criteria. Initially, to improve the SNR of prostate studies at 1.5 T, receive-only external surface arrays (roESAs) were combined with receive-only endorectal coils (roERCs). However, with the arrival of the clinical 3 T platform, some studies returned to focus on the roESA alone where the field strength-dependent increase in SNR provided a similar performance compared with the combined roERC + roESA coil at 1.5 T (12). Although there are several advantages to the sole use of a roESA, the diagnostic information obtained with the roERC (13) and roERC + roESA coils (6) at 3 T has been demonstrated to be superior.
Optimizing radiofrequency (RF) coil configurations for ultra-high-field (UHF) applications, i.e., higher than 3 T, requires attention to additional issues not present on lower field systems. For example, UHF whole body MRI systems typically do not make use of whole-body RF transmit coils because of the increased power requirements and power deposition in tissues, both roughly proportional to B (14). In the absence of a whole-body volume transmit coil, local transmit coils must be developed, and typically each of these transmit coil elements is also used as receive coil elements (then referred to as transceiver coil elements).
Although the previously described coil configurations are all options for prostate imaging at 7 T, the challenges of optimizing these coils in terms of performance and safety greatly increase with field strength. These challenges are a result of the quadratic relationship between B0 and RF power as well as the increasingly complex B fields. With respect to performance, it was sufficient at 3 and 4 T to predetermine B shimming solutions determined from the geometric design of trESA RF coils (15–17). However, this one size fits all approach becomes far less optimal at 7 T, where the RF wavelength becomes as short as ∼12 cm in the body, so that B fields vary much more rapidly and are increasingly affected by body geometry and coil position. This has been experimentally demonstrated in the work done at 7 T using a trESA coil, where a subject-dependent B shimming method (rather than a unique, coil-based method) was used (18). With respect to safety, the E fields are responsible for RF-induced heating in the body, and, at 7 T, they surpass the B fields in term of spatial complexity. An important consequence of the latter is that at UHF the most constraining limits on RF power, with regards to patient safety, are typically the result of local rather than whole-body heating. These local effects become even more prominent when using transmit coils that are positioned close to, as well as inside, the body as in this study. Understanding these local E fields and/or local heating is of paramount importance for determining safe operating limits while not unnecessarily limiting power which would compromise performance.
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- MATERIALS AND METHODS
On closed-bore clinical scanners, a body coil is typically used to generate a relatively homogenous transmit RF field, while local receive arrays are typically used for reception. However, whole-body transmit coils, standard on most clinical scanners, become increasingly challenging for applications at 7 T resulting from increased power absorption and decreased wavelengths. Even though the feasibility of whole-body RF coils has been demonstrated at high fields including 4 (37) and 7 T (38), the limits on maximum peak transmit B1 and strong field inhomogeneity have not yet been adequately addressed for general use. The decreased efficiency and homogeneity at high field is a result of destructive interferences exacerbated by the short wavelengths in tissue at 300 MHz (∼12 cm) (39).
The trERC provides improved peak transmit and receive B1 but at the expense of reduced homogeneity compared with the trESA. With respect to B in the reference volunteer, the trERC exhibits approximately a fivefold advantage near the loop coil compared with the trESA and has an rSNR1distance of 2.8 cm. Although trERC receive inhomogeneity results in correctable display issues, the most deleterious effects originate from the inhomogeneous transmit B1, which was measured at 34% in vivo and 66% in simulation. For reasons previously explained, the simulated inhomogeneity is most likely closer to what is actually being realized in vivo. As shown in the vertical and horizontal profiles in Fig. 4, the B inhomogeneity is not only dependent on the distance normal to the coil but is also asymmetric across the coil from left to right. As shown in Fig. 7, the B inhomogeneity of the trERC makes it difficult to achieve a uniform contrast across the entire prostate. Close to the coil, B insensitive adiabatic pulses can be used to achieve good localization in spectroscopic studies as previously demonstrated by Klomp et al. (22). For T2w imaging, however, a previously presented adiabatic RARE sequence (43) is acknowledged to have a much high power deposition than the TSE sequence used in this study, which was already close to the allowable local SAR limit. Again, combining adiabatic RARE with SAR reduction strategies such as VERSE may help mitigate these issues to a level sufficient to allow reasonable anatomic imaging.
The roERC + trESA coil combines several benefits of both coils and has resulted in the best anatomic imaging of the prostate to date at 7 T as shown in Fig. 8. Combining the roERC with the trESA improves the imaging of critical structures such as the posterior prostate capsule, neurovascular bundle, and peripheral zone in several ways. First, the trESA provides a homogeneous B resulting in uniform contrast across the prostate. Meanwhile, the roERC increases receive sensitivity, reduces prostate/rectum motion, and allows control over susceptibility by filling the coil balloon with a perfluorocarbon. The challenge with this configuration is the same as the trESA coil alone, a relatively low peak B.
The SAR limits for the trESA were chosen based on simulation as opposed to heating measurements for several reasons. First, temperature measurements in vivo are impractical as the location of maximum heating is at first unknown making it difficult to accurately place probes on the surface and unreasonable to place probes subcutaneously. Second, it is difficult to build a phantom to reasonably approximate the widely varying tissue properties which have a tremendous effect on E-field distributions. Finally, the multichannel transmitter further complicates the issue as there are complex E fields generated by each transmit element which constructively and destructively interfere. The situation is similar to the issues faced with B but additionally complicated by the influence of tissue boundaries and varying electrical properties. Therefore, to incorporate this complexity, the human body model was used in simulations, accepting its overall size and shape would differ from in vivo measurements.
To determine TAP limits for the trERC, the phantom study was most appropriate as it allowed both simulation and experimental testing. Experimentally, the phantom allowed interrogation of the temperature at multiple locations in the polyacrylamide gel immediately adjacent to the coil. Along with simulation, it was deemed important with this internal coil to measure the maximum local heating along the conductor which, except for a latex balloon, is immediately adjacent to the tissue. Temperature measurements confirmed simulation results that the location of maximum heating occurs directly above the capacitors (Fig. 6e), and the resulting local SAR measurements from the heating study were similar to the local SAR predicted in the phantom model from a single cell in the simulation. These results along with similar B profiles in both simulation and in vivo gave confidence in the calculation of TAPlimit,trERC from the simulated SARpeak_norm.
Limitations on Transmit B1
Considering the local SAR constraint alone, more power can be used for the trESA than for the trERC by the factor TAPlimit,trESA/TAPlimit,trERC = 2.54/1.34 = 1.9. Despite this apparent advantage, the average B in the prostate is still lower for the trESA than the trERC. For demonstration purposes, assuming we are at the SAR limit with 1 W input power for the trERC, the trESA could use 1.9 W. Therefore, for the reference volunteer, the trESA coil could generate a relative field of 0.152×(16 × 1.9)0.5 = 0.84 μT compared with the average trERC value of 1.24 μT. In the average male, the generated field by the trESA would be reduced by 20% compared with the reference volunteer resulting in a B of 0.67 μT.
Parallel Imaging Performance
It has been previously demonstrated that the 16-channel stripline array used in this work performs quite well for parallel imaging across the entire human torso (27, 44). In this study, local g-factors were determined in the prostate and external iliac artery as these locations are the most relevant for prostate DCE-MRI studies. Even at the location of its maximum contribution in receive B1, the roERC only modestly improves g-factors and this benefit is seen primarily in the anterior–posterior direction. In the artery, where the contribution of the trERC is much less, there is almost no difference when compared with the trESA alone as would be expected. It is important to mention that if the AIF from the external iliac artery is desired for pharmacokinetic modeling, then using a trESA or at least a local transmit coil is an absolute requirement. On the receive side, improving local parallel imaging of the prostate could be improved further in this configuration through the use of longitudinal (45) and horizontal (46) ERC arrays.
Coil Design Considerations
With a modification of the current coil design, there is the potential to further exploit inductive coupling to augment the local B performance in the region of the prostate. Such methods have been previously published to increase local transmit B1 in the body and head (47, 48). Some of these effects were observed in the data presented in the coil characterization results. When transmitting with the trESA and using the roERC and trERC as receive-only coils, the average transmit B1 in the prostate increased 20 and 73%, respectively. Although not unexpected, the current coil design was not specifically designed for this purpose and is an area for future investigation.
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- MATERIALS AND METHODS
Three different coil configurations to investigate the prostate at 7 T were compared in this study: a trERC, a 16-channel trESA, and a trESA combined with a receive-only ERC (roERC), which will be referred to as trESA + roERC. The advantages of the trERC are higher peak B performance, improved SNR, the ability to perform reduced FOV imaging, decreased prostate motion and control over susceptibility mismatches with the rectum, better visualization of the neurovascular bundle, and patient size-independent performance. Our results suggest that trERC could play a significant role in cases where peak B value is the most critical factor at least close to the coil, such as in spectroscopic studies. However, the B asymmetry across the coil and rapid decrease of B with increasing distance from the coil limit the advantage of the peak B performance and compromise anatomic and other functional imaging studies. The trESA benefits from B and B homogeneity over the prostate area, increased anatomical coverage and capability of performing both parallel transmit and receive methods along with improved patient acceptance. However, in its current implementation, the trESA configuration is limited in power on the transmit side and in SNR on the receive side. The roERC + trESA coil combines many of the best attributes of each such as the trESA's homogenous B and parallel imaging performance along with the ERC's superior SNR to obtain optimal image quality. Another limiting factor for both the trESA and the combined trESA + roERC is the subject size-dependent available B in the prostate that decreases as the coil elements move further away from the target anatomy. Improving transmit chain and coil efficiency in the trESA configuration would further extend the promise of the trESA + roERC for prostate studies.