In this study we present the design and characterization of a 128-channel receive-only cardiac coil for highly accelerated cardiac MRI at 3T. The characterization of the coil performance included the evaluation of noise correlation, SNR, and G-factor maps in a mixture of phantoms and human experiments. In addition, we show that high-quality 2D images can be acquired with this coil with 1D acceleration factors of 7, raising the possibility that high-quality 3D datasets using 2D acceleration with signifi-cantly higher acceleration factors than previously obtained will be able to be acquired with this coil.
The primary rationale for the construction of this experimental 128-channel coil was to explore potential issues in constructing a highly parallel array for cardiac imaging. Our goal was to evaluate the potential gains in SNR and acceleration of increasing the number of channels beyond that currently available in clinical systems. We constructed this proof-of-concept array using a rigid body former in order to allow us to assess the ability of highly parallel reception without some of the complicating issues faced in developing a flexible commercial array.
Measurements of the transmitted RF field with and without the 128-channel coil in the scanner showed a 5% transmit voltage difference. Although one might expect transmit power to be dissipated in the copper used in the elements, cables and cable trap shields, and cable common modes, requiring a higher transmit power when the array is present, we have also observed small decreases in the needed transmit voltage in other arrays. This could reflect an incomplete detuning of the array elements causing surface coil focusing by induced fields in the coil, it could also arise from the an impedance shift in the RF body coil causing the body coil to be slightly better tuned and/or matched with the array present.
The measured SNR from one element in the 128-channel array was approximately 43% of the SNR measured with only a single-element coil present on the phantom. This SNR difference might be explained by eddy current losses from the copper in the surrounding cables and coils, as has been shown previously in a 96-channel brain array (18
). The unloaded-to-loaded Q ratio of 3.0 for the top elements of the array coil indicates that an SNR gain of only 20% would be expected in the case that the isolated coil had no copper loss (infinite unloaded Q). A second source of signal loss might arise from coupling between the elements in the array, essentially making the coil element act like a much larger diameter coil when in the array (when coupling partners are present). Some of the effects of coupling, however, are addressed when the optimal reconstruction (in which the noise covariance due to coupling is taken into account) method is used. Future advances in wireless or optical technology and more generalized availability of optimal reconstruction algorithms will thus be favorable developments for high-element arrays, such as the 128-channel coil.
A comparison of the SNR and G-factor results of our 128-channel coil with the results from commercially available 24- and 32-channel coils suggests that the 128-channel approach can produce significantly more favorable G-factors than either a 24-channel thoracoabdominal coil or a dedicated 32-channel cardiac coil ( and ). Use of the 128-channel coil also provided a secondary benefit by boosting image SNR for unaccelerated imaging in the subjects that fit well in the rigid coil. As shown in , significant portions of the heart lie close enough to the surface of the chest wall to derive a major gain in SNR from the use of high-element arrays such as the 32- and 128-channel coils, both of which were significantly more sensitive than the 24-channel coil, which contains only 12 elements overlying the heart.
The ability of the 128-channel coil to provide increased sensitivity compared to the flexible 32-channel array appears to be correlated with the ability of the rigid former to fit the subject. For the subject used to form the mold (subject #1), the unaccelerated SNR gain in the apex compared to the 32-channel array was 1.3-fold. All other subjects studied showed smaller gains or, in the case of the two subjects who were a poor fit to the 128-channel former, reduced SNR compared to the 32-channel coil. This suggests the importance of a flexible, tight-fitting coil former to insure the elements over the heart are as close to the body as possible. It also suggests that for unaccelerated imaging, 32 appropriately-sized elements overlying the heart will likely produce close to the optimal SNR profile with some benefit possible in the apex if a flexible 128-channel coil could be constructed without loss of performance. Recent results from a 128-channel flexible body array suggests that highly parallel detection can be successfully coupled with a flexible former (11
Many of the additional elements in the 128-channel coil were distant from the heart and likely contributed little to SNR in unaccelerated myocardial imaging. This is in contrast to the 32-channel array, which compactly covered only the thorax above and below the heart. While the additional elements distal to the heart likely did not benefit the myocardial SNR in unaccelerated imaging, the G-factor maps obtained from the chest-sized phantom suggest a significant role for distal elements in highly-accelerated cardiac imaging. Our results suggest that this effect can yield more than a factor of 2 in SNR in highly-accelerated imaging compared to a coil with elements concentrated on the heart, such as the 32-channel coil used here. The 24-channel coil, in contrast, covered a larger anatomical area, but appeared to suffer from the large element size required.
The SNR maps produced by the 128- and 32-channel coils are in qualitative agreement with theoretical and experimental investigations of the SNR behavior of coils with different number of coil elements (8
). For example, simulation studies predicted that the SNR near the coil elements should increase significantly with the number of RF channels with lower gains distant from the elements (8
). Moreover, an experimental comparison of a 32-channel head coil with a commercially available 8-channel head coil at 3T found a very high SNR increase of up to a factor of 3.5 near the coil plane (corresponding to the chest-wall in thoracic imaging) and a factor of 1.4 in the center of the head (16
The noise correlation between the coil elements in the 128-channel array revealed a low mean coupling value of 5.5%, but five pairs showed correlations over 50%. Three of these element pairs were located on the anterior section of the coil and two of them on the posterior part. All of these coupling-element pairs were adjacent to each other and the anterior pairs were located at the neck and near the arms of the volunteers, where the array has high curvature. We hypothesize that the degree of coil overlap needs to be further optimized in these high-curvature areas. The coupling of the two element pairs on the posterior part of the array is likely due to a leak from the output coax of one element into the preamplifier input of another.
Image reconstruction exploiting both coil sensitivity and noise correlation information, the so-called optimum reconstruction method, is able to reduce the impact of such element coupling within the array. It should be noted, however, that the in vivo images were not reconstructed with this method but rather with a standard sum-of-squares reconstruction.
Several studies have previously been published using 32-channel coils for cardiac imaging at 1.5T (12
). Niendorf and Sodickson (12
) investigated the SNR difference in coronary artery and phantom images acquired with a four-channel cardiac coil, an eight-channel cardiac coil, and their 32-channel coil. Acceleration factors up to rate 4 in one direction and 12 in two directions were used, beyond which image quality deteriorated significantly. Wintersperger et al. (14
) also used a 32-channel coil for accelerated cine SSFP protocols at 1.5T in combination with TSENSE. Their study showed that accurate volumetric evaluation was possible until an acceleration of R = 4 was reached. The improved ability to accelerate cardiac imaging (up to R = 7 in 1D) shown in the 128-channel cine images compared to the 32-channel results obtained at 1.5T may reflect the benefits of increased field strength as well as the increased number of elements.
Although the 128-channel coil was designed primarily for accelerated 3D imaging, the benefits of the low G-factors it produced could be clearly seen in the 2D cine images in this study. The female volunteer () showed acceptable high-resolution (6-mm slice) cine image quality at R = 7 acceleration even though this subject was the poorest fit in the anterior coil former (which was molded to a male chest) and showed the lowest myocardial SNR of the four subjects studied in the 128-channel coil. In addition, phase encoding in short axis cardiac images occurs partially along the anterior-posterior (AP) direction, along which the array has the lowest distribution of coil intensities and thus creates conditions least likely to demonstrate the benefits of increased elements. In contrast, 3D imaging accelerated in two phase encode directions with a significant component of one or more phase encode directions along the right-left (R-L) and H-F direction, should produce further acceleration benefits. The experimental 128-channel system used to acquire the images in this study used a modified version of the operating system that did not support some cardiac features such as retrospective gating and spatiotemporal compression schemes such as TSENSE and time-adaptive GRAPPA (TGRAPPA). These spatiotemporal compression algorithms have the potential to produce even further acceleration benefits compared to SENSE or GRAPPA alone.
The results obtained with this prototype coil suggest several improvements to the 128-channel design. A flexible anterior component to more closely fit varying body shapes will likely increase the sensitivity and clinical potential of the coil. Careful attention to the sources of the highly coupled coil pairs could improve SNR in the local regions dominated by these coils. On-going development in cardiac pulse sequence design will likely facilitate full use of the acceleration capabilities of this coil.
In conclusion, we have developed and tested a 128-channel receive-only coil for cardiac MRI at 3T. SNR and G-factor measurements in phantoms and healthy subjects revealed potential SNR gains from the mid-ventricle to the apex and significantly lower G-factors in highly-accelerated imaging compared to commercially available 24- and 32-channel cardiac coils. The ability to acquire high-quality, highly-accelerated images with this coil has been demonstrated in normal volunteers, with diagnostic quality 2D cine images obtained with up to R = 7 with GRAPPA image reconstruction. As 3D cardiac imaging with 2D acceleration becomes more widely available, highly parallel cardiac arrays, such as the 128-channel coil described in this study, will have the potential to prove of benefit in both the research and clinical settings