Evaluation of Image Quality of a 32-Channel versus a 12-Channel Head Coil at 1.5T for MR Imaging of the Brain

Discussion

Multichannel head coils have 2 major clinical applications: 1) acceleration of scanning times without sacrifices in SNR and spatial resolution via PI techniques, and 2) improvements in spatial resolution, contrast, and SNR via standard processing. Specifically, phased-array head coils have been shown to have superior SNR profiles compared with TEM and CP head coils.²¹ The advantages of increasing the coil element number for phased-array coils that have been either theoretically or experimentally described are many. Significant gains in sensitivity are realized at small distances from the coils, and the addition of more coil elements reduces the sensitivity loss at farther distances.^1,14,22 An initial study with a 32-channel receive-only phased-array head coil showed gains in SNR throughout the field compared with a similar 8-channel head coil.¹⁴ In contrast to the superiority of SNR profiles of phased-array coils, signal-intensity homogeneity can be slightly inferior to TEM and CP coils.²¹ Additionally, noise can be directly proportional to coil element numbers due to induction of current in neighboring coils (coupling) and signal averaging from each channel.^14,23

The results of this study affirm that gains in SNR and spatial resolution achieved with a 32-channel receive-only phased-array head coil at 1.5T result in improvement in image quality as assessed in most regions of the brain by neuroradiologists. At the same time, the results indicate that objective gains in SNR and spatial resolution do not always or uniformly correlate with subjective preference or diagnostic value. Chapman et al (1999)¹⁹ found similar results in their evaluation of several MRA techniques. Even in the evaluation of vessels, increasing signal intensity and decreasing noise could only predict subjective preference until a certain threshold. Beyond, this threshold, raters favored more robust edge detail at the expense of objectively measured SNR.

Furthermore, objective improvements in image quality may compete with observer experiential preferences. In fact, 2 of our neuroradiologists during the initial training period found the images from the 32-channel coil to be esthetically displeasing and “cartoonlike” due to increased contrast and edge detail. Objectively, however, they did find these images to be superior as indicated by our results. Our results also indicate that while T1WIs are clinically useful in delineating anatomy, the added value of the 32-channel coil to these images in this respect was less readily apparent (Fig 3). In favorable contrast, the improvements were more readily apparent for FLAIR and T2WIs, and even more apparent for DWI, for which all parameters of image quality and delineation of anatomy improved significantly with the 32-channel coil (Fig 5).

In our study, the most drastic improvements with the 32-channel coil were seen in the DWI and FLAIR sequences in which almost all parameters of image quality were rated as “subtle to apparent improvement,” and many preferences indicated “immediately apparent to marked improvement.” In contrast, T1WIs yielded the least preference and T2WIs had more subtle improvements in several parameters.

Lesion and artifacts evaluation was an important aspect of this study because these were the only 2 sections in which direct evaluation of the overall diagnostic value of 1 coil compared with the other was possible. While images acquired by using the 32-channel coil were, on average, of better quality, only 4 of 41 lesions were rated by at least 1 neuroradiologist as having improved diagnostic value. Thus, it may be that improvements in spatial resolution, contrast, or SNR must reach a higher threshold before there is a clearer impact on the diagnostic value of the resultant images. That threshold remains to be determined in the current escalation of elements in a phased-array multichannel head coil.

An important issue identified in this study was central loss of signal intensity on 7 T1WI stacks and 1 FLAIR stack (Fig 6). The regular-shaped area of reduced signal intensity was most apparent at the level of the midbrain. As previously stated, multichannel surface coils confer gain in SNR at the surface with predictable dropoff with distance from each element coil, thus introducing signal-intensity inhomogeneity.¹⁴ Regardless, normalization of signal-intensity inhomogeneity is required to produce clinically uniform images.²⁴ We hypothesize that the artifacts are the result of imperfect normalization in the postprocessing of these images. In fact, this was a predictable problem, and effort (though as yet not sufficient) was made to correctly normalize signal intensity before the beginning of this study.

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Fig 6.

T1WIs from 12-channel (A, C) and 32-channel (B, D) head coils showing central signal-intensity loss in the 32-channel coil. This figure is representative of the artifacts observed in several of the T1WIs. Note that section positioning is inexact, but the artifacts are apparent throughout the series.

A challenging aspect of this study relates to its design and, more specifically, the method of scaling for improvements in image quality and determination of parameters that should be evaluated. Previous studies that have qualitatively evaluated image quality in MR imaging have used scales of poor-to-excellent without direct comparison.^15–17 In contrast, we aimed for a design that would directly compare differences in image quality. While direct comparison and ranking are robust and efficient, they may not necessarily reflect the diagnostic value of perceived improvements and are subject to observer bias.¹⁹ Thus, we attempted to carefully define the parameter to be compared and confirmed these definitions during the training period. When determining the parameters for evaluation, we prioritized the evaluation of both the contrast and edge detail/sharpness. We also attempted to stratify structures closest and farthest from the element coils to capture previously reported SNR characteristics.^3,14

While our study design was intended to capture differences in parameters that are routinely evaluated in diagnostic neuroradiology, it may not fully extrapolate to conclusions about all aspects of diagnostic value. Although we were able to capture diagnostic value through lesion and artifacts evaluation, a study could more ideally compare lesions in a systematic manner. Examples of such studies are those that count multiple sclerosis plaques or, in musculoskeletal imaging, correlate the imaging measurements of the depth of cartilage defects to those found at dissection.^17,25 Our study was also limited by section positioning, especially in the evaluation of cranial nerves and small lesions. As with all human observer studies, our study was also subject to observer bias, though we attempted to reduce this effect by standardizing the way in which disparities in section positioning and angle were handled during the training session.

Furthermore, from the results of this study, 2 additional avenues for future studies are apparent. The first avenue is to specifically investigate the utility of the 32-channel coil in specific protocols such as temporal lobe seizure, internal acoustic canal, or MRA. The second avenue is to evaluate the performance of the 32-channel coil with other sequences such as fast and ultrafast spin-echo.