In this study we present the design and characterization of a high-density whole-body mouse phased-array coil with 20 receive-only elements on a cylindrical former. The coil was developed and used on a clinical 3T MRI system and characterized with bench tests as well as phantom and in vivo rodent imaging. Bench tests included the evaluation of unloaded-to-load Q-ratio, tuned-detuned isolation, and nearest neighbor coupling. Imaging validations included pixel-wise SNR maps, G-factor maps, and noise correlation as well as highly accelerated whole-body mouse images.
Special attention was given to both mechanical and electrical construction, to provide a reliable RF-system for long term use in MRI murine application. However, a number of technical issues arise in the implementation of a large channel-count array with small element size. In particular, the inter-element decoupling, QU
and SNR performance became more challenging. Prior to the coil construction, we investigated different conductor types to maximize QU
for the case when the surrounding elements were present. The flat circuit board loop showed the largest QU
-ratio when examined as a single isolated loop (no neighbors). This is attributed to the stronger capacitive coupling (measured down-shift of loaded resonance) of the flat inductor into the sample and therefore, its increased dielectric losses. Arranged in an array configuration, the flat conductor showed the highest QU
loss compared to the wire and tubular conductors. Thus, the flat loop is significantly negative impacted due to eddy currents induced by its neighboring conductive material. This yields in an increased resistance and a decreased inductive reactance (measured up-shift of unloaded resonance) of the flat conductor loop. This is consistent with recent studies, which show that eddy current losses in the conductors of neighboring elements can be significant and the optimum configuration for an array uses spatially sparser conductors than would be optimum for a single element alone (9
). Additionally, the circuit board elements utilize twelve additional solder joints for the overpasses required to bridge the conductors of neighboring coils, each joint adding additional losses (28
). The other test-loops were made out of solid wire or a tubular conductor and showed better QU
in the test, where neighboring elements were present. Here, the bridges over the conductors of adjacent coils were formed by bending the wire rather than soldering an additional bridge, thus minimizing the number of solder joints. Lower losses in the tubular conductor configuration are likely due to the slightly bigger cross-sectional diameter of the tubular conductor. Since the current flows on the surface of the conductor and is largest toward the inner edge of the loop coil (29
), a larger diameter conductor is expected to have lower losses. Finally, since capacitive coupling between elements is largely determined by the area and spacing of the cross-over regions, the wire and tubular design reduce this source of capacitive coupling by minimizing the area and increasing the spacing between the crossing conductors. We also found that the ability to mechanically optimize the overlap between two loops by bending these wire bridges facilitated the element decoupling procedure.
Despite optimization, our QU/QL = 2, shows that the sample and component losses contribute almost equally to the image noise, for these small diameter loops with limited tissue volume within under each element. This suggests that significant improvement could be gained by reducing component losses (e.g. through coil cooling), or increasing the relative tissue load (e.g. using multi-turn coils) but significant changes in design strategy are likely needed to significantly improve the unloaded-to-loaded Q-ratio.
One of the primary aims of the current study was to investigate, if a 20-channel MRI phased-array with small receiving elements would result in SNR gains compared to a commonly used circularly polarized birdcage resonator with similar geometry. Our findings in SNR comparison between the array and the birdcage coil suggest that a significant improvement in SNR is possible even at the coil center, when the birdcage has similar geometry. In contrast, when compared to a shorter birdcage coil (aspect ratio of 1:1), the SNR was almost equal at the coil center, however, the SNR benefit at the edge of phantom was still apparent (2-fold gain). Thus, the highly parallel array can be thought as added extended coverage and gains in peripheral SNR compared to an optimized birdcage, as well as the ability to accelerate image encoding. A competing approach is to form an array of extended birdcages to extend coverage while optimizing central SNR (30
Comparison with a standard root sum-of-squares reconstruction showed less benefit in SNR performance. This was likely due to the coupling between the coil elements, is not accounted for in the sum-of-squares reconstruction. For example, a maximum noise correlation of 36% was seen for elements facing each other across the FOV. Part of the SNR gain over the birdcage coil likely arises from the smaller over-all size of the 20-channel phased-array (conductive elements on a 95 mm/34 mm length/diameter cylinder) compared to the birdcage (95 mm/40 mm). The larger birdcage is typically used to avoid transmit B1
-field inhomogeneity in the animal. In contrast, receive-only arrays, which utilize a homogeneous transmit coil (the RF body coil in our case) are not constrained by this consideration and can be made with a tighter geometry. Secondly, our requirement for whole-mouse body imaging requires a length to diameter ratio of nearly 2:1, which is larger than the optimum ratio (0.7:1) for a birdcage (22
The measured SNR from an element in the 20-channel array with other elements detuned was approximately 11% higher compared to that element’s SNR in the uncombined image data, obtained with the other elements active. This sensitivity loss might result from residual coupling between the elements not eliminated using geometrical and preamp decoupling. Furthermore, the measured SNR from the single active element (all others detuned) was 94% of the SNR obtained from a single isolated receive element (no other loops present). This SNR drop of 6% might be attributed to eddy current losses from the surrounding coils and preamplifiers.
In most small animal MRI, whole-body coverage is not required. Thus, literature about coil development for mice is focusing mainly on dedicated local applications (e.g. brain, cardiac, or abdominal imaging). These localized MRI examinations have no benefit at the coil center for non-accelerated imaging compared to an optimized birdcage coil. Likely they would only have moderate peripheral sensitivity benefits compared to an optimized single channel surface coil or small array. But we note that increasing the channel count of array of small surface coils is that adding coverage does not detract from sensitivity. If the channels are present (as they increasingly are on clinical scanners), then adding coverage down the body by adding more coils comes only at the cost of increased construction complexity. However, several technical coil studies addressed already whole-body mouse imaging using long birdcages (31
) or solenoid coils (32
), capacitive-decoupled overlapped array (35
) or array coils (14
). Not only in research, but also several commercial coil vendors included 8-channel whole-body coils for small animals in their portfolio. Whole-body mouse imaging applications are widely used for body composition examinations in rodents (e.g. fat quantification) (36
). Furthermore, some multi modal mouse imaging uses MRI for anatomical information to match with nuclear-medicine imaging. In particular, when bio-distributions of radio-labeled tracers are addressed, whole-body information is necessary (38
). Multimodal small animal imaging (e.g. PET-MRI, SPECT-MRI,) are very time consuming imaging procedures and could take up to several hours. Therefore, accelerated MR-imaging is desirable for in-vivo
mouse experiments since it reduces scan time significantly. Several tumor mouse models are very sensitive to anesthesia, and so MR-imaging has to be performed as fast as possible. Additionally, high thru-put mouse imaging likely requires accelerated image encoding. High density array coils might solve these problems and provide sufficient image quality with short scan times through the use of high acceleration rates.
The accelerated imaging performance of the 20-channel phased-array is expressed through the G-factor, which reflects the SNR penalty of ill-conditioned image reconstruction in parallel imaging. Distributing the coils in all spatial directions makes the 20-channel coil well-suited to the use of highly accelerated parallel imaging for 3D whole-body mouse acquisitions. Even six-fold (R=3 in-plane phase encoding (PE) direction, R=2 through-plane PE direction) 2D accelerated data shows sufficient image quality in in-vivo mice imaging (). Note, for G-factor calculations the FOV was cropped as tight as possible to the phantom. In typical in vivo MR-experiments the FOV is usually set slightly bigger around the sample, which will result in a better G-factor. The calculation of average G-factor values listed in and also includes background area of the images.
Alteration to the present coil design in order to achieve higher performance in accelerated parallel image acquisition could be possible. First, our simulations suggest that potentially higher performance can be achieved by using twenty channels rather than eight. Our G-factor simulation showed the lowest noise amplification, when the coils are arranged with an inter-element spacing, rather than overlapped. Those designs provide better distinguishable sensitivity profiles in magnitude and phase from the coil elements in order to encode spatial information (6
). But acceleration in superior-inferior direction does not show any benefit in non-overlapped array design (41
). Furthermore, non-overlapped arrays provide less baseline SNR at the image center (but usually higher SNR close to surface, assuming sample noise domination). However, the gapped vs.
overlapped choice becomes less critical for large coil arrays. Wiesinger et al.
showed a break-even number of 16 channels arranged around a spherical phantom, when gap design becomes less favorable for accelerated imaging (7
). Although, the 20-channel overlapped array provide 17% and 12% less average G-factor at acceleration R
=3, compared with gap design and shared conductor layout, respectively, after image reconstruction the SNR was slightly higher in the center as in the non-overlapped arrays (). Thus, the larger and overlapped elements overcome the improved G-factors afforded by the gap and sheared conductor in the center regarding SNR, which is the metric we ultimate care about. In our simulations, while the 20-channel gapped array had an improved G-factor compared to the overlapped design, it had slightly reduced central SNR for R
=3 accelerated imaging.
Commercially available small bore high field animal scanners provide significantly better gradient performance for small animal experiments compared to human clinical MR systems. Only a few small animal systems can accommodate 20-channel coils, nevertheless, the spatial resolution obtained using the 20-channel coil in combination with a clinical 3T system is sufficient for many studies. Furthermore, the MR signal is influenced by a number of parameters, including proton density, water diffusion, blood flow, T1 relaxation time, T2 relaxation time and magnetic susceptibility. The latter cases revealed considerable dependence in the magnetic field strength. The use of small animal MRI scanners with high field strength can therefore make it hard to directly address clinical questions in mouse models. Whereas, imaging results can potentially be transferred directly to the clinical setting, if high quality animal images can be obtained from clinical scanners. In particular, contrast agent evaluations benefit from field strength matched image acquisitions. Thus, the presented 20-channel mouse coil gives an effective solution for those who work on clinical MR scanners but also require accelerated small animal MR imaging.