Uncompromised MRI of knee cartilage while incorporating sensitive sodium MRI

Abstract

Sodium imaging is able to assess changes in ion content, linked to glycosaminoglycan content, which is important to guide orthopeadic procedures such as articular cartilage repair. Sodium imaging is ideally performed using double tuned RF coils, to combine high resolution morphological imaging with biochemical information from sodium imaging to assess ion content. The proton image quality of such coils is often harshly degraded, with up to 50% of SNR or severe acceleration loss as compared to single tuned coils. Reasons are that the number of proton receive channels often severely reduced and double tuning will degrade the intrinsic sensitivity of the RF coil on at least one of the nuclei. However, the aim of this work was to implement a double-tuned sodium/proton knee coil setup without deterioration of the proton signal whilst being able to achieve acquisition of high SNR sodium images. A double-tuned knee coil was constructed as a shielded birdcage optimized for sodium and compromised for proton. To exclude any compromise, the proton part of the birdcage is used for transmit only and interfaced to RF amplifiers that can fully mitigate the reduced efficiency. In addition, a 15 channel single tuned proton receiver coil was embedded within the double-resonant birdcage to maintain optimal SNR and acceleration for proton imaging. To validate the efficiency of our coil, the designed coil was compared with the state-of-the-art single-tuned alternative at 7 T. B1+ corrected SNR maps were used to compare both coils on proton performance and g-factor maps were used to compare both coils on acceleration possibilities. The newly constructed double-tuned coil was shown to have comparable proton quality and acceleration possibilities to the single-tuned alternative while also being able to acquire high SNR sodium images.

Keywords: 7 T; cartilage; knee; sodium.

Figure 1

Coil configuration, showing the double‐tuned four‐ring birdcage. The dimensions are in millimeters. The following capacitor values are used: C1 = 5.6 + 2 pF, C2 = 5.6 + 2.4 pF, C3 = 91 pF and C4 = 91 + 8.2 pF

Figure 2

Noise correlation matrix of the double‐tuned coil. A small amount of coupling can be observed between channel 1 and 4, and 1 and 3

Figure 3

Axial g‐factor maps of double tuned coil, with SENSE acceleration factors up to 3 in both directions

Figure 4

Axial g‐factor maps of the single tuned coil, with SENSE acceleration factors up to 3 in both directions

Figure 5

Axial (a) and sagittal (B) image slices for double tuned coil with corresponding g‐factor maps (E & F), and axial (C) and sagittal (D) image slices for single tuned coil with corresponding g‐factor maps (G & H)

Figure 6

SNR maps of double tuned coil (left) and single tuned coil (right)

Figure 7

Sodium image overlaid on proton density weighed anatomical scan