7 T renal MRI: challenges and promises

Abstract

The progression to 7 Tesla (7 T) magnetic resonance imaging (MRI) yields promises of substantial increase in signal-to-noise (SNR) ratio. This increase can be traded off to increase image spatial resolution or to decrease acquisition time. However, renal 7 T MRI remains challenging due to inhomogeneity of the radiofrequency field and due to specific absorption rate (SAR) constraints. A number of studies has been published in the field of renal 7 T imaging. While the focus initially was on anatomic imaging and renal MR angiography, later studies have explored renal functional imaging. Although anatomic imaging remains somewhat limited by inhomogeneous excitation and SAR constraints, functional imaging results are promising. The increased SNR at 7 T has been particularly advantageous for blood oxygen level-dependent and arterial spin labelling MRI, as well as sodium MR imaging, thanks to changes in field-strength-dependent magnetic properties. Here, we provide an overview of the currently available literature on renal 7 T MRI. In addition, we provide a brief overview of challenges and opportunities in renal 7 T MR imaging.

Keywords: 7 T MRI; Kidney; RF shimming; Renal MRI.

Fig. 1

a Anatomy of the kidney and b the nephron. In the glomerulus, the blood is filtered. In the proximal convoluted tubule, most of the filtrate—both water and ions—is reabsorbed. The loop of Henle (consisting of the proximal straight tubule, the descending thin limb, and ascending thin and thick limb) concentrates the preurine and in the distal convoluted tubule more NaCl is reabsorbed [16]

Fig. 2

Coronal T ₁w 2D FLASH images: a arrow adrenal gland b arrow renal vasculature with high signal intensity (Umutlu et al. unpublished results)

Fig. 3

Coronal T ₁w gradient echo images a without B₁ ⁺ shimming, b with local B₁ ⁺ shimming and c a high-resolution version of b (FOV 240 mm, slice thickness 2.2 mm, remaining parameters the same) (Metzger et al., reprinted with permission from [14])

Fig. 4

Coronal TFE images with Dixon reconstruction; a water image, b zoom of a, and c fat image (Hoogduin et al. [37])

Fig. 5

Axial T ₂w TSE images acquired in two subjects: a strongly impaired and b with medium image quality (Umutlu et al. unpublished results)

Fig. 6

a Coronal T ₂w TSE images; b zoom of a (Hoogduin et al. [37])

Fig. 7

a, b ss-FSE images for T ₁ measurement with inversion time 100 and 150 ms, respectively; c, d T ₁ maps with and without ROIs for T ₁ estimation; e, f ss-FSE images for T ₂ measurement with effective echo time 20 and 40 ms; g, h T ₁ maps with and without ROIs for T ₁ estimation (all ss-FSE images were acquired at six different inversion times or effective echo times to minimize short-term SAR) (Li et al., reprinted with permission from [8])

Fig. 8

a TOF MRA and b maximum intensity projection of TOF MRA (Umutlu et al. unpublished results)

Fig. 9

a Unenhanced and b CE 3D FLASH. Wide arrow right renal vein, slim arrow right renal artery, dashed arrow left renal artery (Umutlu et al. unpublished results)

Fig. 10

Axial (left) and cropped coronal (right) MIPs from multiple volunteers with different shimming strategies: a phase-only homogeneity shim both for saturation and conventional pulses; b efficiency shim for saturation pulse; c, d trade-off solution for saturation pulse, magnitude, and phase homogeneous shim for conventional pulses (Metzger et al., reprinted with permission from [14])

Fig. 11

T ₂*-weighted images: a coronal images (echo times 4.9, 9.9 and 14.8 ms); b transversal image (echo times 4.9, 9.9 and 14.8 ms); c, d corresponding R ₂* maps; e histogram of the R ₂* values. In the compartmental method, the sum of a Gaussian function (red) representing the cortex and a gamma function (purple) for the medullary values is fitted to the histogram. Arrow distinct peak of medullary voxels, not visible on 3T data (Hoogduin et al. [37])

Fig. 12

a Theoretical simulations of renal perfusion SNR efficiencies at 3 T and 7 T for renal perfusion imaging using FAIR EPI. TR represents repetition time. b One subject’s proton (left) and normalized perfusion-weighted (right) images from perfusion study using FAIR-EPI at 7 T with 2 × 2 × 5 mm³ resolution. ΔM represents perfusion-weighted signal evaluated as the signal difference between label and control images, and M ₀ the fully relaxed renal tissue signal (Li et al. [48])

Fig. 13

PASL images acquired with ss-FSE readout: a proton density; b control image; c labelling d perfusion-weighted imaging normalized to proton density (Li et al. [50])

Fig. 14

Coronal images: a ²³Na image on 7 T with scale representing ²³Na SNR in arbitrary units and b corresponding T ₂-weighted proton image in the same subject on 3 T with scale representing signal intensity in arbitrary units, c T ₂-weighted image with overlaid colour-encoded ²³Na image (Haneder et al., reprinted with permission from [51])

Fig. 15

Different array coils and setups for renal 7 T imaging: a dorsal array of 8-channel array with microstrip meander elements used by Umutlu et al. (reprinted with permission from [36]). These elements consist of a central conductor over a ground plane, which are connected to each other by capacitors at both ends of the element while the element is fed in the centre. At each end of the element, extra inductance is added by a meander in the central conductor that effectively lowers SAR and reduces inter-element coupling; b setup of a; c two coil elements and d setup used by Hoogduin et al. [53]. The array consists of eight ‘fractionated dipole antennas’. Here, the legs of each dipole are divided into segments and the segments are interconnected by meanders (inductors). This element structure also reduces SAR levels and coupling in comparison to plain dipoles; e anterior array of 16-channel microstrip array used by Metzger et al. [47] (reprinted with permission from [55]). Here, a conductor is placed over a ground plate with capacitors connecting the two at both ends of the element (f). Capacitive coupling is used between the conductors and ground planes of adjacent elements to permit closer element spacing and higher element density. This element is driven from one side, over one of the connecting capacitors

Fig. 16

Survey pre (upper row) and post (lower row) RF shimming. Only phase shimming was performed. Arrows region of destructive interference in the left kidney in two of three images acquired before RF shimming, disappearing after shimming (Hoogduin et al. [37])

Fig. 17

Local SAR distributions (10 g averaged) for two phase-amplitude settings that are both designed for constructive interference of B1 in the kidneys. Distributions are in the transverse plane crossing the maximum value in the distribution. Values are for 8 × 800 W, with 1 % duty cycle (Raaijmakers, unpublished results)