Recent advances in DWI have facilitated the application of IVIM model in the separate estimation of tissue perfusion and diffusivity [1]. IVIM parameters have shown sensitivity to renal allograft rejection and vascularity and cellularity of renal masses. The sequence most commonly used in renal DWI is based on single-shot echo-planar imaging (SS-EPI), which is prone to artifacts and distortions related to susceptibility and eddy currents. Reducing these artifacts and distortions in SS-EPI generally requires a reduced field of view (rFOV) in the phase-encoding direction and/or reduced spatial resolution. To resolve these problems, a 2D rFOV-DWI sequence was introduced for imaging and further IVIM modeling. With a 2D-RF excitation pulse [2], the required number of k-space lines in the PE direction is decreased, enabling higher resolution imaging with a proper scan time.We used a combined IVIM-rFOV method to evaluate renal diffusion and perfusion.

MR Imaging: All the MRI measurements were carried out on a 3.0 Tesla MR scanner (Achieva, Philips Medical Systems, Best, Netherlands). DWI was performed using full-FOV DWI and rFOV-DWI sequences, with the following b-values used for each acquisition: 0, 20, 40, 60, 80, 100, 150, 200, 300, 400, 500, 600, 700, 800, 900 and 1000 sec/mm². The diffusion gradients were applied in three orthogonal directions. Other imaging parameters are demonstrated in Table 1. For renal DWI imaging, a nonselective fat saturation pulse was applied to suppress fat signal, and respiratory triggering was applied to make the sequence motion insensitive.

Study Protocol: To demonstrate the efficacy of this rFOV-DWI technique, we performed a comparison study using a quality control phantom. The phase-encoding (PE) is along left-right direction. For renal imaging, twelve healthy young volunteers (22-26 years) were enrolled in this IRB-approved study. To assess the within-session and between-day reproducibilities of the the proposed method, both full-FOV DWI and rFOV DWI sequences were repeated in a 20 min interval and on a second day.

Quantitative Analysis: To mitigate respiratory motion artifacts, two-dimensional affine registration of DWI images was performed separately for the right and left kidney with an in-house software. ADC was also calculated using a monoexponential model with 16 b values. True diffusion coefficient D, pseudodiffusion (perfusional) coefficient D*, fraction of pseudo-diffusion f, were estimated using bixeponential model (least squares fitting) according to the following equation: M = M₀(f·exp(-b·D)+(1-f)·exp(-b·D*)).

Two independent observers (with 7 years and 6 years of experience in abdominal MRI) rated the image quality for both the full-FOV DWI and the rFOV-DWI images and assigned a score with a 5-point scale (1 = poor image quality; 2 = acceptable image quality; 3 = fair image quality; 4 = good image quality; 5 = excellent image quality).

Image quality: DWI images of phantom and coronal kidney acquired using full-FOV DWI and rFOV DWI sequences are shown in Fig.2 and Fig.3. The rFOV-DWI sequence (Fig.1 (b,d)) effectively reduced spatial distortion at the edge of the phantom. For renal DWI, the full-FOV DWI image (Fig.2a) showed strong spatial blurring along the PE direction, and this blurring was dramatically reduced in the rFOV-DWI image (Fig.2b) due to higher spatial resolution and shorter ETL. Table 2 summarizes the image quality scores of full-FOV DWI images and rFOV-DWI images.

IVIM parameters: The rFOV-DWI based original images (τ = 0 ms) and calculated parametrical maps showed significantly better image quality with sufficient depiction of the cortico-medullary structure of the kidneys (Table 1). The ADC, D, and f values derived from rFOV-DWI images were each significantly higher in the cortex than in the medulla (P < 0.05 for each comparison). These results were consistent with previously reported literature values [3]. The within-session CVs of ADC measurement were 8.05% in the cortex and 5.02% in the medulla, while the between-day CVs of OEF measurement are 8.45% in the cortex and 5.02% in the medulla.

With phantom and volunteer studies we demonstrated the feasibility of using rFOV-EPI techniques for single renal DWI imaging to either reduce artifacts and distortion caused by susceptibility differences or limit spatial blurring. Compared with conventional full-FOV single-shot DWI techniques, rFOV-DWI methods generally produced images of superior quality. With the application of IVIM model, it is possible to evaluate single renal diffusion and perfusion simultaneously.