Low-Tube-Voltage CT Urography Using Low-Concentration-Iodine Contrast Media and Iterative Reconstruction: A Multi-Institutional Randomized Controlled Trial for Comparison with Conventional CT Urography

Abstract

Objective: To compare the image quality of low-tube-voltage and low-iodine-concentration-contrast-medium (LVLC) computed tomography urography (CTU) with iterative reconstruction (IR) with that of conventional CTU.

Materials and methods: This prospective, multi-institutional, randomized controlled trial was performed at 16 hospitals using CT scanners from various vendors. Patients were randomly assigned to the following groups: 1) the LVLC-CTU (80 kVp and 240 mgI/mL) with IR group and 2) the conventional CTU (120 kVp and 350 mgI/mL) with filtered-back projection group. The overall diagnostic acceptability, sharpness, and noise were assessed. Additionally, the mean attenuation, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and figure of merit (FOM) in the urinary tract were evaluated.

Results: The study included 299 patients (LVLC-CTU group: 150 patients; conventional CTU group: 149 patients). The LVLC-CTU group had a significantly lower effective radiation dose (5.73 ± 4.04 vs. 8.43 ± 4.38 mSv) compared to the conventional CTU group. LVLC-CTU showed at least standard diagnostic acceptability (score ≥ 3), but it was non-inferior when compared to conventional CTU. The mean attenuation value, mean SNR, CNR, and FOM in all pre-defined segments of the urinary tract were significantly higher in the LVLC-CTU group than in the conventional CTU group.

Conclusion: The diagnostic acceptability and quantitative image quality of LVLC-CTU with IR are not inferior to those of conventional CTU. Additionally, LVLC-CTU with IR is beneficial because both radiation exposure and total iodine load are reduced.

Keywords: Computed tomography; Contrast media; Double dose reduction; Low dose; Urography.

Figures

Fig. 1. Inclusion criteria, exclusion criteria, and flowchart for enrollment of study population.

Nineteen patients withdrew consent prior to CTU. Twelve patients were excluded for following reasons: 1) age > 70 years (n = 4); 2) abnormal renal function test results (n = 3); 3) metformin usage (n = 3); and 4) history of urinary tract obstruction (n = 2). Additionally, 8 patients were excluded for following reasons: 1) protocol violation (n = 5); 2) follow-up loss (n = 2); and 3) limitation of image assessment owing to incidental detection of urinary tract obstruction by urinary stone (n = 1). CTU = computed tomography urography, FBP = filtered-back projection, IR = iterative reconstruction

Fig. 2. Quantitative measurements of urinary tract, renal parenchyma, and psoas muscle.

Mean attenuation values (HU) were measured in contrast-filled regions of urinary tract including major calyx (A), renal pelvis and parenchyma (B), upper ureter and psoas muscle (C), lower ureter (D), and urinary bladder (E) with manually drawn circular ROIs (red circles). Sizes of ROIs for contrast-filled pelvocalyces and ureters were approximately 10–20 mm2 in axial or coronal images that better visualized urinary tract. Sizes of ROIs for renal parenchyma, psoas muscle, and bladder were approximately 40–100 mm2. Care was taken to avoid vessels, prominent artifacts (i.e., streak artifacts), and heterogeneous enhancing areas in renal parenchyma (i.e., focal scarring) and to place ROI in most homogeneous area. Urinary tract was measured on both sides separately (only right side was shown). HU = Hounsfield unit, ROIs = regions of interest

Fig. 3. Image sharpness and image noise for LVLC-CTU with IR and conventional CTU with FBP.

Noise and sharpness scores for LVLC-CTU with IR (2.55 ± 0.24) were significantly lower than those for conventional CTU with FBP (2.66 ± 0.17, p < 0.001). However, LVLC-CTU with IR showed at least acceptable noise and average sharpness (score of 2). BMI = body mass index, LVLC = low-tube-voltage and low-iodine-concentration-contrast-medium

Fig. 4. Mean attenuation measured using LVLC-CTU with IR and conventional CTU with FBP.

Mean attenuation value for LVLC-CTU with IR was significantly higher than that for conventional CTU with FBP in all pre-defined segments of urinary tract including pelvocalyces on both sides and urinary bladder.

Fig. 5. Multi-planar reconstructed images of excretory phase in CTU.

In conventional CTU with FBP images acquired from patients with high BMI (A) and low BMI (B), filling defects due to polypoid tumor (arrow) (A) or trabeculated muscle (arrow) (B) were well visualized with high contrast-to-noise ratio. In LVLC-CTU with IR images acquired from patients with high BMI (C) and low BMI (D), collecting systems, including calyces and urinary bladder, showed high attenuation. Diagnostic acceptability scores were comparable between two protocols. In maximum intensity projection images of urinary tract involving LVLC with IR (E) or conventional CTU with FBP (F), urinary tract was well visualized with LVLC-CTU compared to conventional CTU, despite injection of iodine contrast with low osmolality.