Comparison of MDCT, MRI and MRI with diffusion-weighted imaging in evaluation of focal renal lesions: The defender, challenger, and winner!

Abstract

Purpose: To compare the diagnostic performance of multidetector computed tomography (MDCT), magnetic resonance imaging (MRI), and MRI with diffusion-weighted imaging (DWI) in the characterization of focal renal lesions. We also compared MDCT and MRI in the staging of renal cell carcinoma (RCC).

Materials and methods: One hundred and twenty adult patients underwent MDCT (40-row and 128-row scanners), MRI (at 1.5 T), and DWI (at b-values of 0 and 500 s/mm²) for characterization of 225 renal lesions. There were 65 malignant neoplasms (44 RCCs), 25 benign neoplasms, 25 abscesses, 45 pseudotumors, 15 hemorrhagic cysts, and 50 benign cysts. A composite gold standard including histology, typical imaging criteria, and follow-up imaging was employed. To determine the diagnostic performance of imaging modalities, area-under-curve (AUC) was calculated by receiver-operating-characteristic analysis and compared. Fisher's exact test was used to compare the diagnostic accuracies and confidence levels with MDCT, MRI, and MRI + DWI. Cross-tabulation was used to assess the precision of MDCT and MRI in RCC staging.

Results: AUC for MDCT (0.834) and MRI (0.841) in the classification of benign and malignant lesions were within corresponding 95% confidence interval (CI) (P = 0.88) whereas MRI + DWI had significantly better performance (AUC 0.968, P = 0.0002 and 0.0004, respectively). Both CT and MRI had low specificity (66.9% and 68.8%, respectively), which increased substantially with DWI (93.8%) owing to correct diagnosis of pseudotumors. MRI was superior to CT in diagnosing necrotic RCC and hemorrhagic cysts. MRI + DWI had the highest accuracy (94.2%) in assigning the definitive diagnosis and 97.6% lesions were diagnosed with very high confidence, significantly better than CT and MRI. Both CT and MRI had the same accuracy (86.1%) in RCC staging and evaluation of intravascular thrombi.

Conclusions: Characterization of renal lesions was most accurate with MRI + DWI. The latter is also the most suitable modality in diagnosing pseudotumors and evaluating patients with renal dysfunction. CT and MRI were equivalent in RCC staging.

Keywords: Contrast media; MRI; diffusion-weighted MRI; focal renal lesion; multidetector CT; renal cell carcinoma.

Figure 1

Graphical comparison of ROC curves for MDCT (dotted black line), MRI (dashed red line), and MRI + DWI (dot and dash purple line) in the differentiation of benign and malignant renal lesions. AUC is the largest for MRI + DWI (0.968) and similar for CT (0.834) and MRI (0.841). Straight diagonal green line spanning the middle of the graph indicates an AUC of 0.5

Figure 2 (A-C)

Pseudotumour in diseased kidney in a 45-year-old female patient, not diagnosed on CT or MR but correctly interpreted on DWI. (A) Coronal unenhanced CT image shows ill-defined, mildly hypodense, exophytic mass lesion (asterisk) in the lower pole of the right kidney. Contrast could not be administered because of renal dysfunction. (B) Axial T2-weighted MR image shows that the lesion is hyperintense (asterisk). Solid “ball-type” morphology and hyperintensity on T2-weighted image raised the suspicion of malignant mass on CT and MRI. (C) Axial ADC map (generated from DWI) demonstrates the lesion to be hyperintense (asterisk) suggesting no diffusion restriction within the lesion. Absence of diffusion restriction within a solid mass lesion ruled out malignancy. Ultrasound-guided biopsy did not reveal any evidence of malignancy

Figure 3

Distribution of the degree of diagnostic confidence on various imaging modalities. MRI + DWI diagnosed 97.6% of the renal lesions with very high confidence

Figure 4 (A-D)

A 49-year-old male patient presented with right flank pain, fever, and polymorphonuclear leukocytosis. (A) Axial contrast-enhanced CT image shows an ill-defined heterogeneously enhancing mass lesion (arrowhead) in the medial cortex of interpolar region of the right kidney with infiltration into perirenal space. Considering the clinical details, diagnosis of abscess was made but confidence level of making this diagnosis was just average. (B) Axial T2-weighted MR image adds to the diagnostic confidence by demonstrating central hyperintense region surrounded by thick hypointense rim (arrowhead). Note is also made of a hypointense band in the lateral cortex of the kidney (arrow) and perirenal fascial thickening. (C) Axial ADC map demonstrates very dark signal in the lesion (arrowhead) indicative of marked restriction of diffusion, especially in the T2 hyperintense fluid component, thus making the diagnosis of renal abscess almost certain. The hypointense band seen previously on T2-weighted MR image also shows restricted diffusion. (D) Axial ADC map (at cranial level) shows multiple bands (some of them are wedge-shaped) of restricted diffusion (arrowheads) in the renal parenchyma suggesting acute pyelonephritis, further favouring the inflammatory etiology. Follow-up MR imaging (not shown) after 6 weeks of intravenous antibiotics showed near-complete resolution of the abscess

Figure 5 (A-E)

Comparison of MDCT, MRI, and DWI in demonstrating malignant intravascular thrombus. (A) Coronal arterial phase CT image shows lobulated mass in the right kidney with contiguous thrombotic extension into right renal vein and inferior vena cava (IVC). The intravascular thrombus is showing arterial enhancement (asterisk), indicative of neovascularity, suggesting malignant thrombus. (B) Coronal venous phase CT image shows heterogeneous enhancement in the primary mass as well as the contiguous thrombus. Note that the thrombus is seen as nonocclusive filling defect in the IVC and is causing expansion of the involved segments. Also note that the entire extent of the thrombus is well seen on different phases of MDCT and clearly depicted in coronal reconstructions. (C) Coronal T2-weighted MR image beautifully demonstrates the mass and the thrombus to be of similar high signal intensity, further favoring malignant thrombus. Corresponding to the areas of neovascularity in the malignant thrombus, flow voids are seen on T2-weighted images (asterisk). (D) Coronal venous phase MRI image shows heterogeneous enhancement in the primary mass as well as the contiguous thrombus and gives similar information as contrast-enhanced CT. (E) Axial ADC map shows dark signal within the renal mass (asterisk) as well as the IVC thrombus (arrowhead), indicating restricted diffusion. The ADC values of primary mass and IVC thrombus were comparable [1.12 and 1.09 (×10^-3 mm²/s), respectively]. Presence of restricted diffusion with ADC values similar to that of primary mass endorse the diagnosis of malignant thrombus. The malignant thrombus is extending into the retrohepatic IVC below the level of diaphragm and the cranial most extent is same in both CT and MR (A-D)