MR evaluation of solid renal masses

A practical imaging classification for the non-invasive differentiation of renal cell carcinoma into its main subtypes

AIM

Renal cell carcinoma (RCC) is a heterogeneous disease which encompasses various subtypes that exhibit differing biologic behavior and imaging findings. Non-invasive subtype differentiation by imaging facilitates prognostication and treatment selection. The aim of the study was to evaluate the performance of a diagnostic imaging key based on tumor morphology, T2 signal intensity on MRI, and tumor vascularity for differentiating RCC into its subtypes.

MATERIALS AND METHODS

Using a custom-designed diagnostic imaging key, three blinded fellowship-trained abdominal radiologists independently evaluated the cross-sectional imaging of 50 histologically proven RCCs and categorized these into subtypes in two sessions. The diagnostic performance of the imaging key was evaluated and compared to the baseline performance without the key.

RESULTS

The 50 RCCs comprised 20 (40%) clear cell, 17 (34%) papillary, and 13 (26%) chromophobe tumors. All expert readers demonstrated an improvement in diagnostic accuracy by an average of 5.3% with the use of the key. The readers showed good to excellent diagnostic performance for clear cell RCC (area under the receiver operating curve, AUROC of 0.86-0.91) and papillary RCC (AUROC of 0.82-0.87), and fair performance with chromophobe RCC (AUROC of 0.67-0.77). The Reader-to-SOR (standard of reference) agreement increased from 0.53 (moderate) to 0.67 (good) with the use of the key.

CONCLUSION

The diagnostic imaging key facilitates RCC subtype characterization and can be used as a decision support tool.

Vascular staging of renal and adrenal malignancies with a noncontrast enhanced steady state free precession technique

PURPOSE

To compare a noncontrast enhanced balanced steady state free precession (bSSFP) MRI technique with a conventional dynamic contrast-enhanced (DCE) three-dimensional (3D) spoiled gradient recalled echo (SPGR) imaging in the vascular staging of renal and adrenal malignancies.

MATERIALS AND METHODS

Sixty-three MRIs with both bSSFP and DCE acquisitions performed for initial staging of renal and adrenal malignancies were retrospectively evaluated for presence and extent of thrombus in the renal veins and inferior vena cava (IVC). Thrombus characterization was also evaluated. DCE imaging was used as the standard-of-reference. Histopathology was available in 46 of 63 cases as an additional external standard.

RESULTS

There is very good agreement between bSSFP and DCE imaging for determining the presence or absence of thrombus in the renal veins (r = 0.95; P < 0.0001) and IVC (r = 0.91; P < 0.0001). BSSFP is less successful at distinguishing bland from tumor thrombus.

CONCLUSION

Noncontrast enhanced bSSFP is an acceptable alternative to DCE imaging for vascular staging of locally advanced renal/adrenal malignancies, with somewhat limited ability to distinguish bland from tumor thrombus.

Magnetic resonance urography

Excellent contrast resolution and lack of ionizing radiation make magnetic resonance urography (MRU) a promising technique for noninvasively evaluating the entire urinary tract. While MRU currently lags behind CT urography (CTU) in spatial resolution and efficiency, new hardware and sequence developments have contributed to a resurgence of interest in MRU techniques. By combining unenhanced sequences with multiphase contrast-enhanced and excretory phase imaging, a comprehensive assessment of the kidneys, ureters, bladder, and surrounding structures is possible with image quality rivaling that obtained with other techniques. At the same time, formidable challenges remain to be overcome and further clinical validation is necessary before MRU can replace other forms of urography. In this article, we demonstrate the current potential of MRU to demonstrate a spectrum of urologic pathology involving the kidneys, ureters, and bladder while discussing the limitations and current status of this evolving technique.

Lower limits of detection using magnetic resonance imaging for solid components in cystic renal neoplasms

OBJECTIVES

We determined the positive predictive value (PPV) for malignancy in complex renal cysts with focal nodular enhancement seen on magnetic resonance imaging (MRI).

METHODS

A surgical database was reviewed to identify all patients having both a preoperative 3 dimensional (3D) renal MRI and a radical or partial nephrectomy from January 2000 through April 2004 at our hospital. A group of 21 patients were identified with focal nodular enhancement within cystic renal masses. Pathologic correlation was made in each case.

RESULTS

We performed 286 nephrectomies during the study period. Of these patients, 159 (56%) had a preoperative MRI studies. There were 21 of 159 (13%) patients with complex cystic lesions that displayed focal nodular enhancement, 14 of which (67%) measured 10 mm or larger in size. Twenty (95%) of the 21 lesions were renal cell carcinoma. The single, benign lesion was a cystic nephroma. Fuhrman grade 1 or grade 2 cancers were found in the majority of patients (80%), and there were no grade 4 cancers. Fifteen patients had a preoperative computerized tomography (CT) scan as well and nodular enhancement was suspected in only 4 patients (27%). MRI findings upgraded these lesions in 11 patients (73%).

CONCLUSIONS

The demonstration of solid enhancing nodular components with high-resolution 3D MRI provides excellent positive predictive value for diagnosing neoplastic cystic renal lesions, including a large percentage 10 mm or larger in size. Our experience suggests a 95% likelihood that cystic renal lesions with focal nodular enhancement are malignant. We recommend that such lesions be considered malignant.

MR urography: techniques and clinical applications

Magnetic resonance (MR) urography comprises an evolving group of techniques with the potential for allowing optimal noninvasive evaluation of many abnormalities of the urinary tract. MR urography is clinically useful in the evaluation of suspected urinary tract obstruction, hematuria, and congenital anomalies, as well as surgically altered anatomy, and can be particularly beneficial in pediatric or pregnant patients or when ionizing radiation is to be avoided. The most common MR urographic techniques for displaying the urinary tract can be divided into two categories: static-fluid MR urography and excretory MR urography. Static-fluid MR urography makes use of heavily T2-weighted sequences to image the urinary tract as a static collection of fluid, can be repeated sequentially (cine MR urography) to better demonstrate the ureters in their entirety and to confirm the presence of fixed stenoses, and is most successful in patients with dilated or obstructed collecting systems. Excretory MR urography is performed during the excretory phase of enhancement after the intravenous administration of gadolinium-based contrast material; thus, the patient must have sufficient renal function to allow the excretion and even distribution of the contrast material. Diuretic administration is an important adjunct to excretory MR urography, which can better demonstrate nondilated systems. Static-fluid and excretory MR urography can be combined with conventional MR imaging for comprehensive evaluation of the urinary tract. The successful interpretation of MR urographic examinations requires familiarity with the many pitfalls and artifacts that can be encountered with these techniques.

MRI of the kidney-state of the art

Ultrasound and computed tomography (CT) are modalities of first choice in renal imaging. Until now, magnetic resonance imaging (MRI) has mainly been used as a problem-solving technique. MRI has the advantage of superior soft-tissue contrast, which provides a powerful tool in the detection and characterization of renal lesions. The MRI features of common and less common renal lesions are discussed as well as the evaluation of the spread of malignant lesions and preoperative assessment. MR urography technique and applications are discussed as well as the role of MRI in the evaluation of potential kidney donors. Furthermore the advances in functional MRI of the kidney are highlighted.

Dynamic contrast-enhanced magnetic resonance imaging for assessing tumor vascularity and vascular effects of targeted therapies in renal cell carcinoma

Traditional cross-sectional tumor imaging focuses solely on tumor morphology. With the introduction of targeted biological therapies in human trials, morphologic change may lag behind other physiologic measures of response on clinical images. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is a new imaging method for assessing the physiologic state of tumor vascularity in vivo. DCE-MRI, which uses available imaging techniques and contrast agents, assays the kinetics of tumor enhancement during bolus i.v. contrast administration. Modeling of the temporal enhancement pattern yields physiologic variables related to tumor blood flow and microvessel permeability. Changes in these variables after vascular-targeted therapy can then be quantified to evaluate the tumor vascular response. As these responses may precede morphologic tumor shrinkage, DCE-MRI might serve as a noninvasive means of monitoring early tumor response to vascular-targeted therapy. Renal cell carcinoma provides an excellent model for assessing the effect on DCE-MRI in clinical trials. The vascular richness of renal tumors provides a large dynamic scale of DCE-MRI measures. Patients with disseminated renal cell carcinoma frequently present with one or several large tumors, creating an easy imaging target for DCE-MRI evaluation. Finally, renal cell carcinoma is clearly susceptible to therapies that target tumor angiogenesis. DCE-MRI can be used to monitor the vascular changes induced by such therapies. Future efforts must be directed to standardizing image acquisition and analysis techniques to quantify tumor vascular responses.