Arterial input functions for dynamic susceptibility contrast MRI: requirements and signal options.

Cerebral perfusion imaging using dynamic susceptibility contrast (DSC) has been the subject of considerable research and shows promise for basic science and clinical use. In DSC, the MRI signals in brain tissue and feeding arteries are monitored dynamically in response to a bolus injection of paramagnetic agents, such as gadolinium (Gd) chelates. DSC has the potential to allow quantitative imaging of parameters such as cerebral blood flow (CBF) with a high signal-to-noise ratio (SNR) in a short scan time; however, quantitation depends critically on accurate and precise measurement of the arterial input function (AIF). We discuss many requirements and factors that make it difficult to measure the AIF. The AIF signal should be linear with respect to Gd concentration, convertible to the same concentration scale as the tissue signal, and independent of hematocrit. Complicated relationships between signal and concentration can violate these requirements. The additional requirements of a high SNR and high spatial/temporal resolution are technically challenging. AIF measurements can also be affected by signal saturation and aliasing, as well as dispersion/delay between the AIF sampling site and the tissue. We present new in vivo preliminary results for magnitude-based (DeltaR2*) and phase-based (Deltaphi) AIF measurements that show a linearity advantage of phase, and a disparity in the scaling of Deltaphi AIFs, DeltaR2* AIFs, and DeltaR2* tissue curves. Finally, we discuss issues related to the choice of AIF signal for quantitative perfusion imaging.

Small renal cell carcinomas: correlation of size with tumor stage, nuclear grade, and histologic subtype.

OBJECTIVE: Our goal was to correlate the size of renal cell carcinoma with tumor stage, nuclear grade, and histologic subtype in patients treated using partial or radical nephrectomy. MATERIALS AND METHODS: We retrospectively reviewed 213 consecutive renal cell carcinomas resected at our institution from 1995 through 1999. Three groups of lesions stratified by size (< or = 3 cm, > 3-5 cm, > 5 cm) were compared with regard to pathologic findings. Statistical significance was assessed using Fisher's exact test. RESULTS: Of 50 lesions 3 cm or smaller, 19 (38%) had extension outside the renal capsule (T3 or T4) and 14 (28%) were a high nuclear grade (Fuhrman grade 3 or 4). Lesions 3 cm or smaller and those greater than 3 cm to 5 cm did not differ statistically with regard to T stage or nuclear grade. Lesions larger than 5 cm showed a statistically higher T stage (p < 0.001) and nuclear grade (p = 0.001) than the other smaller lesions. More non-clear cell tumors were found in the two groups of smaller lesions (p = 0.105) but without statistical significance. The majority (58%) of the tumors were asymptomatic and had been detected incidentally on cross-sectional imaging. Lesions larger than 5 cm were significantly more likely to be symptomatic (p < 0.001). Seventy-nine percent of the tumors 3 cm or smaller were incidental, and these lesions did not differ significantly from the symptomatic lesions with regard to stage, grade, or histology. CONCLUSION: In our study population, renal cell carcinomas up to 3 cm, including asymptomatic lesions, showed a significant incidence of high nuclear grade and tumor extension beyond the renal capsule; these findings support aggressive management of small lesions. Symptomatic status was not an adequate discriminator to guide management. A longitudinal study is necessary to further evaluate the efficacy of current patterns of therapy.