CT of thoracic aortic disease.

The role of CT continuous to expand. However, competing modalities and their value and limitations for the same clinical indication should also be understood. In addition, CT radiation patient dose must be justified in the context of adding diagnostic value. Techniques for quantitative CT and their utility for specific applications are important, and protocols must be tailored to answer the clinical need. Finally, newer post-processing techniques, including surface rendering, maximum intensity projections (MIP), 3-D displays and multimodality image integration can certainly be helpful for diagnosis and usually they improve the referring clinicians' comprehension, however these methods may not necessarily provide additional diagnostic information.

Valvular heart disease.

Congenital and acquired valvular disease remains a frequent cause of morbidity and mortality. It presents a diagnostic challenge in all age groups, and often occurs in conjunction with other types of heart disease. Traditional chest radiography provides the earliest opportunity for radiologic diagnosis, hence the need for skill and knowledge in interpreting the radiographic findings. Echocardiography with color flow Doppler measurements is frequently the next modality applied. CT and MR imaging can simultaneously display cardiovascular morphology with greater spatial resolution than ultrasound, and at the same time provide quantitative assessment of cardiac function. The role of diagnostic imaging is therefore crucial, both for primary diagnosis and in the management of valvular heart disease. Furthermore, it is fundamental in evaluating the results of all forms of interventional therapy.

In vivo imaging of extraction fraction of low molecular weight MR contrast agents and perfusion rate in rodent tumors.

Tissue uptake of a fully extractable MR detectable tracer, deuterated water (D2O), was compared with that of a less extractable contrast agent, Gadolinium-DTPA-dimeglumine (Gd-DTPA), in rodent tumor and muscle tissue. This dual tracer method allowed calculation of relative (to muscle) tissue perfusion and extraction fraction of Gd-DTPA in each image pixel in vivo. Solutions of Gd-DTPA and D2O were injected intravenously into Fisher female rats (n = 9) with R3230 mammary adenocarcinomas implanted in the hind limb. Perfusion rate was approximately two times greater (P < 0.005 by paired t test) in tumor than in muscle. Gd-DTPA extraction fraction at the interface between tumor and muscle was 2.0 times the extraction fraction in normal muscle (P < 0.005 by paired t test). Extraction fraction at the tumor center was 1.6 times the extraction fraction in muscle (P < 0.01 by paired t test). High extraction fraction of Gd-DTPA correlated with high capillary permeability determined from Evans Blue staining. Low molecular weight Gd-DTPA derivatives are widely used in clinical practice, and their extraction fractions are crucial determinants of image contrast during the first few passes of the contrast agent bolus. Therefore spatially resolved measurements of contrast agent extraction fractions obtained in vivo have significant clinical utility. The data demonstrate that extraction of low molecular weight tracers is sensitive to increased permeability in tumor vasculature and that this increased permeability can be imaged.

Splenic blood flow: evaluation with computed tomography.

RATIONALE AND OBJECTIVES: To study splenic perfusion with use of computed tomography (CT). METHODS: Twenty-six control patients without splenoportal disease, six with cirrhosis, and seven with other splenic disease were examined with electron-beam CT. Twenty-five milliliters of iohexol (300 mg of iodine per milliliter) was given intravenously at 10 mL/sec followed by a saline bolus. Multiple single-level axial sections were acquired 8-90 seconds after injection. Perfusion was calculated by dividing maximal splenic enhancement by the area under the circulation-corrected aortic time-enhancement curve. Subjective assessments of enhancement heterogeneity were made, and regional perfusion was calculated in 10 patients with heterogeneous enhancement. Total splenic volume and blood flow were computed in 21 patients. RESULTS: Mean perfusion (controls: 1.29 mL/min/mL, miscellaneous group: 1.07 mL/min/mL) was close to predictions. There was a trend toward lower perfusion in cirrhotic patients (0.87 mL/min/mL), but the difference was not statistically significant. Total splenic blood was increased in patients with cirrhosis (P < .01). Marked perfusion heterogeneity was observed in 41% of spleens, but by 2 minutes splenic enhancement was uniform. CONCLUSION: CT shows promise in the study of splenic blood flow.

Differentiating between T1 and T2* changes caused by gadopentetate dimeglumine in the kidney by using a double-echo dynamic MR imaging sequence.

Dynamic MR images of the passage of gadopentetate dimeglumine through the kidneys of normal rats are obtained using a dual gradient-echo sequence. The amplitudes of gradient echoes are defined by local T1 and T2* values in the tissue. The ratio of these amplitudes, primarily defined by local T2*, can be used to differentiate between T1 and T2* effects. This is particularly important with regard to renal studies because, due to a highly inhomogeneous distribution of gadopentetate dimeglumine in the kidney, T2* shortening can impede MR data analysis. To study changes in the observed signal caused by gadopentetate dimeglumine, curves of MR renal intensity versus time were obtained in the cortex and medulla after administration of the contrast agent. Using T2* compensation, distinct temporal peaks were observed in the cortex and outer medulla, indicating a high concentration of gadopentetate dimeglumine in the vascular phase. The authors conclude that this technique can be a useful tool for studying renal function noninvasively.

Liver perfusion studied with ultrafast CT.

OBJECTIVE: Our goal was to quantify absolute hepatic arterial and portal venous perfusion noninvasively in patients with and without liver disease using ultrafast CT. MATERIALS AND METHODS: A single slice through the porta hepatis was repeatedly scanned after bolus injection of 25 ml of iohexol 300 mg I/ml, followed by a 25 ml saline "chaser" intravenously at 10 ml/s. Thirty-nine controls, 7 cirrhotic patients, and 5 patients with known metastases on the slice plane were studied; hepatic arterial perfusion was determined in 41 patients and portal venous perfusion in 24. Time-attenuation curves from regions of interest drawn over the liver, spleen, aorta, and portal vein were analysed. Hepatic arterial perfusion was calculated by dividing the peak gradient of the liver time-attenuation curve prior to the time of peak splenic attenuation by the peak aortic CT number increase. Splenic perfusion was calculated by dividing the peak gradient of the splenic time-attenuation curve by the peak aortic CT number increase. Portal perfusion was derived by scaling the splenic time-attenuation curve by the ratio of hepatic arterial/splenic perfusion. This scaled curve was subtracted from the liver time-attenuation curve to give a portal curve. The peak up-slope of this curve was divided by the peak rise in splenic or portal vein density. RESULTS: Hepatic arterial perfusion averaged 0.19 ml/min/ml (n = 31) in controls and was raised in cirrhosis to 0.25 ml/min/ml (n = 6) and metastases 0.43 ml/min/ml (n = 4). Portal venous perfusion was 0.93 ml/min/ml (n = 19) in controls and 0.43 ml/min/ml (n = 4) in cirrhosis. Reproducibility has been confirmed. CONCLUSION: Dynamic ultrafast CT shows potential in quantifying arterial and portal hepatic perfusion. The technique may be adaptable to dynamic bolus MRI.

Magnetization transfer magnetic resonance imaging of parenchymal lung disease.

RATIONALE AND OBJECTIVES: Magnetic resonance imaging with magnetization transfer (MT) contrast recently has been described as a method that may provide additional information about the macromolecular composition of tissue. Magnetization transfer contrast images were compared to conventional gradient-recalled echo images in a variety of pulmonary parenchymal diseases and normal lung. METHODS: Single-slice gradient echo images were obtained with and without an off-resonance radio frequency pulse on a 0.1T MR scanner. The change in signal intensity between identical regions of interest on non-MT and MT images was determined in 13 patients with known lung disease, five healthy volunteers, and three postmortem atelectatic dog lungs. RESULTS: No significant change in signal intensity (MT effect) was observed in fat, flowing blood, normal lung, atelectatic lung, or in acute pulmonary edema. Chronic parenchymal lung disease showed the greatest MT effect, 37.7% +/- 7.5. Acute infectious lung disease showed an intermediate degree of MT effect, 19.5% +/- 3.0. CONCLUSIONS: Magnetization transfer contrast magnetic resonance imaging of pulmonary disease is feasible at low field strength and may be useful in the characterization and differentiation of pulmonary parenchymal abnormalities. Magnetization transfer contrast appears to be proportional to the amount of interstitial fibrosis in lung parenchyma, while acute inflammatory cell infiltration exhibits less MT effect and acute pulmonary edema exhibits very little.