Sanjay Saini

Hepatobiliary-specific MR Contrast Agents: Role in Imaging the Liver and Biliary Tree

Hepatobiliary-specific contrast agents are one of several classes of contrast agents available for magnetic resonance (MR) imaging of the liver. These agents are taken up by functioning hepatocytes and excreted in the bile, and their paramagnetic properties cause shortening of the longitudinal relaxation time (T1) of the liver and biliary tree. The three contrast agents that have been developed are mangafodipir trisodium (Mn-DPDP), gadobenate dimeglumine (Gd-BOPTA), and gadoxetic acid (Gd-EOB-DTPA). These three MR contrast agents vary in mode of administration and dose, mechanism of cellular uptake, degree of excretion through the biliary pathway, and imaging characteristics. In the liver, hepatobiliary-specific agents can be used to improve lesion detection, to characterize lesions as hepatocellular or nonhepatocellular, and to specifically characterize some hepatocellular lesions, notably focal nodular hyperplasia. Biliary excretion of these agents can be used to evaluate the anatomic structure and function of the biliary tree. In the future, hepatobiliary-specific contrast agents may have wider applications, such as grading of cirrhosis and quantification of liver function.

Comparison of Angular and Combined Automatic Tube Current Modulation Techniques with Constant Tube Current CT of the Abdomen and Pelvis

OBJECTIVE The objective of our study was to compare image quality and radiation dose associated with abdominopelvic CT using combined modulation, angular modulation, and constant tube current. CONCLUSION Compared with using a constant tube current to scan the abdomen and pelvis, the use of a combined modulation technique results in a substantial reduction (42-44%) in radiation dose with acceptable image noise and diagnostic acceptability.

Cystic pancreatic neoplasms Evaluation by CT and magnetic resonance cholangiopancreatography

CT provides limited assistance in the differentiation between serous and mucinous neoplasms. Because of the variability in the radiographic appearance of serous cystadenomas and overlap in CT characteristics with mucinous neoplasms, most serous neoplasms still require ancillary testing such as biopsy to reach a definitive diagnosis. MRCP is useful in differentiating benign and malignant mucinous tumors including IPMT of the pancreas. The presence of mural nodules is suggestive of malignancy; however, the absence of mural nodules does not indicate that the tumor is benign. A maximum main pancreatic duct diameter of greater than 15 mm and diffuse dilatation of the main pancreatic duct are suggestive of malignancy in main duct-type tumors. Among branch duct-type tumors, malignant tumors tend to be larger than benign tumors; however, this finding is variable. The presence of main pancreatic duct dilatation may be helpful in determining malignancy of branch duct-type tumors.

Using the K-edge to improve contrast conspicuity and to lower radiation dose with a 16-MDCT: a phantom and human study.

Purpose: To study the effect of tube current (milliamperes; mA) and potential (peak kilovolt; kVp) on the attenuation values and contrast-to-noise ratios of iodine- and gadolinium-based contrast media (CM) for computed tomography angiography (CTA). Materials and Methods: Phantom Study: A water-filled phantom with five 20-mL syringes filled with 1:20 dilution of 282, 300, 370, and 400 milligrams of iodine per milliliter concentration CM and gadopentetate dimeglumine (Magnevist, Berlex Laboratories, Wayne, NJ, 0.5 mol/L) was scanned with a 16-multidetector CT using 80, 100, 120, and 140 kVp and 500-millisecond gantry rotation time. The milliampere was either fixed at 100, 200, 300, and 380 or automatically adjusted with noise indices of 15, 20, and 25 or manually adjusted to maintain a constant image noise. The attenuation value (Hounsfield unit; HU) and its standard deviation of CM in each syringe and of the water phantom were obtained. Statistical analysis was performed to determine difference between attenuation values and contrast medium-to-water contrast-to-noise ratios at various kVp and mA selection. Human Study: Three groups of patients had CTA for abdominal aortic aneurysm with similar computed tomography parameters, varying only in kVp selection of either 100 (group A), 120 (group B), or 140 (group C). Another group (group D) had CTA at 100 kVp but with the CM volume reduced to 60%. The CTA studies were evaluated for the quality of axial and 3D images; attenuation values in the aorta, superior mesenteric artery, and iliac arteries; image noise; and scanner-estimated radiation dose. Statistical analysis was performed to determine the difference in image quality and radiation dose among the groups. Results: Phantom Study: In comparison with 140 kVp, regardless of selected milliampere or noise indices, images acquired at 80, 100, and 120 kVp showed 90.8% to 94.2%, 47% to 49.7%, and 18.9% to 20.7% (P < 0.0001) increase in HU of iodine-based CM, respectively, and 62.9%, 39.6%, and 16.8% (P < 0.0001) increase in HU of gadolinium-based CM, respectively. Human Study: The axial images in all the groups were diagnostically acceptable. There was significantly inferior quality of axial images associated with greater image noise in group A and group D (P < 0.01) in comparison with group C, but there was no difference in the quality of 3D images among the 4 groups. In comparison with group C, there was significantly higher attenuation of the aorta, superior mesenteric artery, and iliac arteries in group A (P < 0.01), group B (P < 0.05), and group D (P < 0.01). The radiation dose (CT dose index volume) decreased to 12 ± 4 in groups A and D compared with 17 ± 4 in group B and 24 ± 5 in group C. Conclusions: Lower kVp increases the attenuation of iodinated and gadolinium CM. CTA of the abdominal aorta performed at low kVp results in higher attenuation in aorta with reduced radiation dose and without degrading the diagnostic image quality. The iodinated CM volume can be reduced by reducing kVp during CTA.

Imaging of liver metastases: MRI

Metastases are the most common malignant liver lesions and the most common indication for hepatic imaging. Specific characterization of liver metastases in patients with primary non-hepatic tumors is crucial to avoid unnecessary diagnostic work-up for incidental benign liver lesions. Magnetic resonance (MR) is rapidly emerging as the imaging modality of choice for detection and characterization of liver lesions due to the high specificity resulting from optimal lesion-to-liver contrast and no radiation exposure. Improvements in breath-hold T1-weighted fast spoiled gradient echo and rapid T2-weighted single shot echo-train acquisition enable imaging of the liver in a single breath-hold with high spatial resolution. Most metastases are hypo- to isointense on T1 and iso- to hyperintense on T2-weighted images. MR contrast agents provide critical tumor characterization and can be safely used in patients with iodine contrast allergy and renal failure. Other agents, including newly developing gadolinium-chelates or iron oxide agents may provide additional benefits in selected applications. The degree and nature of tumor vascularity form the basis for liver lesion characterization based on enhancement properties. Liver metastases may be hypovascular or hypervascular. Colon, lung, breast and gastric carcinomas are the most common tumors causing hypovascular liver metastases, and typically show perilesional enhancement. Neuroendocrine tumors including carcinoid and islet cell tumors, renal cell carcinoma, breast, melanoma, and thyroid carcinoma are tumors most commonly causing hypervascular hepatic metastases, which may develop early enhancement with variable degrees of washout and peripheral rim enhancement.

Staging of colon carcinoma using water enema CT.

Objective To determine the accuracy of water enema CT (WECT) for staging colon carcinoma. Materials and Methods Thirty patients with colon cancer, diagnosed by barium enema and/or colonoscopy, underwent preoperative WECT. Dynamic contrast enhanced CT studies were performed after rectal administration of 2 L lukewarm tap water. Images were prospectively analyzed for depth of tumor invasion, nodal involvement, and distant metastases by investigators blinded to the results of barium enema and colonoscopy. Surgical/pathologic proof was obtained in all cases Results Using WECT, 23 of 30 patients were correctly staged. Correct staging occurred in 2 of 2 patients with Stage A, 3 of 3 patients with Stage B1, 6 of 9 patients with Stage B2, 1 of 2 patients with Stage C1, 5 of 8 patients with Stage C2, and 6 of 6 patients with Stage D tumors. Of the patients incorrectly staged, 4 were understaged and 3 were overstaged; all were due to errors in predicting lymph node involvement. Sensitivity and specificity for evaluating nodal involvement were 60 and 79%. respectively. Conclusions Water enema CT allows for accurate depiction and staging of colon carcinoma. Aqueous distention of the colon avoids artifacts seen with positive contrast agents yet allows accurate evaluation of the bowel wall and pericolonic structures.

Postoperative Imaging in Liver Transplantation: What Radiologists Should Know

Liver transplantation is now frequently used in the treatment of end-stage liver disease. Therefore, it is important that radiologists be aware of common anastomotic techniques and expected postoperative imaging findings. Imaging is most useful in evaluating for posttransplantation complications, which are broadly classified into vascular, biliary, and other complications. Hepatic artery thrombosis is the most significant complication and is often associated with graft failure. Radiologists have multiple modalities at their disposal for optimal evaluation. Doppler ultrasonography (US) is the preliminary imaging modality for gross evaluation of the liver parenchyma, biliary tree, and vasculature for abnormalities. When US findings are indeterminate or there is persistent clinical suspicion for an abnormality, computed tomography (CT) is often performed. The major indications for CT are detection of bile leak, hemorrhage, and abscess, but CT is also useful in the assessment of the vasculature. T-tube cholangiography and magnetic resonance cholangiopancreatography are the best noninvasive imaging tools for evaluating for biliary stricture. Some investigators would argue that endoscopic retrograde cholangiopancreatography (ERCP) is a better diagnostic imaging modality; however, ERCP is invasive. Hepatobiliary scintigraphy is optimal for the evaluation of biliary leakage. Early detection of posttransplantation complications will help lower morbidity rates and will likely allow graft salvage in selected cases.

Liver-specific MR imaging contrast agents.

Liver-specific MR imaging contrast agents consist of iron oxide particles or specially designed paramagnetic complexes targeting either the reticulo-endothelial system of the liver or the hepatocytes. These agents enhance the relaxation of water molecules in normal liver tissue and are excluded from abnormal tissue, such as metastases. Relaxation enhancement provides a map of normal liver function, increasing conspicuity of focal abnormalities. Understanding the indications and use of these agents is a central challenge for radiologists practicing liver MR imaging.

Multidetector computed tomography technology: current status and emerging developments.

By virtue of its speed and isotropic resolution of thin slices, multidetector-row computed tomography (MDCT) offers unique advantages over previous single-detector-row CT, with improved temporal and spatial resolution. This review describes the historical perspective and growth trends of MDCT scanners and highlights the current status and emerging developments in multidetector technology based on presentations at the symposium “Advances in Multidetector CT,” under the auspices of the Institute for Advanced Medical Education in Washington, DC.

16-MDCT angiography in living kidney donors at various tube potentials: impact on image quality and radiation dose.

OBJECTIVE The objective of our study was to compare the performance of 16-MDCT angiography at various peak kilovoltage (kVp) settings and the impact of the different settings on image quality and on radiation dose in adult kidney donors. MATERIALS AND METHODS Sixty-two renal donors (32 men, 30 women) who underwent 16-MDCT were divided into three groups: 18 subjects were studied at 140 kVp (group A); 20, at 120 kVp (group B); and 24, at 100 kVp (group C). Other constant scanning parameters were as follows: detector collimation, 0.625 mm; table feed, 9.375 mm/rotation; gantry rotation time, 500 milliseconds; and automatic current tube modulation (ATCM) using a noise index of 15. A total of 135-140 mL of iodinated contrast material (300 mg I/mL) was administered at 5 mL/s via an 18-gauge cannula, and arterial phase scanning was initiated using a bolus-tracking technique. Two observers evaluated image quality of the axial and 3D images and the visibility of branch order in the superior mesenteric artery (SMA) and renal arteries. Attenuation (in Hounsfield units [H]) in the aorta, SMA, and main renal artery was also measured by placing a region of interest. Radiation dose measurements were based on the scanner-generated CT dose index volume (CTDI(vol)). Each parameter tested was compared among the three groups using a nonparametric analysis of variance test, and a p value of 0.05 was considered significant. RESULTS Differences in the quality of the axial images existed between groups A and C (p < 0.001) and between groups B and C (p < 0.01); the image quality of the 3D images and the visibility of branch order in the SMA and renal arteries were comparable for all groups. The difference in mean attenuation of the aorta, SMA, and renal arteries was significant between groups A and C (p < 0.001) and between groups B and C (p < 0.01). All groups had 100% diagnostic accuracy in identifying the number of renal arteries on the side of nephrectomy. The mean radiation dose in CTDI(vol) was 25 +/- 3 mGy at 140 kVp, 17 +/- 4 mGy at 120 kVp, and 12 +/- 3 mGy at 100 kVp (p < 0.001). CONCLUSION Our initial observations suggest that the image quality of 16-MDCT angiography performed at 120 kVp is similar to that of CT angiography (CTA) performed at 140 kVp in adult kidney donors but with a significant radiation dose reduction. CTA at 100 kVp results in higher image noise but provides diagnostically acceptable images with significant radiation dose reduction compared with CTA at 120 or 140 kVp.