Research News: 2002 - 2003 - 2004

What’s new in Abdominal Imaging

Fortunately, our ability to visualize the interior of the abdomen has advanced considerably since biblical times. Virtual endoscopy and other forms of three-dimensional imaging have been revolutionized by the near isotropic resolution of multidetector CT. Other developments have included the emergence of functional modalities such as MR spectroscopy and perfusion imaging.

The Abdominal Imaging Section at UCSF is dedicated to promoting excellence in the evidence-based application, performance, and interpretation of abdominal imaging studies and to researching new or improved techniques and applications for abdominal imaging. Areas of particular and current interest include MRI/MRSI of prostate cancer, CT cholangiography, fetal MRI, MRI/MRSI of diffuse liver disease, tumor perfusion imaging, virtual colonoscopy, and three-dimensional CT.

MRI/MRSI of Prostate Cancer
Despite the frequent and somewhat glib statement that “men die with prostate cancer, not of it,” almost as many men die of prostate cancer as women die of breast cancer. Unfortunately, a man with newly diagnosed early stage prostate cancer is faced with considerable uncertainty, since surgery, radiation, and observation may all be reasonable treatment choices. At UCSF, the use of MRI and MRSI (MR spectroscopic imaging) for the local evaluation of prostate cancer extent and aggressiveness has been pioneered in a collaborative effort between the Magnetic Resonance Science Center and the Abdominal Imaging Section. While the technology remains in evolution, it is arguably the best available modality for prostate imaging (Figure 1) and provides a unique “one-stop” combined anatomic and metabolic tumor assessment. Through the efforts of the multidisciplinary team at UCSF, this technology is now being incorporated into routine practice at many centers across the United States, and is the subject of an active multi-institutional trial being conducted by the American College of Radiology Imaging Network.

Figure 1A. T2-weighted axial MRI in a 75-year-old man with a PSA of 18.2 ng/mL (normal less than 4). A large area of low T2 signal intensity in the right side of the gland is consistent with tumor. Figure 1B. Corresponding MR spectroscopic imaging overlay shows multiple voxels (asterisks) with elevated choline peaks (the choline is the initial peak in the spectral trace when examined from left to right), concordant with the MRI findings. Biopsy confirmed the presence of extensive right-sided cancer.

CT Cholangiography
Despite the increasing use of MR cholangiopancreatography, the non-invasive evaluation of small non-dilated intrahepatic bile ducts is difficult, but can be crucial to clinical management, such as in patients who are being evaluated as potential living-related liver donors.

The Abdominal Imaging Section has addressed this problem by exploring the use of CT cholangiography, in which the patient is injected with a radiopaque contrast agent that is excreted by the liver into the bile ducts. In the past, this contrast agent was imaged using conventional radiography, and the procedure was known as intravenous cholangiography. The procedure fell into disuse, because of limited results. However, we have found that when patients receiving this contrast are imaged with CT rather than conventional radiographs, excellent visualization of the biliary system can be obtained (Figure 2).

Initial results suggest this procedure, known as CT cholangiography, is safe, accurate, and reliable. Preoperative CT cholangiography has virtually eliminated the need for intraoperative cholangiography in the liver transplant program at UCSF. The utility of CT cholangiography is now under study for numerous other biliary and surgical diseases. A UCSF success story!

Fetal MRI
Ultrasound remains the primary modality for prenatal imaging, but there are some situations in which ultrasound is inconclusive or unhelpful and where fetal MRI is of clinical value. Fetal MRI is a vital tool for the UCSF fetal surgery and treatment group. One of the exciting applications currently being studied is the use of in vivo MR spectroscopy to detect choline (a major constituent of surfactant) within amniotic fluid and fetal lung (Figure 3). Surfactant is produced by the fetal lung during the third trimester, and is required for normal pulmonary function after birth. Fetal lung maturity is currently assessed by performing an amniocentesis in late pregnancy, so that a specimen of amniotic fluid can be directly tested for surfactant. It is possible that MR spectroscopy may ultimately lead to the elimination or reduction in the need for such late amniocenteses. This project is another example of groundbreaking research at UCSF that results from the close cooperation of referring physicians, clinical radiologists, and basic scientists.

MRI/MRSI of Diffuse Liver Disease
The epidemic of obesity in the United States, affecting both children and adults, has triggered an interest in the associated hepatic conditions known as non-alcoholic fatty liver disease and non-alcoholic steatohepatitis. Up to 70% of obese individuals develop non-alcoholic fatty liver disease, which may progress to non-alcoholic steatohepatitis and ultimately lead to cirrhosis, end-stage liver disease, and transplantation.

The UCSF Division of Gastroenterology is one of seven sites participating in the National Institutes of Health-funded non-alcoholic steatohepatitis Clinical Research Network, a group of leading academic centers collaboratively studying the etiology, contributing factors, natural history, complications and therapy of this common liver disease. The Abdominal Imaging Section is working closely with the Division of Gastroenterology to improve the imaging evaluation of non-alcoholic fatty liver disease, with a particular focus on the role of MR imaging and MR spectroscopic imaging. Better imaging assessment of the liver may facilitate prediction of prognosis and may help in monitoring therapeutic intervention. Fat saturation, in and out of phase gradient echo, diffusion, and spectroscopy are various MR techniques being investigated to non-invasively measure the degree of fatty infiltration, fibrosis, and inflammation in the liver.

Tumor Perfusion Imaging
The response of abdominal cancers to chemotherapy is typically monitored by measuring tumor size, even though changes in tumor size provide only a gross indication of tumor biology and may lag behind therapeutically important changes in tumor perfusion. This may be particularly true for modern anti-angiogenesis treatment that targets growing blood vessels within the tumor. The rapid serial acquisition of CT or MR images through a fixed area of tumor after the administration of intravenous contrast can provide a wealth of additional data over and above the information provided by inspection of traditional contiguous planar images. Such data includes physiological features related to tumor vascularity such as blood flow, vascular permeability, and blood transit time (Figure 4). This extra functional data provides an insight into tumor biology that may someday allow us to assess the response of tumors to treatment before the tumor changes size, allowing faster and more tailored changes to chemotherapy regimens.

Figure 4. CT perfusion in a 56-year-old woman with breast cancer metastatic to the lung. Post-processed CT blood volume perfusion image shows a metastasis (arrow) in the right lung (A). Figure 4B. Repeat CT perfusion study after treatment with an angiogenesis inhibitor demonstrates a marked decrease in blood volume in the lung metastasis (B).

CT Colonography
CT colonography (also known as virtual colonoscopy) is an exciting breakthrough in the detection of colorectal polyps and cancer that is now available at Moffitt Hospital. Our state-of-the-art CT scanners can perform volumetric acquisitions through the entire colon with 300 to 350 thin isotropic slices obtained in 10-12 seconds. These high-resolution images are used to create a three-dimensional model of the colonic lumen that can be navigated in an interactive fashion, resembling the view through an endoscope. Early studies by Dr. Judy Yee at the San Francisco Veterans Affairs Medical Center and others have shown the accuracy of virtual colonoscopy approaches that of optical colonoscopy for polyps larger than 5-10 mm. (See Virtual Colonoscopy article.)

Screening colonoscopy is recommended for all patients over 50 years, but many patients are reluctant to undergo optical colonoscopy. A greater percentage of patients may be willing to undergo the less invasive procedure of CT colonography. Work is in progress to eliminate the need for unpleasant bowel cleansing preparation prior to the study. Fecal and fluid tagging with contrast is employed at Moffitt Hospital, which allows “electronic bowel cleansing” by computer subtraction of the high density tagged bowel content. Other areas being investigated include computer-assisted diagnosis for the detection of colonic polyps. The future appears bright for virtual colonoscopy.

Three-Dimensional CT
One thousand full-resolution CT images in five minutes — 8- and 16-slice multidetector CT scanners can churn out a blizzard of images in a hurry! To interpret such studies by scrutiny of each individual axial image is becoming increasingly impractical, and three-dimensional post-processing of these enormous datasets appears to be the only viable option. Dedicated graphics computers can extract and display structures of interest interactively as three-dimensional models, whether these structures are the blood vessels (CT angiography), urinary tract (CT urography), biliary tract (CT cholangiography), hollow viscera (CT endoscopy), or solid organs (CT volumetry). The same technology can be used for MRI studies. Optimization of CT scanning technique to supply data to these computers, and conversely, optimization of graphical reconstruction of CT output, is in rapid evolution. The resultant three-dimensional models offer analysis of select structures with unprecedented anatomic accuracy, challenging radiologists and surgeons to revisit their approach to patient management.

The Abdominal Imaging Section is actively investigating different hardware and software approaches to determine the optimal method of handling these large datasets in a way that extracts the maximal amount of clinically important information in as timely and efficient a manner as possible.