Ultrasound | Doppler | CT scan | MRI | PET
Watch this video to learn the pros and cons of ultrasound, how to prepare for an ultrasound and basics on how to read the scan.
Ultrasound (or sonography) is commonly used as the first imaging study for liver and bile duct diseases. If you are at an increased risk for developing liver cancer, your doctor may recommended that you have regular ultrasound exams to screen for liver cancers in their early stages. Ultrasound is widely available, non-invasive, painless, safe and less expensive than CT or MRI.
In the examination room, the patient will lie down on the examination table. The examiner (ultrasound technologist or radiologist) will spread some gel on the patient’s skin and then press a small hand-held probe against the abdomen and move it around to see structures inside the abdomen. Ultrasound can show solid organs such as liver, kidneys, spleen, and pancreas.
Image 1: An ultrasound of the liver. The arrow points to a 3cm mass, likely a tumor.
Image 2: Ultrasound showing dilated bile ducts (darker areas) due to obstructing cancer (arrow).
If needed, a Doppler ultrasound exam can be performed to show blood flow through blood vessels.
If there is any tumor in the liver, it will be seen on ultrasound as a bright or dark area. The appearance of the liver mass on an ultrasound exam may suggest a specific diagnosis, but a CT scan or MRI is usually required to provide more detail.
An ultrasound exam is very helpful to identify blockages in the gallbladder and large bile ducts. If there is any blockage, called obstruction, bile ducts will become enlarged (dilated) and will be easily seen as dark tubes on the ultrasound scan.
CT scans (computer aided tomography or CAT scan) use X-rays to produce cross-sectional (slices through the body in a horizontal direction) images of internal organs. CT scan is performed on a large machine with a tunnel in the center.
A CT scan of the abdomen usually involves an injection of 100-150cc of contrast material into the vein that requires insertion of a small needle into the arm. Oral contrast will also be given before the scan in most cases. These contrast materials are important to evaluate abdominal organs and blood vessels. There is some minimal risk of allergic reactions and kidney problems from contrast material, but radiology departments are well equipped to take care of these rare events. During the scan, there may be a mild warm sensation and slightly unpleasant taste because of the injection of contrast.
Image 3: CT scan shows a mass (arrow) in the liver which is brighter than normal liver. The mass was diagnosed as primary liver cancer.
Image 4: This 3-dimensional CT scan shows normal abdominal blood vessels as branching structures.
MRI (magnetic resonance imaging) is usually performed when a CT and/or ultrasound scan is not conclusive, or if you are allergic to CT contrast material. MRI exams are more complicated and take a longer time than CT or ultrasound scan. MRI uses a strong magnetic field and does not involve exposure to radiation, unlike CT scans. Patients with any metal devices implanted in their body, such as a pacemaker, will not be able to undergo an MRI if the device is not MRI compatible.
The MRI machine appears similar to a CT scanner, and the patient will lie down on a narrow table that slides into a tunnel. However, the size of the tunnel is smaller, the machine is louder, and the exam takes longer time than a CT scan. Individuals who experience claustrophobia may need a relaxing medication before MRI exam. There are open MRI machines that can be helpful for large or claustrophobic patients. The risk of allergic reaction to MRI contrast is much lower than to CT contrast material.
Image 5: MRCP (magnetic resonance cholangio-pancreatography) which is a special MRI of the gallbladder, bile ducts and pancreatic ducts.
PET (positron emission scan) scanning is a relatively new type of imaging study. The major difference between CT and PET or PET-CT, is that a different type of contrast material (intravenous contrast agent) is used in PET scanning. PET scanning takes advantage of the fact that most cancers are hyper-metabolic, which means that cancer cells are dividing more rapidly than other normal cells.
Cells that are actively growing require nutrition in the form of glucose, or sugar. The contrast material (intravenous contrast agent) that is used for PET scans is attached to a form of sugar, so the dye will be absorbed by cells that are actively growing.
Before undergoing a PET scan, the patient will be asked to follow a strict low-carbohydrate diet for a few days. This is to help the PET scan distinguish between cancer cells, and other normal structures that also actively take up glucose, such as the liver and intestines. PET scans are very helpful in imaging certain, but not all types of cancers. An individual’s oncologist will decide if PET scanning is appropriate.
Image 6: PET scan
Last Updated on 11/26/2012 5:32:09 PM