What Special Tests Do I Need to Have?
- Ultrasound (US)
- Computed Tomography (CT)
- Magnetic Resonance Imaging (MRI)
- Positron Emission Tomography (PET) Scan
- Radionuclide Imaging
- Tumor Markers
Ultrasound (US)
Ultrasound images are made by placing a hand-held device, called a transducer, (approximately the size of a cell phone) directly on the patient's skin. This device emits sound waves and measures the "echoes" they make on the liver. The strengths of this technique include the safety of the method and the high resolution of fluid-filled "sacs" (cysts). The weaknesses of ultrasound include poor image quality in patients who are large or obese, patients with excess bowel gas, and patients with cirrhosis (chronic liver disease). In addition, the technique requires a high level of operator experience. Some lesions in the liver may also possess a similar echo pattern to the background liver (in other words, looks the same) and thus they are difficult to see.
In liver surgery, ultrasound is often the first choice for screening and follow-up as it is relatively inexpensive and it is portable to the bedside as a mobile system. It can also be used to understand the patterns of blood flow (Doppler mode) and as a means of guiding biopsy needles and drainage catheter placement.
From the surgical point of view, intraoperative sonography (IOUS) - done laparoscopically or with open surgery - is the most important application of ultrasound, offering direct contact of the sonographic probe to the liver surface and allowing detection of lesions as small as 3 mm. Furthermore, it gives exact anatomic resolution, notably in determination of the relationship of lesions to the vessels and the hilum.
Computed Tomography (CT)
Computed tomography (CT) is a machine that is a donut-shaped ring composed of varying numbers of x-ray sources and detectors through which the patient passes on a sliding table. Images are acquired of tissues and structures based on differing attenuations of the x-ray beam by these entities. Advantages of computed tomography include excellent demonstration of calcified structures, for example kidney stones; excellent demonstration of the lungs and other structures in the chest, and reproducible, consistent image quality. Weaknesses of the technique include the dangers associated with x-ray exposure, allergic reactions, and kidney damage due to the iodine-based intravenous contrast agents employed. CT, by virtue of the large number of machines in the community and the general familiarity in the medical community with its images, is the most commonly used machine to investigate and image liver disease.
Since its introduction in the 1970s, CT has been shown to have wide applications within all the radiological subspecialities. Spiral (or helical) CT moves the patient through a rotating continuous fan beam exposure, in this way a spiral volume of tissue is examined. Its advantages are speed, exact contiguity of images, smaller volume of contrast medium and improved 3 dimensional (3D) images. It does require, however, a co-operative patient and a very precise timing of enhancement.
Normally, the liver receives approximately 75 per cent of its blood from the portal vein and 25 per cent from the hepatic artery. In contrast, hepatic tumors, whether they are primary or secondary, receive their blood supply predominantly from the hepatic artery. Thus, with a hepatic arterial injection, tumors will enhance to a greater degree than normal liver tissue.
Magnetic Resonance Imaging (MRI)
The magnetic resonance machine looks like a CT machine except that the tunnel is longer, usually at least 3 feet in length. Magnetic resonance imaging uses a strong magnetic field and radiofrequency pulses. These have not been shown to have harmful effects on biological tissues - in order words, it has not been shown to cause cancer from repeated or prolonged exposures. Advantages of magnetic resonance imaging include the safety of the imaging machinery, safety of the contrast agents employed (typically an intravenous contrast agent that is based on the rare metal gadolinium), the ability to acquire images in many different planes, high soft tissue contrast resolution, and more than one type of data information is acquired.
In MR images, typically data acquisitions termed 'T1' and 'T2' images are acquired, and images are acquired after the administration of intravenous contrast agents - immediately afer administration and then a little later on. In comparison, ultrasound and CT can only make images based on one type of signal - ultrasonic for US and X-ray for CT. One of the advantages of MRI over ultrasound and CT, then, is the fact that MRI obtains several types of data acquisitions resulting in more detailed images. Thus, liver lesions can be more precisely defined.
Disadvantages of MRI include the possibility of patient claustrophobia (approximately 5% of patients), and longer imaging times and less consistent image quality than CT. Also, patients with iron-containing metal blood vessel clips in the brain and cardiac pacemakers generally cannot have an MRI due to the dangers of these "metallic devices" being placed in the strong magnetic field.
On state-of-the-art MR systems, the long study time and less consistent image quality can be overcome by using short duration sequences that can be acquired while a patient holds their breath. A routine MRI examination performed on the liver should include non-contrast enhanced T1 and T2 images in at least two planes of acquisition, typically transverse and coronal (looking across the liver from side to side, and up and down the liver from back to front.) T1 images are then acquired early and later following the intravenous administration of a gadolinium chelate (MRI contrast agent) compound. This type of MRI protocol is best performed on current high field MRI systems that have a powerful magnetic system. As MRI is less widely used than CT for the investigation of liver disease, there is also greater variability in experience in interpretation of MRI studies and in performing MRI studies compared to CT. MRI is however unsurpassed in diagnosing benign liver tumors that do not require further investigation or treatment, or diagnosing small malignant tumors such as metastases or primary liver tumors.
Positron Emission Tomography (PET) Scan
Positron emission tomography (PET scan) using F-18-fluorodeoxyglucose (18FDG-PET) is valuable at evaluating the response of liver metastases to therapies such as chemotherapy, radiotherapy and chemoembolization, because uptake of 18FDG mirrors metabolic activity. Thus, presence of viable tumor in treated lesions can be determined by detection of cellular glucose metabolism. PET has been approved for use in evaluating metastatic colo-rectal cancers. It should be performed in all patients with metastatic colorectal cancer to the liver before a surgical intervention.
Radionuclide Imaging
Although many radionuclide techniques have been described, very few indications in a selected group of patients are left for this diagnostic modality: 99mTc (Technetium) - labeled red cells for hemangioma, DISIDA (diisopropyl-immunodiacetic acid) for assessment of the biliary tree and 99mTc-sulfur colloid scan for visualization of the reticuloendothelial cells in focal nodular hyperplasia. Hepatic Gallium scans are rarely done because of their lack of specificity. 111In-octreotide imaging provides a sensitivity and specificity of about 80 to 90 % for the detection of gastrinoma metastases to the liver, which remain difficult to detect by CT or MRI.
Tumor Markers
Tumor markers such as AFP (alpha-Fetoprotein), CEA (Carcinoembryonic Antigen) and CA (Carbohydrate Antigen) 19-9 may be used to warrant the diagnosis of a malignancy but always have to be interpreted in context with other diagnostic modalities. Although AFP is nondiagnostic when slightly or moderately elevated in patients with active hepatitis or cirrhosis, high levels and a liver mass on CT or MRI are virtually diagnostic of HCC. CEA is most sensitive for hepatic colorectal metastases (overall 75-90% of these patients have an elevated CEA) and relatively insensitive for peritoneal involvement. A rise in CEA usually predicts recurrence of colorectal cancer 6-8 months before clinical signs are present. Slowly rising CEA generally indicates a local recurrence, whereas rapidly rising CEA suggests liver metastases. A rise above 5 ng/ml is occasionally found in benign conditions, but levels of >10 ng/ml almost always indicate recurrent colorectal cancer.
