Nuclear Medicine
Nuclear Medicine examinations involve the injection of a small quantity of a radioactive material which is designed to specifically target a region of interest in the body. After a variable period of time, the radioactive substance accumulates in the target organ. By placing the region of interest under a gamma camera, the radioactivity emitted from the patient can be detected and a composite picture built up over time. Unlike conventional X-ray images, the spatial detail of nuclear medicine images is not great. Conversely, nuclear medicine is very sensitive in detecting abnormalities, sometimes long before X-rays can. Stress fractures, tumours or infections in bones can all be demonstrated earlier than with X-rays. Nuclear medicine provides information on the function of an organ where as conventional imaging techniques are generally more concerned with the structure of an organ. The radioactive materials used (or radiopharmaceuticals) have short half lives. This means that the radioactivity only persists for a short period of time. The radiation dose from most nuclear medicine procedures are small and are comparable to the dose from other X-ray imaging procedures.
Nuclear Medicine Procedures are tailored to the region of interest. The radioactive substance is attached to other compounds which are designed to target specific organs in the body. In this way many different kinds of examinations can be performed using the same basic equipment. A specially trained nuclear medicine technologist interviews patients prior to the examination to obtain relevant medical history. Adrenal Scans Bone Scans Gallium Scans Lung Scans Parathyroid Scans Renal Scans Thallium Scans Thyroid Scans
Tumours of the adrenal glands often produce specific hormones. Technetium can be attached to substances that mimic precursors of these hormones. Adrenal tumours take up these radiopharmaceuticals and can be diagnosed by demonstrating a 'hotspot' over the involved adrenal gland. This technique can be used to diagnose 'phaeochromocytomas', an adrenaline secreting tumour which can cause severe hypertension. Different radiopharmaceuticals are used to diagnose tumours that produce steroids as in 'Conn's Syndrome' and 'Cushing's Syndrome'. No preparation is required for adrenal scans.
A bone scan involves an injection in the arm which may be performed either in the injection room or under the gamma camera if the pattern of blood flow to the region of interest is required. The radiopharmaceutical used is distributed via the bloodstream and is taken up by actively metabolizing bone. After three hours, the images are obtained. The scans take up to one hour to complete and usually the entire skeleton is imaged by positioning the gamma camera over multiple regions. Occasionally a special tomographic study (SPECT) may be required in which the gamma camera rotates around the region of interest. This takes additional time. Bone scans can detect most diseases of bone usually earlier than other techniques and are more sensitive than x-rays in depicting stress fractures, infections of bones and joints, infections of artificial joints and most tumours. No special preparation is required for a bone scan. Fluids are required to be drunk between the injection and scan to help clear the radiopharmaceutical from the soft tissues which enhances the quality of the subsequent bone scan. The bladder must be emptied before the scan.
Gallium is a radioactive substance which is taken up in certain types of tumours and in areas of active infection. Gallium scans can therefore be used to follow the progress of these tumours or to document the presence of active infection such as in an artificial joint. Occasionally a scan is also performed to study inflammation in areas such as the lungs, heart etc. The gallium is administered by a vein injection. Several scans are obtained between two and five days after the injection. The first scan is the most detailed and may take more than an hour in total. No preparation is required for a gallium scan.
There are two parts to a lung scan. The 'ventilation' scan is first obtained in which the patient breathes a gas labelled with technetium. A series of images is obtained depicting the pattern of ventilation in the lungs. Next an injection of technetium is administered, attached to minute particles that are temporarily trapped in the capillaries (tiny blood vessels) of the lungs. Another series of images is obtained depicting the pattern of perfusion in the lungs. By comparing the two scans, regions where there is a ventilation / perfusion mismatch can be defined. This enables the diagnosis of pulmonary emboli where blood clots from the legs travel into the lungs and obstruct the pulmonary arteries. No preparation is required for a lung scan.
Tumours of the parathyroid take up thallium but not technetium, whereas the thyroid gland takes up both. By performing scans of the neck with both radiopharmaceuticals, two sets of images are obtained. Using the system's computer, the technetium scan can be digitally subtracted from the thallium scan leaving an image of the parathyroid tumour alone. This is a useful technique because parathyroid tumours are often small and difficult to image on CT or ultrasound. No preparation is required.
There are two basic types of renal scan both of which are designed to evaluate the kidneys. A 'DMSA' scan involves an injection of a radiopharmaceutical that binds to the renal substance. The images obtained give an indication of the function of each portion of the kidney and can define areas of kidney scarring related to previous infection. A 'DTPA" scan involves an injection of a radiopharmaceutical that is excreted by the kidney into the renal collecting systems and ureters and into to the bladder. By tracking the radioactivity from the substance of the kidney and from the collecting systems over a period of time, graphs of kidney function can be obtained. Delay in function of a kidney can be defined by comparing the graphs obtained from each side. This technique is used to assess overall renal function where a kidney has been damaged in the past. Scans can also be used to assess obstruction of the kidneys, during which an injection of a diuretic (Lasix) is administered late in the scan. DTPA scans are also used to diagnose narrowing of the renal arteries in patients with hypertension. When assessing for hypertension, a drug known as Captopril may be administered after the first scan and the scans subsequently repeated.
Thallium scans are used to evaluate coronary artery disease. Coronary artery disease is a major cause of death and morbidity in Western cultures. Narrowing of the blood vessels supplying the heart leads to a lack of oxygen. The heart muscle cannot function properly without an abundant supply of oxygen. Under periods of increased cardiac workload during exercise, the lack of blood supply can lead to chest pain known as angina. If there is a severe or sudden reduction in blood supply a myocardial infarct may result where the heart muscle dies. Thallium is injected into the bloodstream about one minute prior to the actual scan and enters the heart muscle via the coronary arteries. The gamma camera is placed over the heart and tomographic (SPECT) pictures are obtained depicting the level of radioactivity in each portion of the heart muscle. The level of radioactivity is directly related to the perfusion or blood flow in that portion of the heart. Two separate scans are obtained, each requiring an injection of thallium. The scans are performed at rest and after exercise on a treadmill or bicycle to define areas of reversible 'ischaemia' i.e. areas where the muscle is still viable but the blood supply compromised. During the exercise an ECG trace will be monitored and blood pressure and pulse rate checked. In preparation for the test, products containing caffeine should be avoided for twelve hours beforehand. A four hour fast is required immediately before the scan to ensure an empty stomach. Smoking should also be ceased four hours prior to the scan. Usual medication should be continued.
An injection of technetium. Is given via an arm vein. After half an hour, the thyroid gland is placed under the gamma camera and images obtained. Thyroid scans demonstrate the structure, location and size of the gland. The pattern of function in the gland is also depicted and any 'hot' or 'cold' areas defined. The pattern of activity is usually correlated with other imaging studies, particularly ultrasound. Any cold areas that correspond to a nodule seen on ultrasound must be biopsied to rule out a thyroid cancer. An estimate of overall thyroid function (thyroid uptake) is also obtained during the examination.
