Background
The principles on which nuclear medicine rely on are fundamentally different from some of the other imaging modalities used in radiology. In nuclear medicine radioactive material is administered to the patient converting them into the radiation source, and the images are obtained using a camera. Therefore, this is different from a CT scan or plain radiograph where an external radiation source is directed towards the patient and the rays that pass through are captured using various detectors. In nuclear medicine, small quantities of radioactive materials are administered to the patient and function as radiotracers. These radiotracers can be bound to different compounds depending on the purpose of the study. The compounds have a unique ability of allowing the radiotracer to be guided to a specific target. Once the radiotracer has accumulated in the area of interest, a camera is then used to capture the emitted radiation.
Another fundamental difference of nuclear medicine is that it focuses more on function, not structure. CT or conventional radiographs provide a picture of the human anatomy. Nuclear medicine delivers information on the functioning of these anatomical structures. The radiotracers and their bound compounds are function dependent. An example would be the HIDA scan that works by using Technetium 99m (the radiotracer work horse in nuclear medicine). This is usually tagged with iminodiacetic acid (the compound) that leads to Tc 99m being taken up by hepatocytes. If the patient’s liver is not functioning adequately, their hepatocytes will not take up the radiotracer. These changes can be seen before anatomical abnormalities can be appreciated on other imaging modalities1.
Injecting radioactive compounds are not risk free. The two major factors to consider are the type of radiotracer, and the dose that is being administered. All these radiotracers want to be stable. Thus, for each radiotracer this means reaching a certain number of protons and neutrons. The way these radiotracers reach the ‘zone of stability’ is by either gaining or losing a neutron or a proton. Depending on the initial number of protons/neutrons each radiotracer has, and by the amount of energy that is available to that isotope it will decay into a more stable compound by emitting radiation which can then be captures and processed to produce an image.
I-131 is an example of a radiotracer which emits high levels of radiation. Some of its uses include treating hyperthyroidism caused by Graves’ disease, or for ablating residual tumor after surgical excision. The patients that receive I-131 are counselled by radiation safety officers and are given strict instruction upon discharge from the hospital. Some of these instructions include limiting contact with family members, not sharing towels or washcloths, and rinsing the tub and sink upon use. This high level of radiation can also be detected by the police, and these patients are given specific documents upon discharge to explain this high level of radiation.
Radiotracers use in nuclear medicine have variable physical half-lives ranging from a few seconds (Rubidium-82, 75 seconds) to days (I-125, 60 days). Physical half-life means the amount of time in which the radioactivity level decreases to half of its original value. Fortunately, biological half life is another factor which decreases the time the radiotracer remains in the human body. Biological half-life is the time taken for half of radiotracer to be excreted from the human body. An example is Xenon-133 which is used to perform the ventilation portion of a ventilation perfusion scan to assess for pulmonary embolism. The physical half-life of Xe-131 is 5.3 days, but the biological half life is 30 seconds. This is because the radiotracer is exhaled rapidly reducing the total radiotracer dose in the human body.
Description
Bone scan is an imaging technique used to image the osseous structures. Usually, Technetium 99m is tagged with either methylene diphosphonate (MDP) or with hydroxy diphosphonate (HDP). In this example, MDP and HDP are the compounds used. Alternately, a bone scan can also be obtained using F-18 sodium fluoride PET/CT which demonstrates higher sensitivity and specificity2.
To perform a bone scan, the patient is given an intravenous injection of radioactive material and, after an appropriate time interval, is scanned with a gamma camera, which is sensitive to radiation emitted by the injected material. About half of the radioactive material is localized by bone. Bone scans work on the principle of bone turn over (resorption and then replacement by new bone) which occurs continuously through an individual’s life span3. Focal areas which have increased abnormal bone turn over will be see as ‘hot spots’, standing out in a background of mild bone turn over seen in the rest of the normal skeletal system. Unfortunately, this can be nonspecific as multiple pathologies may show increased bone turn over. Pathologies which rely on decreased bone turn over demonstrate ‘cold spots’, and typically are more difficult to identify. Therefore, it is important to have a narrow differential diagnosis before obtaining a bone scan.
The study can be obtained as a single phase (after a single time interval post injection) or as a multi-phase (at multiple time intervals) study depending on the clinical concern. Some of the clinical indications for obtaining a bone scan are detection of occult fractures, skeletal metastases (sclerotic metastasis), osteomyelitis, joint prosthesis infection or loosening, reflux sympathetic dystrophy, heterotopic ossification and Paget’s disease. For the evaluation of occult fractures, skeletal metastasis, and Paget’s disease, a single-phase bone scan is adequate. Bone scans are considered more sensitive for occult fractures which may go undetected on radiographs, but typically require 1 to 10 days after the fracture before the bone scan will detect increased uptake caused by the fracture healing. This time interval also varies depending on the patient’s age and bone health.
Multi-phase bone scans require the patient to be imaged at different time points. The first phase images typically show perfusion to a lesion, second phase images show blood flow to the area, and third phase images best show the extent of bone turnover associated with a lesion. Occasionally, an additional delayed scan may be obtained after 24 hours in patients with vasculopathy. These studies are typically reserved for bone infections and reflux sympathetic dystrophy.
The radiation dose used for a bone scan is minimal (15-40 mCi), and even lower in pediatric patients (0.25mCi/kg). The amount of radiation is so small that there is no risk to people who the patient comes into contact. There is always the minimal risk of damage to cells or tissue from exposure to any radiation dose. Typically, bone scans are not performed on pregnant woman in order to avoid exposing the developing fetus to radiation.
