Bone Scan - Uses, Findings, Limitations, Preparation, Instructions, and Risks

Introduction:

A bone scan, also known as skeletal scintigraphy, is a diagnostic tool that uses nuclear medicine in which a special dye called radioactive substances (radiotracers or radiopharmaceuticals) is used to detect bone diseases.

What Are the Radiotracers Used in the Bone Scan?

• Technetium-99m medronate (methylene diphosphonate [MDP]) - commonly used at the dose of 740 Mbq (20 mci) for adults.

• Technetium-99m oxidronate (hydroxy methylene diphosphonate [HMDP]).

• Hydroxyethylene diphosphonate (HDP).

What Are the Uses of a Bone Scan?

A bone scan is used to detect:

• Bone cancer.

• Bone metastasis (cancer in any part of your body that has spread to the bones. Common cancers which spread to bones are the cancer of the breast, lung, thyroid, prostate, and kidney).

• Bone pain without a known reason.

• Bone infection.

• Bone fractures that are not visible in the X-ray.

• Arthritis (swelling of the joints).

• Paget's disease (a bone disease that affects the normal bone recycling process).

• Location of abnormal bone in complex bone structures such as the foot or spine.

• Fibrous dysplasia (a rare bone disease in which normal bone tissue is replaced by abnormal scar-like tissue).

• Avascular necrosis (death of bone cells due to lack of blood supply).

• Osteomyelitis (swelling of the bone caused by infection).

What Are the Normal Findings of a Bone Scan?

• Tc99m-MDP should be evenly distributed throughout the bony structures of a healthy person.

• Increased uptake in children in long bones is normal, which shows growth centers.

• Weight-bearing areas such as the articular surface of the hip, knees, ankles, and feet may show increased uptake of radiotracers due to its constant remodeling process.

• Increased uptake in kidneys and bladder is normal as they excrete the radiotracers. If the kidneys and bladder are not visible in the scan (“super scan”), it may be suspicious of cancer spread.

What Are the Factors To Be Considered Before a Bone Scan?

• Allergy- Allergic reactions to radioactive tracers are rare. Inform your technician about any other allergies you are having or might have experienced allergy in your previous nuclear medicine imaging.

• Pregnancy - Pregnant women are not allowed for bone scans. Radiation exposure during bone scans may cause birth defects in the baby in the womb.

• Breastfeeding- Radiotracer can be transmitted to the baby through breastmilk. You are advised not to breastfeed for about 24 hours after the scan.

• Pain in Injection Site- Radiotracer injection during the scan may cause mild pain and redness.

• Radiation Exposure- Some patients may worry about radiation exposure during the scan. The average radiation dose is approximately 4.2mSv.

• Claustrophobia- Afraid of closed spaces. If you are claustrophobic, you will be given medicine to make you feel sleepy and reduce your anxiety.

How Do You Prepare for a Bone Scan?

• You can normally eat and drink before your bone scan.

• Inform your doctor about your medical conditions and the medicines you are regularly taking. You will be asked not to take medicines containing barium or bismuth, which can affect the scan results.

• You should remove the metal objects or jewels before the scan.

How To Take a Bone Scan?

• Before the scan, you will be asked to wear a hospital gown. You need to empty your bladder before the test as it may hide the view of pelvic bones.

• For your bone scan or skeletal scintigraphy, the doctor may inject a small number of radioactive tracers into the vein in your arm. The radioactive tracers may enter your blood and will be absorbed by the bone cells. It may take about two to three hours to absorb completely.

• During skeletal scintigraphy, you will lie down flat on an imaging table till imaging from head to toe is taken. The camera in the scanner may rotate or stay in position to take the series of images. You might be asked to change your body position during the scan. While taking the image, you will be asked not to move to obtain clear images.

• Children may feel discomfort from having to remain still during imaging. Family members or caretakers of the children may be allowed to stay in the room. For some types of scans, images are taken immediately after injecting the radiotracer and three to four hours after the injection. This type of test is called a three-phase bone scan.

• After the scan, your radiologist will review the images for results. If the images are not clear or need additional images, you may be asked to repeat the test.

What Are the Instructions Given to You After the Bone Scan?

After your bone scan, the radioactive materials injected into your body may remain for about a few hours to days. A few instructions will be given after the bone scan, such as:

• Limit your contact with pregnant women and infants.

• Drink plenty of water which helps to flush out the tracers from your body. Usually, it takes about two to three days to come out completely.

• Inform your doctor if you are facing any discomfort or pain or swelling at the injection site.

What Are the Limitations of a Bone Scan?

• Time-consuming.

• It may require additional images such as computed tomography (CT), magnetic resonance imaging (MRI), positive emission tomography (PET), or biopsy.

• Image resolution is less when compared to CT and MRI.

What Are the Interfering Factors in the Bone Scan?

• Poor Renal Function- Radiotracers should be cleared from soft tissues by the kidney for effective imaging of the bone. If the kidney is not functioning properly, overlying soft tissue may affect the visualization of the bone.

• Free Technetium-When, the labeling efficiency of Tc99m-MDP, has reduced, it results in the formation of unbound technetium 99m called free technetium. It may hide the abnormalities in interpretation.

• Flare Phenomenon- Flare phenomenon or osteoblastic flare phenomenon is the interval visualization of bone lesions with sclerotic rim previously not visible on a radiograph in the follow-up of cancer patients. If there is an increase in radiotracer uptake, it shows the healing process, which is usually seen between two to three months of chemotherapy and can be misinterpreted as bone metastasis.

What Are the Risks and Complications Associated With Bone Scans?

• Patients with bone or joint pain may have discomfort from staying long without movement during the scan. They can relax by taking a deep breath.

• Injection of radiotracers mistakenly into radial arteries results in abnormal uptake of radiotracers in the palm, and the thumb called the “glove” phenomenon.

• When the bone absorbs minimal radiotracer, the underlying disease will not be properly visible in the scan.

• Methyldiphosphonate may cause low blood pressure in some patients. Monitor the patients carefully during the scan.

Conclusion:

Bone scan or skeletal scintigraphy is an effective and versatile diagnostic tool for detecting bone diseases. The radiation exposure of bone scan is approximately half the dose of the abdominal CT scan. This scan is very sensitive in the detection of osteomyelitis up to 94 percent. When the bone scan is combined with additional images called a three-phase bone scan, pathology will be identified better.