A bone scan or bone scintigraphy is a nuclear scanning test to find certain abnormalities
in bone. It is primarily used to help diagnose a number of conditions relating to bones,
including: cancer of the bone or cancers that have spread to the bone, locating some sources
of bone inflammation, the diagnosis of fractures that may not be visible in traditional X-ray
images, and the detection of damage to bones due to certain infections and other problems.
Nuclear medicine bone scans are one of a number of methods of bone imaging, all of which are
used to visually detect bone abnormalities. Such imaging studies include magnetic resonance
imaging, X-ray computed tomography and in the case of 'bone scans' nuclear medicine.
However, a nuclear bone scan is a functional test: it measures an aspect of bone metabolism
or bone remodeling, which most other imaging techniques cannot. The nuclear bone scan competes
with the FDG-PET scan in seeing abnormal metabolism in bones, but it is considerably less expensive.
Nuclear bone scans are not to be confused with the completely different test often termed
a "bone density scan," DEXA or DXA, which is a low-exposure X-ray test measuring bone
density to look for osteoporosis and other diseases where bones lose mass, without any
bone-rebuilding activity. The nuclear medicine scan technique is sensitive to areas of unusual
bone-rebuilding activity because the radiopharmaceutical is taken up by osteoblast cells that build
bone. The technique therefore is sensitive to fractures and bone reaction to infections
and bone tumors, including tumor metastases to bones, because all these pathologies trigger
osteoblast activity. The bone scan is not sensitive to osteoporosis or multiple myeloma
in bones; therefore, other techniques must be used to assess bone abnormalities from
these diseases. Technique
In the nuclear medicine technique, the patient is injected with a small amount of radioactive
material such as 740 MBq of technetium-99m-MDP and then scanned with a gamma camera, a device
sensitive to the radiation emitted by the injected material. Two-dimensional projections
of scintigraphy may be enough, but in order to view small lesions especially in the spine,
single photon emission computed tomography imaging technique may be required. In the
United States, most insurance companies require separate authorization for SPECT imaging.
A disruption of bone turnover by a pathologic process on the order of 5 to 15% from normal
can be detected by bone scintigraphy. Specificity of bone scintigraphy can be increased by performing
an indium 111-labeled white blood cell test combined with a technetium-99m-MDP injection.
About half of the radioactive material is localized by the bones. The more active the
bone turnover, the more radioactive material will be seen. Some tumors, fractures and infections
show up as areas of increased uptake. Others can cause decreased uptake of radioactive
material. Not all tumors are easily seen on the bone scan. Some lesions, especially lytic
ones, require positron emission tomography for visualization.
About half of the radioactive material leaves the body through the kidneys and bladder in
urine. Anyone having a study should empty their bladder immediately before images are
taken. In evaluating for tumors, the patient is injected
with the radioisotope and returns in 2–3 hours for imaging. Image acquisition takes
from 30 to 70 minutes, depending if SPECT images are required. If the physician wants
to evaluate for osteomyelitis or fractures, then a Three Phase/Triphasic Bone Scan is
performed where 20–30 minutes of images are taken during the initial injection. The
patient then returns in 2–3 hours for additional images. Sometimes late images are taken at
24 hours after injection. The three phase bone scan detects different
types of pathology in the bone. The first phase is also known as the nuclear angiogram
or the flow phase. During this phase, serial scans are taken during the first 2 to 5 seconds
after injection of the Technetium-99m-MDP. This phase typically shows perfusion to a
lesion. Cellulitis shows up more in phase 1 and phase 2 scan, but not in phase 3. Pathology
that is more moderate to severe will show more in the first two phases. Pathology that
is chronic or partially treated will be more pronounced in the third phase of a triphasic
scan. The second phase image, also known as the
blood pool image is obtained 5 minutes after injection. This shows the relative vascularity
to the area. Areas with moderate to severe inflammation have dilated capillaries, which
is where the blood flow is stagnant and the radioisotope can "pool". This phase shows
areas of intense or acute inflammation more definitively compared with the third phase.
The third phase, delayed phase, is obtained 3 hours after the injection, when the majority
of the radioisotope has been metabolized. This phase best shows the amount of bone turnover
associated with a lesion. Pregnant patients should consult with a physician
before consenting to radioactive injections.
A typical radiation dosage obtained during a bone scan is 6.3 mSv.
Footnotes
References "Bone scans". WebMD. Retrieved July 9, 2008.
"SkeletalScintigraphy.com".