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The Bone Scan
The bone scan is a well established procedure in Nuclear Medicine.
Areas of bone injury or bone destruction are usually associated
with ongoing bone repair and consequent increased metabolic activity and
calcium turnover. The
radionuclides which mimic the metabolic behavior of calcium will localize
in this region of bone repair in increased concentration relative to
normal bone. In the past 85Sr
and 18F were the primary radionuclides used.
However, various phosphate compounds labeled with 99mTc are the
agents of choice. The most
commonly used radionuclides are Methylene Diphosphonate (MDP) and 99mTc
HDP. The exact mechanism of
bone uptake of these radionuclides is not clear. However, current
knowledge suggests that it is a combination of bone blood flow and
extraction
Bone imaging studies are useful in a variety of
disorders. These include
obscure or switching leg lameness, trauma, negative or ambiguous
radiographs, occult fracture, not lame enough to block, flexion tests
equivocal, anesthetic blocks are negative, minimal or no change with
medication, difficulty visualizing areas of the skeleton radiographically
such as the hips, pelvis and parts of the skull. Scintigraphy has been
known to be useful in tracking the overall skeletal health of horses in
arduous training and also as part of a pre purchase exam.
Studies are helpful in tracking degenerative disease, exercise
related trauma, assessment of significant x-ray findings, vascular
abnormalities, bone tumors, cysts or abscesses and osteomyelitis.
The radiation dose to the skeleton and the whole
body depends on the rate of clearance of the radionuclides by the kidneys.
In humans the estimated dose to the skeleton with an injected dose
of 20 mCi, is 0.7 rads, to the
kidney 0.8 rads, to the bladder with a 2 hour void 2.60 rads and to the
whole body is 0.1 rad.
Three phase Bone Imaging
A three phase bone imaging study is usually
indicated in suspected infection (osteomyelitis and septic arthritis) and
in trauma or suspected vascular abnormality.
Radiopharmaceutical and Dose
200 mCi 99mTc HDP or other diphosphonate
compound is administered intravenously via the jugular vein of the adult
horse. The dose for foals and other small animals 3 to 5mCi /
kg is adequate. Minimum
dose is 2 mCi is suggested.
Instrument
A
large field of view (LFOV) gamma camera with a low energy all purpose
collimator. Energy window at 20% centered at 140keV. Nuquest Nuclear Medicine
computer. Alfanuclear data and image processor.
Procedure
Once the appropriate area of interest has been
established, the collimator is positioned over that area for a dynamic
flow study. The radionuclide
is injected as a bolus. Data
is collected soon after the injection with each frame of the dynamic flow
set for a duration of 2-5 seconds for approximately 30-60 frames.
It takes approximately 15 seconds for the injected bolus to reach
the terminal aorta and 45 seconds to arrive in the distal hind limb of a
horse. In the average patient the dynamic flow component of the study
could be terminated 30-45 seconds after it appears in the area of
interest. Immediately after the flow study, a static image is obtained
with about 200-300k counts.
One hour to one and a half hours after injection 200mg of
furosemide in administered intravenously to facilitate faster renal
excretion of the radionuclide and hence better target to non-target ratio.
The patient returns for the delayed images
approximately three hours post injection.
Static images are obtained of the area of interest.
Distal limbs require 100-150k counts, upper limbs and axial
skeleton usually require 150-250k counts. Less counts may not be
diagnostic. This phase should include regular views, magnified views and
additional projections of the area of interest.
Another option is all the images of the upper limb and the axial
skeleton are acquired with motion correction applications for 45-60
seconds.
Care must be taken when acquiring images of the
hips, pelvis and sometimes the stifles.
If there is significant amount of activity in the urinary bladder a
lead bladder blocker should be placed over the bladder to obscure most of
the radioactivity. In some cases this does not help the image quality and
the horse must be taken back to the stall until the bladder is emptied and
returned later to acquire the needed images.
Note
A patient history sheet should be completed
with pertinent information and relative factors of the study and should be
submitted with the images for interpretation.
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Workstation
Large field of view detector is lowered into a pit
in the floor to acquire images of the distal limbs. A shield is
placed around the medial aspect of the other leg to prevent visualization.
Acquiring solar views of the foot while standing
on the small field of view camera mounted in the floor. A lead
"apron" is wrapped around the horsed foot to isolate the foot
from the rest of the leg. Additional aprons (not shown) are scattered over
the field of view to prevent visualization of the upper body.
Anterior view of the front feet and
fetlocks. Shielding is needed between the front and hind limbs.
Posterior view of the hind feet and
fetlocks. Shielding is needed between the hind limbs and the front limbs.
Transverse view of the pelvis.
A slight caudal tilt is applied to the
detector realigning the bladder away
from the tuber sacrale and the ilium.
Lateral view of the carpus.
The detector should always be positioned
as close to the patient as possible.
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