Case Author(s): Anton J. Johnson, M.D., Ph.D. and Keith Fischer, M.D. , 11/8/96 . Rating: #D2, #Q5

Diagnosis: Camera off peak (Co-57 instead of Tc-99m)

Brief history:

81 year old woman with right hip pain after a recent fall.


Delayed anterior and posterior whole body images.

View main image(bs) in a separate image viewer

Full history/Diagnosis is available below

Diagnosis: Camera off peak (Co-57 instead of Tc-99m)

Full history:

81 year old woman with right hip pain after a recent fall.


20.4 mCi Tc-99m MDP i.v.


Apparent diffusely increased soft tissue activity with poor visualization of the bones. This appearance raised the question of a technical problem. The gamma camera was checked and its energy peak found to be set erroneously low at 122 keV, the energy of Co-57.


Co-57 has a relatively long half-life (270 days) and an energy peak (122 keV) similar to that of Tc-99m, two attributes that make it desirable for use in quality control. At our institution, Co-57 flood sources are used for daily field homogeneity quality control. A minor drawback is that you have to remember to reset the energy peak of the gamma camera before you begin imaging patients. If this is not done, subsequent studies using Tc-99m (or higher energy) labeled compounds will yield low resolution images with apparent poor radiopharmaceutical localization. This is because the incorrectly set camera is imaging lower energy photons that have been scattered (Compton) from random sites throughout the patient's body. Normally, most of these lower energy Compton scattered photons are rejected (not used for image construction) when the camera's energy peak and window are set properly.

An alternative to Co-57 is to use a solution containing Tc-99m pertechnetate as the flood source. Although this obviates the need to reset the energy peak for subsequent Tc-99m studies, a new solution has to be prepared every day because of Tc-99m's six hour half-life. The added work of daily preparation, along with the potential problems which can arise if the Tc-99m pertechnetate solution is not homogeneous or of uniform thickness, have led many nuclear medicine departments to prefer Co-57 flood sources.


The camera's energy peak was reset to the correct value used for Tc-99m containing radiopharmaceuticals (140 keV) and the patient re-imaged. The findings include a right femoral prosthesis with no evidence of fracture.

View followup image(bs). Delayed anterior and posterior images with the camera set on the correct peak (140 keV).

Major teaching point(s):

Do not forget to include 'camera off-peak' in the differential diagnosis of a bone scan with apparent poor skeletal radiopharmaceutical uptake.

Differential Diagnosis List

Once incorrect energy peak has been excluded, the general differential diagnosis for poor skeletal localization of Tc-99m MDP includes poor radiopharmaceutical preparation, congestive heart failure, bisphosphonate therapy, osteoporosis, and incorrect radiopharmaceutical administration (e.g., Tc-99m DTPA instead of Tc-99m MDP for bone scintigraphy).

ACR Codes and Keywords:

References and General Discussion of Bone Scintigraphy (Anatomic field:Skeletal System, Category:Normal, Technique, Congenital Anomaly)

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Case number: bs069

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