Proponents of [99mTc]sulfur colloid for GI bleeding studies argue that, although labeled red blood cells are useful for intermittent bleeding, they are not capable of detecting low bleeding rates. Studies of dogs with experimental GI bleeding have indicated bleeding rates of 0.05 ml/min can be detected with [99mTc]sulfur colloid. Since similar data in the dog model were unavailable for 99mTc-labeled red blood cells, we undertook this study. To simulate lower GI bleeding, catheters were inserted into the bowel lumen. Each dog's blood was labeled with 99mTc using an in vitro technique. Venous blood was then withdrawn and re-infused into the lumen of the bowel using a Harvard pump. Fourteen dogs were studied, ten receiving a bleeding rate from 4.6-0.02 ml/min in the descending colon and four with proximal jejunal bleeds of 0.20-0.02 ml/min. Bleeding rates of 4.6-0.2 ml/min were detected within 10 min in the colon and bleeding rates as low as 0.04 ml/min were seen by 55 min. Slower bleeding rates were not detected. Similar findings were noted for proximal jejunal bleeds. Based on the time of appearance, a minimum volume of approximately 2-3 ml labeled blood was necessary to detect bleeding. We conclude that 99mTc-labeled RBCs are sensitive for low bleeding rates in the dog model. The rates are comparable to those described for [99mTc]sulfur colloid in this experimental setting. The time of appearance of activity is related to the bleeding rate.

Although photon deficient defects on bone scan have received a great deal of interest, such defects in bones on Indium-111 (111In) leukocyte imaging have not been as well recognized. We therefore undertook a retrospective review to determine the frequency and significance of such "cold" defects on 111In-labeled leukocyte imaging. Three hundred thirty-two scans on 290 patients were reviewed and 40 cases of decreased activity involving bone were found, for an incidence of 12%. The causes of the defects were: fracture (eight), nontraumatic avascular necrosis (eight), solid tumor (six), prostheses and other orthopedic hardware (four), advanced age (four), radiation (three), leukemia (two), osteomyelitis (two), myelofibrosis (one), postlaminectomy (one), and idiopathic (one). To determine the frequency of cold defects in osteomyelitis, all 15 cases of osteomyelitis in this series were reviewed and 12 showed increased activity, two were cold, and one was normoactive. Thus, 14% of cases of osteomyelitis presented as cold defects. We conclude that cold bone defects do occur on 111In-labeled leukocyte scans and that the causes of such defects are similar to those reported for bone and bone marrow scanning.

Stress fractures are a common injury in long-distance runners, and typically involve the lower extremities. Although relatively rare, pubic ramus stress fractures also occur, primarily in female runners. Bone imaging visualized a pubic stress fracture and a tibial stress fracture in a female long-distance runner with groin pain. Pubic stress fractures should be considered in female runners who present with groin pain. Radionuclide bone imaging is useful in diagnosing these lesions.

Although 111In-labeled leukocytes have been shown to be a useful technique for detecting infection, it has been postulated that antibiotic therapy may reduce the sensitivity of the leukocyte scan. Many patients with suspected bacterial infections are placed on antibiotics before a definite site of infection has been identified. Three hundred twelve leukocyte scans on 271 patients were retrospectively reviewed and classified as positive or negative, and as to whether or not they were being treated with antibiotics at the time the leukocyte scan was performed. The overall sensitivity, considering all 312 studies, was 90%. One hundred sixty-nine patient studies were on patients receiving antibiotics; 143 studies were on patients not on antibiotics. The sensitivity of the leukocyte scan was 88.7% in patients on antibiotic therapy; it was 92.1% in those who were not receiving antibiotics. The differences in sensitivity between the two groups were not significantly different (p less than 0.05). We conclude that antibiotic therapy does not affect the sensitivity of the 111In-labeled leukocyte scan.