Radionuclides in the evaluation of urinary obstruction

Radionuclide renography and renal scanning techniques are ideally suited to the initial and follow-up evaluation of patients with obstructive uropathy. While other modalities are superior in their ability to provide anatomic information, the radionuclide study yields functional information for each kidney without the necessity to resort to invasive studies. In addition, the Nuclear Medicine study is well suited to the evaluation of obstruction where serial studies often are required because of a lower radiation burden compared to urography. This lower radiation dose is especially important in obstruction because of the recurrent nature of several kinds of obstructive uropathy and because of the high incidence in the pediatric age group. The ability to control urine flow rate during the procedure through dehydration or administration of diuretics is an additional benefit. Increasing availability of computerization of nuclear medicine procedures as well as interest in studies employing physiologic intervention (including the diuresis renogram) have assured an important place for radionuclide studies in the evaluation of patients with urinary obstruction.     

Radiopharmaceuticals for hepatobiliary imaging

Tests for liver function have by and large centered around clinical laboratory diagnostic procedures for a number of years. Besides these, radiographic imaging procedures, including oral cholecystography and intravenous cholangiography, serve a very useful purpose, but several of them are invasive and involve a certain degree of risk from the administered contrast media as well as discomfort to the patient. The cholescintigraphic procedures, though noninvasive, have not played a significant role in the evaluation of hepatobiliary disorders prior to the introduction of the currently available. 99mTc-labeled IDAs. These new hepatobiliary agents offer many advantages over the previously utilized radiopharmaceuticals (131I-rose bengal in particular) in terms of the high degree of specificity for localization in the gallbladder with rapid extraction rates by the polygonal cells of the liver and very low excretion via the GU tract. A detailed understanding of the structure distribution relationship of the various groups in the complex enable the design of agents with an improvement in hepatobiliary specificity and other desirable characteristics. In many clinical situations, even in patients with high bilirubin levels, the 99m-Fc-labeled IDAs offer far superior clinical information over the alternative diagnostic imaging modalities. Further, the absorbed radiation dose imparted to the critical organs is far lower than with the older agents. Thus, the introduction of the cholescintigraphic procedures with the 99mTc-labeled IDAs have ushered in a new phase in the diagnostic workup of patients with impaired hepatocellular function and other biliary disorders.     

Renal radiopharmaceuticals--an update

Noninvasive radionuclide procedures in the evaluation of renal disease have been accepted increasingly as effective and valuable alternatives to older clinical methods. The development of suitable radiopharmaceuticals labeled with high photon intensity radionuclides and with 99mTc in particular has stimulated this modality during the last few years. Currently several nearly ideal agents are available for anatomical and functional studies of kidney imparting very low absorbed radiation doses. These include 99mTc-GHA and 99mTc-DMSA for renal morphology and differential function evaluation, 99mTc-DTPA for GFR and 123I orthoiodohippurate for ERPF measurements. A suitable agent as a replacement for the latter labeled with 99mTc is actively being sought. Computer-assisted processing of dynamic renal function studies enables the observer to obtain a wealth of information related to the renal extraction, uptake, parenchymal transit and pelvic transit parameters of the agent administered into the bloodstream. Each of these parameters either globally or differentially contributes to a detailed evaluation of renal disease states. Several of these procedures have been validated against classical techniques clinically but more detailed information is being sought with the recently introduced radiopharmaceuticals. With the detailed validation and increasing recognition of the clinical utility of several of the radionuclidic procedures at many centers, it is hoped that radionuclide assessment of renal disorders ultimately will be made available routinely at all medical facilities.     

Radionuclide evaluation of lung trauma

Nuclear medicine imaging procedures can play a significant role in evaluating the pulmonary complications that are seen in trauma patients. A quantitative method for measuring increased pulmonary capillary permeability that uses Tc-99m HSA allows early diagnosis of acute respiratory distress syndrome (ARDS) and accurately differentiates this condition from pneumonia or cardiogenic pulmonary edema. This technique may be of great value in following the response to therapy. The use of 133Xe to diagnose inhalation injury remains an important diagnostic tool, particularly at hospitals with specialized burn units. Regional decreases in ventilation-perfusion images reliably localize aspirated foreign bodies. Radionuclide techniques that are used to demonstrate gastropulmonary aspiration remain controversial and require further clinical evaluation. Pulmonary perfusion imaging, although nonspecific, may provide the earliest clue for correct diagnosis of fat embolism, air embolism, contusion, or laceration. Furthermore, the possibility of perfusion abnormality due to these uncommon conditions must be remembered whenever trauma patients are evaluated for pulmonary thromboembolism with scintigraphy. Occasionally, liver or spleen scintigraphy may be the most appropriate procedure when penetrating chest trauma also involves these subdiaphragmatic organs.     

Bone and marrow imaging in sickle cell disease: Diagnosis of infarction

Sickling of erythrocytes in patients with S-hemoglobin causes marrow and bone infarction. The former can be demonstrated as a lack of 99mTc-sulfur colloid uptake on marrow imaging examination. These defects may resolve or persist long after the acute episode. If the bone is involved in the acute episode, imaging within the first few days of onset of symptoms can show lack of 99mTc-labeled phosphate uptake, usually in a smaller area than that shown by marrow scanning. Follow-up bone imaging shows increased activity, particularly along the circumference of the bone where periosteal reaction can be demonstrated radiographically. Magnification by use of the pinhole collimator provides better definition of the uptake defect and the distribution of the increased reactive uptake. Timing of examination is important. If marrow imaging is performed in an asymptomatic period, the repeat examination during a painful crisis permits differentiation of old and acute marrow infarction. If 99mTc-phosphate imaging is performed after about 2 days of symptoms, acute infarction can be differentiated from osteomyelitis, which it may mimic clinically. Although osteomyelitis may cause no increased activity in the first 48 hr after onset of symptoms, it is characterized by intense focal activity thereafter (see article by Handmaker in this issue). To assist in differentiating bone infection in a site of marrow infarction demonstrated by marrow imaging, serial bone imaging with magnification may be useful. The uptake defect, followed in several days to 2 weeks, by circumferential increased activity, is a different pattern than the homogeneously intense activity of osteomyelitis, but the peripheral distribution may not be apparent on routine imaging. It is hoped that the utilization of these techniques can decrease the emotional and economic costs of prolonged hospitalization for suspected infection and can also expand our knowledge of the complex pathophysiologic changes of sickle cell bone disease.     

Radionuclide studies of the spleen in trauma and iatrogenic disorders

The first task of radiocolloid spleen imaging, after trauma, is to determine if the organ is functioning. If this cannot be demonstrated, splenic avulsion must be considered. The second task is to document splenic injury (not to suggest splenectomy, but to serve as a baseline for following later healing). Failure to heal may be a risk factor for subsequent splenic rupture. The third task of spleen imaging is to determine if accessory splenic fragments are present and functional. The radiocolloid spleen scan facilitates the successful accomplishment of all of these tasks.