Nuclear Medicine Skeletal System PDF

Nuclear Medicine Skeletal System Bone scan The bone scan often provides an earlier diagnosis and demonstrates more lesions than are found by radiographic procedures RADIOPHARMACEUTICALS The most widely used radiopharmaceuticals for skeletal imaging are technetium-labeled diphosphonates,...

Nuclear Medicine Skeletal System Bone scan The bone scan often provides an earlier diagnosis and demonstrates more lesions than are found by radiographic procedures RADIOPHARMACEUTICALS The most widely used radiopharmaceuticals for skeletal imaging are technetium-labeled diphosphonates, most often methylene diphosphonate. Why Diphosphonates? They have rapid renal excretion so they provide a high target-to-non target ratio in 2 to 3 hours after injection, with 50% to 60% of the activity localizing in bone and the remainder being cleared by the kidneys. With most diphosphonates, maximal skeletal uptake occurs at about 5 hours. The biologic half-life is about 24 hours. Important points Avoid the injection of air into the mixing vial during preparation of phosphate radiopharmaceuticals because the resultant oxidation of technetium causes poor tagging of the phosphates. If the radiopharmaceutical is administered more than 4 hours after preparation, gastric and thyroid visualization on bone scans may be seen as the result of free pertechnetate. Accumulation of technetium in bone is related to: Blood supply (primary cause, a fourfold increase in blood flow increases bone uptake by 30% to 40%. ) Capillary permeability The local acid–base relation Fluid pressure within bone Hormones Vitamins The quantity of mineralized bone Bone turnover The Bone Scan Indications Infection: Osteomyelitis, septic arthritis Metabolic bone disease Unexplained musculoskeletal pain Paediatrics: suspected non-accidental injury, tumors (primary or secondary) Detection and follow-up of metastatic disease Differentiation between osteomyelitis and cellulitis Determination of bone viability: infarction or avascular necrosis Evaluation of fractures difficult to assess on radiographs (stress fractures, fractures of complex structures) Evaluation of prosthetic joints for infection or loosening Determination of biopsy site TECHNIQUE For routine planar scans, the patient is normally injected intravenously with 10 to 20 mCi (370 to 740 MBq) of the technetium diphosphonate Images are taken 2 to 4 hours later. The site of injection should be distant from any suspected osseous pathology and should be recorded. A slight extravasation of isotope at the injection site causes a focus of increased soft-tissue activity.  In patients suspected of having either osteomyelitis or cellulitis: Three-phase study: a radionuclide angiogram and initial blood pool image are performed after injection, and routine images are obtained at about 2 to 3 hours. In rare cases, additional images are performed 18 to 24 hours after injection (four-phase study). Useful in patients with renal failure who have poor soft-tissue clearance.  Following IV injection MDP circulates in the vascular system for a short time then equilibrates to the extravascular space. Its accumulation in bone is rapid, with excretion of the residual MDP via the urine. Approximately half of the administered dose is eliminated within 4 hours, producing a high bone- to-background ratio of activity, except in situations where renal function is poor. Patient preparation Maintain good hydration with oral fluids (especially after injection, before scanning). Empty the bladder regularly to reduce unnecessary radiation dose to the pelvic organs. The rapid urinary excretion causes large amounts of activity to accumulate within the bladder, which may obscure pelvic lesions Voiding (e.g. incontinent patients), may result in radioactive contamination of skin or clothing; this may obscure underlying pathology or mimic a lesion. Removal of contaminated clothing and cleansing of skin may be necessary to obtain accurate results. No other patient preparation is required. Patient Imaging Two to four hours after injection whole body imaging takes place (using a moving table). This is performed either on a dual-head gamma camera, acquiring anterior and posterior views simultaneously, or on a single head facility, performing spot views Oblique views of the sternum and ribs, lateral views of the lower legs and the pelvis. Magnification views may be useful for improving visualization of the hands and wrists in the adult, and for the hip joints in pediatric patients (using selective pinhole or high-resolution collimator )  Increased vascularity and increased vascular permeability account for the early accumulation of MDP in bone tumors, healing trauma, inflammatory and infected conditions of bone. Bone scan- summary Normal Appearances and Interpretation In the normal adult skeleton individual bones are visualized, and uptake is symmetrical about the midline. There may be some background soft tissue uptake, particularly in an obese patient. Both kidneys and the urinary bladder should be readily identifiable 15-year-old boy: Anterior (left) and posterior (right) images demonstrate markedly increased activity around the epiphyseal plates. This is usually best seen around the knees, ankles, shoulders, and wrists. Areas of common activity Focal maxillary or mandibular alveolar ridge, owing to dental disease. The lower cervical spine, usually representing degenerative changes or simply a result of the lordosis of the cervical spine rather than activity in the thyroid cartilage or the thyroid itself. On the anterior view, prominent visualization of the sternum, sternoclavicular joints, acromioclavicular joints, shoulders, iliac crests, and hips. Increased activity in the knees in older patients (arthritic changes). On the posterior view, the thoracic spine, the tips of the scapulae and the sacroiliac joints are well seen. How do we recognise any abnormality in bone can? Any asymmetric osseous activity should be viewed with suspicion. The kidneys and bladder should be routinely invistigated for focal space- occupying lesions producing photopenic defects in the renal cortex Asymmetric renal activity is not uncommon. Because the scans are usually obtained in the supine position, activity may accumulate in extrarenal pelves. If urinary tract obstruction is suspected, kidney views should be repeated after the patient has ambulated to distinguish obstruction from position-related collecting system activity. Posterior view for the presence and location of renal activity and anterior view, for bladder activity.  If there is extravasation of the radiopharmaceutical at the site of injection, lymphatic drainage may occur, resulting in the visualization of one or more lymph nodes. Activity in axillary lymph node (arrow) after extravasation of injection into left antecubital fossa. Metastatic Disease The cardinal features of skeletal metastatic disease are multiple focal areas of increased MDP uptake, randomly distributed but favoring the axial skeleton, with asymmetric involvement. Over a series of examinations, without therapeutic intervention, these increase in size, number, and intensity of uptake Bone scan compared with radiography For a lytic lesion to be visualized by radiography, localized demineralization of about 30% to 50% must occur Bone scans usually demonstrate metastatic lesions much earlier than radiography does. The false-negative rate of radiographic skeletal surveys may be as high as 50% with certain tumors The false-negative rate of bone scanning for the most common neoplasms may be as low as 2%.  In which cases do we perform a bone scan for cancer patients? For patients with bone pain: about 80% of patients with known neoplasms and bone pain have metastases documented by the bone scan. What about asymptomatic patients? 30% to 50% of patients with metastases do not have bone pain so…. Bone scan is recommended for patients with tumors that have a tendency to metastasize to bone (e.g., breast, lung, and prostate); but for tumors with low rates of osseous metastases (e.g., colon, cervix, uterus, head, and neck), the procedure may not be cost-effective. Follow up bone scans Follow-up bone scans in patients undergoing treatment for advanced breast and prostate cancer should be interpreted with caution. Within the first 3 months of chemotherapy, a favorable clinical response by focal bone metastases may result in healing that causes increased uptake at involved sites, which is usually a good prognostic sign. If not clinically correlated, this flare phenomenon can give the false impression of new lesions or the extension of existing metastatic sites. Bone lesions that appear 6 months or later after treatment almost always indicate disease progression. Malignant Bone Tumors Osteosarcoma In the past, follow-up bone scans were not worthwhile in patients with osteosarcoma because pulmonary metastases almost always develop before osseous metastases. Aggressive chemotherapy has altered the natural history of osteosarcoma, and now about 20% of patients develop osseous metastases before pulmonary disease. Thus follow-up bone scans are now recommended.  Malignant Bone Tumors Osteogenic sarcoma (osteosarcoma) Malignant Bone Tumors Ewing sarcoma: Is a relatively uncommon primary bone tumor Frequently occurring in the pelvis, ribs or femur. Up to 11% of patients present with osseous metastases. On the bone scan, an oblique view of the ribs shows abnormal uptake in the anterior-lateral left third rib. Benign Osseous Neoplasms Most malignant lesions are hyperemic and most benign lesions initially accumulate little radiopharmaceutical. An early blood pool image may therefore be helpful in identifying benign lesions because it may show little or no increased uptake.  The major exception is osteoid osteoma On delayed images, benign lesions may show a wide range of activity. (e.g. osteoid osteomas, usually have intense activity on delayed images and hemangiomas rarely show increased activity and usually cannot be distinguished from normal bone. Trauma Fractures not apparent on routine radiographs may be readily detected with CT, MRI, or radionuclide bone scanning MRI is most useful for occult hip and knee fractures or in cases in which the site of pain is well localized. When multifocal trauma is suspected, bone scanning may be more effective. Fractures Bone scan appearance after fracture : Acute phase: usually lasts from 3 to 4 weeks and demonstrates a generalized diffuse increase in radionuclide activity around the fracture site. Subacute phase: lasts 2 to 3 months, with the activity more localized and intense Healing phase: may occur over a much longer period and is accompanied by a gradual decline in intensity of radiotracer activity.  Pelvic and spine fractures: in the first 3 days, only 30% show increased activity. All recent fractures in the axial skeleton and long bones can be seen by 14 days. Skull fractures: may not show any increase in activity on bone scan. Rib fractures: almost always show intense activity and can often be recognized by their location in consecutive ribs. Single rib fractures are often difficult to distinguish from a metastasis. Rib fractures present as punctate foci of increased activity, whereas neoplastic lesions frequently Rib fractures. These can be have a more linear distribution following the long confidently diagnosed because axis of the ribs. there are consecutive right rib abnormalities, and they have relatively rounded (not elliptical) foci of increased activity.  Occult hip fracture : about 3 days are needed to detect in an elderly patient on bone scan. Thus MRI scan is sometimes preferred to do prompt surgery. Stress fracture Often difficult to visualize on a plain radiograph (for 7-10 days). Radionuclide bone scans are frequently positive at the time of clinical presentation and offer a means of early diagnosis and treatment. Osteomyelitis, Cellulitis, and Septic Arthritis Early involvement of bone by an inflammatory disease process is often difficult to detect on radiographs. MRI is highly effective and has excellent spatial resolution, but it is expensive and its use is limited in patients who have an infected metallic prosthesis. Scanning with radionuclides often demonstrates increased activity, both in the soft tissues and in the underlying bony structures. Radiopharmaceuticals- Osteomyelitis 99mTc-diphosphonate Indium-111- (111In-) or 99mTc-labeled leukocytes 18F-FDG, and gallium-67 citrate (much less common) To differentiate osteomyelitis from cellulitis on a 99mTc diphosphonate bone scan, a radionuclide angiogram and an immediate blood pool image should be obtained after injection, and routine images should be taken at 2 to 3 hours (three-phase). Cellulitis vs Osteomyelitis Cellulitis Increased blood flow (perfusion) and diffusely increased soft tissue on early images, with decreasing activity on later scans. No significant foci of increased bony activity on the delayed images in the area of concern. Osteomyelitis Increased blood flow and blood pool activity with accumulation of bone activity Becomes more focal and intense on delayed scans  Septic arthritis Increased activity in all phases of a three-phase bone scan. Usually, it can be differentiated from osteomyelitis by the presence of diffusely increased bone activity on both sides of the joint, as opposed to osteomyelitis in which the bony activity is typically focal and increased on only one side of the joint. Figure 8-42. Septic arthritis. A, Three-phase bone scan done on this young man who had been bitten over the third metacarpal joint shows increased activity (arrow) on blood pool and B, delayed images. C, Normal radiograph at the time of the bone scan became positive 3 weeks later (D). Note bony destruction (arrows) involving both sides of the joint.  Metabolic Bone Disease: Osteoporosis Osteoporosis is characterized by reduced bone density and increased risk of fracture, and is found most commonly in elderly females. Vertebral compression fractures are the hallmark of this disease. The classic appearance of osteoporotic collapse is a vertebral body that is reduced in height with intense linear MDP uptake across its width.  Another typical fracture in the osteoporotic patient is the sacral insufficiency fracture, which produces a classic appearance on the bone scan. There is vertical linear increased uptake through each sacral ala, bridged by horizontal uptake across the sacral body, forming the pathognomonic “H” sign. The radionuclide bone scan is not used in the assessment of bone density. This is the province of bone densitometry. References Fred A. Mettler, J., MD, MPH and Milton J. Guiberteau, MD. Essentials of nuclear medicine. Sixth edition