Bone Isotope Scan 2015/2016 الدكتور قصي المقبل أستاذ مشارك- الطب النووي كلية الطب-جامعة العلوم والتكنولوجيا الأردنية

Introduction Bone scan is one of the most commonly performed procedures in nuclear medicine. Bone scan often provides an earlier diagnosis and demonstrates more lesions than are found by radiographic procedures. Bone scan has the advantage of screening the entire bone skeleton.

Sensitivity and Specificity Bone scan is very sensitive study but it is not specific. The differential diagnosis of any lesion seen on bon scan is wide including tumor, infection and fracture. The diagnosis is usually made based on clinical history, lesion location, number of lesions, correlation with lab and other imaging data (x-ray, CT, MRI…).

Radiopharmaceuticals They are bone seeking agents. They are labeled with Tc99m. They are phosphate analogs They are given intravenously.

SEM of the Hydroxyapatite Crystal Bone Composition 45-70% inorganic Calcium, phosphorus, magnesium crystalline form of apatite Ca10(PO4)(OH2) 25-30% organic collagen cellular elements 5-25% water The inorganic portion of bone accounts for about 45-70% of its dry wt; with the bulk of the mineral in bone being calcium & phosphate. In an adult this mineral is primarily crystalline in structure and resembles synthetic hydroxyapatite crystals; Minor mineral constituents of bone are magnesium, sodium, potassium, and a number of trace elements (zinc, manganese, fluoride, and molybdenum) considered to be adsorbed on the bone surface; A smaller 25-30% is organic, primarily collagen, ground substance, and cellular elements An even smaller percent is 5-25% water SEM of the Hydroxyapatite Crystal

Mechanism of Localization The radiopharmaceutical binds to the hydroxyapatite [Ca3(Po4)2] structure of bone tissue. Bone remodeling (bone injury) results in more accumulation of hydroxyapatite. So, more radiopharmaceutical will deposit in that region giving “hot” area (active lesion or osteoblastic lesion). 50% of injected dose localizes in the bone and the remain dose is cleared by the kidneys.

Whole body bone scan Inject radiopharmaceutical and image in 2 hours. Bone scan means whole body bone scan. The kidneys are the only soft-tissue organ normally seen on bone scan. Attention should be made to injection site and growth plates in children.

Three Phase Bone Scan It helps in diagnosing soft tissue hyperemia. First phase is the perfusion phase or vascular phase (30 images over 1 min just after injection). Second phase is the blood pool phase or soft tissue phase (image taken 5 min after injection). Third phase is the bony phase (2 hours after injection).

Perfusion

Blood Pool

Hyperemia If there is a focal increased activity in the first and second phases, hyperemia or acute inflammatory process is present.

Third Phase It is the bony phase image obtained in 2 hours post injection. It is the same as whole body bone scan.

Clinical Applications Metastatic bone disease Osteomyelitis Stress fractures Joint Prosthesis

Metastatic Bone Disease Bone scan is an extremely important staging tool during management of cancer patients. Any cancer potentially could cause bone metastatic disease. However, prostate, breast and lung cancers have propensity to metastasize to bone.

Metastatic Bone Disease and Bone Pain About 80% of patients with known cancer and bone pain have metastases documented by bone scan. 30 – 50% of patients with metastatic bone disease do not have bone pain.

Metastatic Bone Disease and Image Findings The hallmark of metastatic bone disease is multiple foci of increased osteoblastic activity in bony skeleton. The distribution of metastatic bony lesions is usually noted in axial skeleton since more blood supply goes to red marrow that resides in axial skeleton in adults.

Note: normal growth plate uptake Bone sarcoma: no bone metastasis Note: normal growth plate uptake Findings: focal uptake in proximal right tibia due to known osteosarcoma

Diffuse multiple foci of increased uptake in axial skeleton = bone metastasis

Stress Fractures It is a fracture (involving less than 50% of the cortex) that is difficult to visualize on a plain x-ray. Fractures may be identified by bone scan as early as 24 hours after occurrence. 3-phase bone scan is the modality of choice. There is hyperemia and osteoblastic process (three phases are positive).

Stress Fracture Blood Pool 2 hour delay Anterior Posterior Left Lat Right Lat Blood Pool Anterior Posterior Left Lat Right Lat 57 year old man with painful left ankle. Plain films showed an area of sclerosis at the distal left tibia. Pertinent Findings: Blood flow images demonstrate an increase in activity in the distal left tibia. The bone scan shows increased activity along the anterior cortex and at the site of sclerosis. Diagnosis: Insufficiency fracture of the distal left tibia. Discussion: Stress fractures are either insufficiency or fatigue fractures. Insufficiency fractures occur in the setting of normal stress applied to abnormal bone. Abnormality of bone includes diminshed elasticity, deficient mineralization. Fatigue fractures result when abnormal stress is applied to normal bone. 2 hour delay

Stress Fracture A fatigue fracture is due to increased repeated stress on normal bone, such as training for a race. Insufficiency fracture is due to usual stress on abnormal bone, such as is seen in osteoporosis.

Fatigue Stress Fracture in the Right Tibia

Insufficiency Stress Fracture of the Right Hip

Acute Osteomyelitis Early plain x-ray signs of osteomyelitis are non-specific and takes as long as 10-14 days to show up. 3-phase bone scan is usually the procedure of choice to differentiate between osteomyelitis and cellulitis.

Acute Osteomyelitis-cont.. If first and 2nd phases are positive (hyperemia) with normal third phase, diagnosis would be cellulitis. In acute osteomyelitis all 3-phases are positive (hyperemia and osteoblastic process in the bone).

Perfusion

Blood Pool

Whole Body Bone Scan

MRI and Bone scan for Osteomyelitis MRI is excellent for the diagnosis of osteomyelitis and stress fracture. Bone scan is a survey imaging technique for the entire bone skeleton Bone scan can be used in patients who have metallic prosthesis or bone hardware.

Child with Right Hip Pain Focal uptake is noted in the right knee consistent with osteomyelitis/septic arthritis

Bone Scan for Hip Replacement Right hip replacement (focal defect) with focal uptake around the prosthesis of osteomyelitis.