technetium-99m-MDP and then scanned with a gamma camera, which captures planar anterior and posterior or single photon emission computed tomography (SPECT) images. In order to view small lesions SPECT imaging technique may be preferred over planar scintigraphy. In a single phase protocol (skeletal imaging alone), which will primarily highlight osteoblasts, images are usually acquired 2–5 hours after the injection (after four hours 50–60% of the activity will be fixed to bones). A two or three phase protocol utilises additional scans at different points after the injection to obtain additional diagnostic information. A dynamic (i.e. multiple acquired frames) study immediately after the injection captures perfusion information. A second phase "blood pool" image following the perfusion (if carried out in a three phase technique) can help to diagnose inflammatory conditions or problems of blood supply. A typical effective dose obtained during a bone scan is 6.3 millisieverts (mSv). PET bone imaging Although bone scintigraphy generally refers to gamma camera imaging of 99mTc radiopharmaceuticals, imaging with positron emission tomography (PET) scanners is also possible, using fluorine-18 sodium fluoride ([18F]NaF). For quantitative measurements, 99mTc-MDP has some advantages over [18F]NaF. MDP renal clearance is not affected by urine flow rate and simplified data analysis can be employed which assumes steady state conditions. It has negligible tracer uptake in red blood cells, therefore correction for plasma to whole blood ratios is not required unlike [18F]NaF. However, disadvantages include higher rates of protein binding (from 25% immediately after injection to 70% after 12 hours leading to the measurement of freely available MDP over time), and less diffusibility due to higher molecular weight than [18F]NaF, leading to lower capillary permeability. There are several advantages of the PET technique, which are common to PET imaging in general, including improved spatial resolution and more developed attenuation correction techniques. Patient experience is improved as imaging can be started much more quickly following radiopharmaceutical injection (30–45 minutes, compared to 2–3 hours for MDP/HDP). [18F]NaF PET is hampered by high demand for scanners, and limited tracer availability. References External links 3D nuclear medical imaging Medical imaging Radiology Scintigraphy