Perhaps the most useful application of planar image fusion is the merging of bone scans with xrays. Combining the high spatial resolution of xrays with the physiologically sensitive bone scan assist on overcoming the limitations of each. Figure 1 presents a 14 year old male with a history of persistent tenderness of the left wrist following a fall (three weeks). The xray was reported as normal. The bone scan demonstrated increased tracer accumulation extending across the distal growth-plate of the left radius as well as the area surrounding the distal portion of the left scaphoid.

Figure 1

Figure 1: Fusion between the xray and the planar bone scan confirming the presence of a recent fracture involving the left distal radial growth plate. The fusion also localized the ‘hot spot' seen distally on the left scaphoid and confirmed a fracture of the left scaphoid tubercle.

It is not uncommon for MIBG and DTPA to be summed for better localization of adrenal glands. Summation of DTPA and MIBG images requires rigorous adherence to protocol. On the final day of imaging and after all other images have been collected, a posterior lumbar spine image is typically acquired for MIBG. Subsequently and without the patient moving, DTPA is administered and an identical image acquired. As a result, the DTPA image is only summed with a single image.

Image fusion would permit DTPA to be administered more conveniently. Indeed, the patient may be administered DMSA and return hours later for imaging to avoid the confounding sometimes associated with DTPA clearance. The DTPA or DMSA image need not be acquired on the same gamma camera or even using the same acquisition parameters (e.g. matrix) (Figure 2). Moreover, DTPA or DMSA images may be acquired in multiple projections and merged with corresponding MIBG projections acquired, not just in the final imaging session, but across all imaging session. This might include the merging the DTPA or DMSA planar image with the whole body MIBG sweep (Figure 3).

Figure 2

Figure 2: A 29 year old female investigated with 123I MIBG for possible pheochromocytoma. Highlighted by this successful fusion is the advantage of merging over summation with not only different size images between MIBG and DTPA but also a different X,Y aspect (0.67 for MIBG and 0.71 for DTPA).

Figure 3

Figure 3: The whole body fusion study of the patient in figure 2.

Similar applications might include, without being limited to: merging blood flow, blood pool and delayed bone images, merging whole body gallium, whole body thallium and whole body bone images, merging previous scans with current scans, merging lung ventilation and perfusion studies, merging white blood cell studies with bone or gallium studies.

On occasion it is difficult to conceptualise the imaged biodistribution to the original object. While the patient might be standing before us, connecting an area of increased tracer accumulation directly to a point of pain or discomfort may be imperfect. Merging planar scintigraphy with a photograph of the patient, however, may provide more of a promotional tool for referring doctors than a tool for improving diagnostic utility. Certainly if colour images are visually appealing to referring doctors, then having a photograph of their patient merged with the scintigraphy study (localized static or whole body) might increase referrals. More importantly, any number of research studies performed in Nuclear Medicine departments might be assisted by fusion of planar images with photographs. This might be particularly useful if the research involves specialists from outside the Nuclear Medicine domain (Figure 4).

Figure 4

Figure 4: The use of 99mTc pertechnetate to investigate flow patterns in grape vines provided interesting results. The fusion study allowed, in particular, the non Nuclear Medicine personnel initiating the research to better understand and interpret findings. An obvious reveal on the fusion study was the correlation of lack of leaf accumulation to those leaves tapped flat to the gamma camera, inhibiting transpiration driven accumulation.

A key limitation of merging scintigraphy with photographs is the ‘pin hole' effect. Like the pinhole collimator, the photograph may distort (parallax error) towards the edge of images. While not generally perceptible to the eye in photographs, high resolution digital photographs taken close to an object or of large objects may be obviously misaligned post fusion. This is despite every effort to ensure the photograph is taken in the same plane as the scintigraphic image.