European Journal of

Nuclear Medicine Letter to the editor

Quantification of iodine-131 distribution by gamma camera imaging

Sir,

We thank Dr. Bice and colleagues for their interesting comments on our recent paper (Green et al. 1990).

The writers note that the planar quantification meth- od was used in its simplest form. They suggest that a background subtraction could have improved the accu- racy of the planar estimations. While we agree that in simple phantom experiments it is trivial to subtract a constant or slowly varying background, this is of little help when dealing with actual patient scans. More com- plicated schemes of background subtraction often in- volve the prior determination of background organ size and shape using technetium 99m compounds (e. g. Eary et al. 1989). Comparison of images taken using these widely different gamma energies often suffers from dif- ferences in image size and registration. If these problems can be overcome, the problem of organ movement and geometric change must be considered. It is not possible to constrain patients to have, for example, stomach and bladder sizes constant at each scanning time over a 3- week study. Eary et al. found that they could obtain good serial estimates using a "realistic human body phan tom " with constant geometry and constant organ activity ratios but, when using a more realistic live ani- mal model, "agreement was less apparent in the two dogs which were imaged and sacrificed 48 h post infu- sion".

We thought it sufficient to show the problems that can arise using planar imaging when background activity is present. The main part of our work was the develop- ment of a suitable method of transaxial imaging, as this is the most effective way of eliminating these problems.

We agree that filtered back-projection reconstruction with dual-window scatter correction and simple attenua- tion correction is not a final solution to the problem

if SPET reconstruction with iodine 131 is used. It is a method which is relatively simple to implement using existing gamma cameras and imaging software. We feel that the use and limitations of this technique should be evaluated as part of our immuno-directed therapy programme.

The writers comment that they do not find it immedi- ately obvious that the corrections will prove satisfactory in complex imaging situations. The results presented in our paper show that good agreement with values mea- sured by other methods can be achieved in both phan- tom work and patient studies with high activities of io- dine 131 by careful application of these techniques.

We agree with the writers that the development of immuno-guided therapy requires accurate estimates of tumor and normal organ doses. The scheme we have presented provides a reasonably accurate basis for this quantification, which can be usefully applied while better imaging methods are being developed.

Alan Green

CRC Laboratories Department of Medical Oncology Charing Cross Hospital Fulham Palace Road London W6 8RF, UK

R e f e r e n c e s

Green AJ, Dewhurst SE, Begent RHJ, Bagshawe KD and Riggs SJ (1990) Accurate quantification of 131I distribution by gam- ma camera imaging. Eur J Nucl Med 16:361-365

Eary JF, Appelbaum FL, Durack L, Brown P (1989) Preliminary validation of opposing view method for quantitative gamma camera imaging. Med Phys 16:382-387

Eur J Nucl Med (1991) 18:144 © Springer-Verlag 1991