Dynamic SPECT/CT HIDA imaging, an improved technique for quantitative Liver function ?

Introduction: Functional evaluation of the liver is an important tool for the optimal management of patients with a diseased liver. Many different clinical and imaging methods are available. including Childs pugh score, ICG and MRI. However , only a few are available to provide both global and regional quantification as do Nuclear medicine techniques. In counties where TC-GSA is available this has been shown to be useful technique, however it is not universally available and dynamic planar TC- HIDA with delayed SPECT has shown to be an acceptable replacement. Unfortunately Tc-HIDA, as opposed to Tc-GSA, is excreted during the delayed SPECT phase of imaging, limiting the accuracy of the study for segmental function. A new emerging technique is Dynamic SPECT/CT. This consists of continuously acquiring SPECT data and then using the CT anatomical data for attenuation corrected post processing and quantification. It been employed mainly for improved cardiac evaluation, but has the potential to help with many different organ systems. Due to the relative constant Liver extraction rate of HIDA between 1 and 5 minutes a physiologic window is available for dynamic SPECT/CT acquisition. We are planning to develop both the acquisition and post processing aspects of this technique to address the limitations of the early excretion of Tc-HIDA by assessing both the global and regional function during the early post injection phase, avoiding the delayed excretory phase and utilizing voxel based attenuation corrected quantification. We hope this will then provide an improved, more accurate assessment of liver function similar to that of Tc-GSA in countries where this is not available. Materials and Results: 15 adult subjects, (9 F,6 M), age 19-65 (mean 44), all underwent HIDA scans for routine clinical indications. Acquisition: Dynamic SPECT /CT : 3 full rotation of 120 step, 3 degrees per step. 1 sec per step. Acquisition frames are rearranged to form a constant revolution direction. Processing: OSEM Iterative reconstruction with attenuation correction is performed on each SPECT set. Sum of the reconstructed sets is used to segments the organs along the registered and fused CT. The summed NM image has better statistics (ease of segmentation) and covers the full dynamic interval. Counts are measured from each segmented VOI (Liver lobes, Heart) from each SPECT set. (time slot) Time Activity Curves are drawn for the organs and the user is prompted to select the portion of the curve showing linear blood excretion into the liver.

Results: Liver clearance index (% extraction per minute): In subjects with known liver disease ranged from 0.83-6.4 (3.6), normal livers 2.55-9.27 (4.79). Discussion: The new Technique showed different result distribution than expected based on prior planar experience and further studies are planned to perform both planar and dynamic SPECT/CT. However, we believe that further development of this new exciting technique will bring incremental improvement in quantification and time reduction in nuclear medicine studies.