Use of the CT scout view (topogram) for accurate attenuation correction and quantification of nuclear medicine planar images

Aim: The CT scout view (topogram) is a 2-dimensional X-ray image acquired using a CT scanner. The topogram is routinely used in clinical CT scanning only to define the scan range of the subsequent CT scan. The research project is looking at whether there is quantitative value in the CT topogram to correct for gamma ray attenuation in nuclear medicine planar images. If successful, it will be possible to quantify the absolute uptake of radiotracer in patients using gamma camera planar techniques which may be useful in assessing response to treatments and/or organ function. Method: A relationship was established between the topogram pixel value (shade of grey) and the amount of attenuation of the X-ray photons using a water phantom. Phantoms and patients containing a known activity of radiotracer were imaged using opposing detectors on a standard nuclear medicine SPECT-CT gamma camera followed immediately by a CT topogram. An established mathematical equation was then used to quantify the activity in the phantom/patient using the geometric mean counts from the gamma camera images coupled with the attenuation correction information from the CT topogram. The derived activity using this technique was then compared against the known (true) activity inside the phantom/patient.

Results: All derived measurements of radiotracer activity inside a phantom or a patient were within ±10% of the true activity at the 95% confidence interval. There was more uncertainty in the measurements for larger phantoms/patients. Conclusion: The CT scout view has quantitative value in performing attenuation correction on gamma camera geometric mean images. This technique can be used to quantify the absolute uptake of radiotracer inside a patient to within 10% accuracy at the 95% confidence interval. The technique has been validated for two radionuclides (99mTc and 111In). Further work would be required to validate the technique for other radionuclides (e.g. 123I, 177Lu).

Outputs

The research concept won third prize at the 2017 annual conference of the British Nuclear Medicine Society (BNMS). https://www.bnms.org.uk/

The department is now looking to implement the technique into routine clinical practice.