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Hepatic perfusion index measured using MRI and scintigraphy.

The Editor—Sir, We were interested to read the recent paper by Totman et al [1] and commentary by Harvey and Blomley [2] on the use of gadolinium-enhanced volumetric MRI to measure the hepatic perfusion index (HPI) in patients with colorectal cancer. Totman clearly demonstrated a raised HPI – the ratio of liver blood flow from the hepatic artery as a fraction of total liver blood flow – in the patient group compared with a group of control subjects who were undergoing spine imaging and who had no evidence of neoplastic disease. The authors refer to dynamic scintigraphy, Doppler ultrasound and CT which have all been used to measure HPI. We developed a radionuclide technique for measuring HPI [3] and demonstrated its efficacy in identifying patients with colorectal cancer and liver metastases (confirmed by laparotomy). We also showed that in patients whose liver appeared normal at their initial surgery but who subsequently developed liver metastases, HPI measured at the time of their original surgery was raised in 87% of cases [4]. Totman et al listed the drawbacks of the radionuclide technique as ‘‘involving exposure to ionizing radiation’’ and that ‘‘the technique cannot easily be incorporated into routine diagnostic scanning’’. The commentary by Harvey and Blomley also refers to the MRI approach as having ‘‘the obvious advantages of the lack of ionizing radiation’’. Referring to the comment that dynamic scintigraphy cannot easily be incorporated into routine diagnostic scanning, the technique requires an intravenous injection, takes about 15 min to complete and uses a standard gamma camera with minimal processing. In relation to the exposure to ionizing radiation, if we use the ARSAC diagnostic reference level for Tc colloid of 80 MBq [5], the effective dose from the scintigraphy is approximately 0.8 mSv [6] leading to a risk of about 1 in 25 000 [7]. The risk a cancer of the colon or rectum carries is greater than 1 in 2 [8]. Surely common sense should be applied to estimates of risk with using ionizing radiation. The higher cost and longer acquisition and processing time of MRI compared with scintigraphy certainly cannot be justified on the basis of the removal of a vanishingly small risk. Yours etc., A PARKIN K E GOLDSTONE P J ROBINSON