Syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of hyponatraemia. There are many causes of SIADH, but investigation tends to focus around the most common causes—particularly diseases of the brain and lung, malignancy and medication-induced SIADH [Ellison and Berl (2007, The Syndrome of Inappropriate Antidiuresis. N Engl J Med., 356, 2064–72]. We describe a case of SIADH secondary to atonic bladder in an 83-year old woman, which was discovered on MRI of the abdomen, performed for further characterisation of a known pancreatic lesion. Insertion of a urinary catheter alleviated retention and resulted in prompt resolution of hyponatraemia. This is an under-recognised cause of this common condition, with important implications for investigation and management.

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Objective Imaged scan length (z-axis coverage) is a simple parameter that can reduce CT dose without compromising image quality. In CT coronary angiography (CTCA), z-axis coverage may be planned using non-contrast calcium score scan (CaCS) to identify the relevant coronary anatomy. However, standardised Agatston CaCS is acquired at 120 kV which adds a relatively high contribution to total study dose and CaCS is no longer routinely recommended in UK guidelines. We evaluate an ultra-low dose unenhanced planning scan on CTCA scan length and effective radiation dose. Methods: An ultra-low dose tin filter (Sn-filter) planning scan (100 kVp, maximum iterative reconstruction) was performed and used to plan the z-axis coverage on 48 consecutive CTCAs (62% men, 62 ± 13 years) compared with 47 CTCA planned using a localiser alone (46% men, 59 ± 12 years) between May and June 2019. Excess scanning beyond the ideal scan length was calculated for both groups. Estimations of radiation dose were also compared between the two groups. Results: Addition of an ultra-low dose unenhanced planning scan to CTCA protocol was associated with reduction in overscanning with no impact on image quality. There was no significant difference in total study effective dose with the addition of the planning scan, which had an average dose–length product of 3 mGy.cm. (total study dose: Protocol A 2.1 mSv vs Protocol B 2.2 mSv, p = 0.92). Conclusion: An ultra-low dose unenhanced planning scan facilitates optimal scan length for the diagnostic CTCA, reducing overscanning and preventing incomplete cardiac imaging with no significant dose penalty or impact on image quality. Advances in knowledge: An ultra-low dose CTCA planning is feasible and effective at optimising scan length.

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