Using the T11 vertebra to minimise the CT-KUB scan field

Abstract

Objectives: Computed tomography scans of the kidney, ureters, and bladder (CT-KUB) are crucial in investigating urinary calculi but impart a substantial radiation doses. Radiation can be limited by minimising the scanning field to the necessary area (i.e. from the kidneys to urethra). Before auditing, the superior limit of CT-KUB scans had not been formally clarified at our trust. Consistently ensuring the upper limit of scans is at or below T10 has been shown to be a viable method of performing CT-KUB scans. This study aimed to assess the overscan length of CT-KUB investigations and modify practice accordingly to minimise it. There were two standards that were set for CT-KUB scanning. First, the mean percentage overscan length (i.e. percentage of the scan above the kidneys) should be <15%. Second, all scans should include the superior borders of both kidneys.

Methods: 90 consecutive CT-KUB scans for ureteric calculus were retrospectively investigated using IMPAX software in the first phase of data collection. After these data were analysed, a newly devised protocol using T11 as the superior scan limit was delivered to radiographers in the department. and 105 in the second phase (re-audit). The analysis parameters were: percentage overscan length, distance between diaphragm and upper border of kidneys, vertebral level at which the scan commenced, and whether both kidneys were fully included.

Results: In the first phase, overscan of >15% was present in 94.4% of scans. The mean percentage overscan length was 28.2%. The superior vertebral limit of 59% of scans was at T10 or below and a lower superior vertebral limit correlated with decreasing overscan. 99% of scans fully included both kidneys. In the second phase (3 months later), the mean overscan percentage reduced to 10.6% (standard deviation = 4.4%). Excessive overscan affected 35.2% of scans. The superior vertebral limit of 8% of scans was at T10 or below. 100% of scans fully included both kidneys.

Conclusion: Excessive overscanning was due to inconsistent technique in capturing CT-KUB scans. Before this audit, the superior limit of CT-KUB scans had not been formally clarified at our trust. By successfully standardising the process with a reproducible method, the overscan target was comfortably met. Therefore, patient dose was minimised without compromising scan quality.

Advances in knowledge: This audit has successfully shown a feasible standardised protocol for CT-KUB investigations which can be used to minimise overscanning of patients.

Figure 1.

The vertebral-landmark protocol developed following the initial cycle of the audit. Staff were advised to carry out the following: (1) this protocol was only for focussed kidney/ureter/bladder scans (i.e. CT KUBs). (2) The midpoint of T11 should be used. This could be easily identified by identifying the articulation of the thoracic vertebrae with the ribs. An alternative method of counting up from the sacrum could be used but whilst noting that lumbosacral anatomical variation could impair the reliability of this method. CT-KUB, CT scan of the kidney, ureters, and bladder.

Figure 2.

Distribution of overscan percentage before and after protocol change.

Figure 3.

Relationship between vertebral level and percentage overscan before and after the protocol change. The data labels above each column indicate the number of scans.

Figure 4.

Relationship between the scan length distance between the diaphragm and kidney (the Diaphragm–Kidney) Distance and percentage overscan length.