Analysis of limited-sequence head computed tomography for children with shunted hydrocephalus: potential to reduce diagnostic radiation exposure

Laboratory investigation

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Object

Despite its diagnostic utility, head CT scanning imparts risks of radiation exposure. Children with shunt-treated hydrocephalus exhibit increased risks of radiation toxicity due to the higher vulnerability of developing, immature tissues and frequent scanning. Several methods have been used to achieve dose reduction, including modifications of CT scanner tube current and potential. This retrospective study explores the use of a newly defined limited sequence of axial head CT slices to evaluate children with shunted hydrocephalus and decrease radiation exposure from diagnostic CT scans.

Methods

Consistent sequences of 7 axial slices were extracted from previously performed standard head CT scans in children with shunted hydrocephalus. Chronologically distinct limited sequences of each patient were blindly, retrospectively reviewed by 2 pediatric neuroradiologists and 1 pediatric neurosurgeon. Limited-sequence CT evaluation focused on the adequacy of portraying the ventricular system, changes in ventricular size, and visualization of the proximal catheter. Reviewers assessed all original full series head CT scans at least 4 months later for comparison. Adequacy and accuracy of the limited-sequence CT compared with the gold standard head CT was investigated using descriptive statistics. Effective dose (ED) estimates of the limited-sequence and standard head CT scans were compared using descriptive statistics and the Mann-Whitney test.

Results

Two serial head CT scans from each of 50 patients (age range 0–17 years; mean age 5.5 years) were reviewed both in standard and limited-sequence forms. The limited-sequence CT adequately portrayed the ventricular system in all cases. The inaccuracy rate for assessing changes in ventricular size by majority assessment (2 of 3 reviewers evaluating inaccurately) was 3 (6%) of 50. In 1 case, the inaccurate assessment would not have altered clinical management, corresponding to a 2 (4%) of 50 clinically relevant inaccuracy rate. As compared with the gold standard complete head CT series, the limited-sequence CT exhibited high sensitivity (100%) and specificity (91%) for portraying changes in ventricular caliber. Additionally, the limited-sequence CT displayed the ventricular catheter in 91.7% of scans averaged across 3 observers. Among all scans reviewed, 97 pairs of standard head CT and complementary limited-sequence CT scans contained adequate dosing information to calculate the effective dose (ED). The ED50 of the limited-sequence CT (0.284 mSv) differed significantly from the ED50 of the standard head CT (4.27 mSv) (p < 0.0001). The limited-sequence CT reflected a median absolute reduction of 4.10 mSv and a mean percent reduction of 91.8% in ED compared with standard head CT.

Conclusions

Limited-sequence head CT scanning provided adequate and accurate diagnostic information in children with shunted hydrocephalus. Techniques including minimization of axial slice quantity and modification of CT scanner parameters can achieve significant dose reduction, maintaining a balance between diagnostic utility and patient safety.

Abbreviations used in this paper:BEIR = Biological Effects of Ionizing Radiation; CTDIvol = volume CT dose index; DLP = dose-length product; EAM = external auditory meatus; ED = effective dose; ED50 = median ED; LNT = linear no-threshold.

Article Information

Address correspondence to: Edward S. Ahn, M.D., Division of Pediatric Neurosurgery, Johns Hopkins University School of Medicine, 600 North Wolfe Street, Phipps 560A, Baltimore, MD 21287. email: eahn4@jhmi.edu.

Please include this information when citing this paper: published online September 20, 2013; DOI: 10.3171/2013.8.PEDS1322.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Case 8. Example of limited-sequence head CT. This compressed view displays the limited sequence of 7 axial head CT slices for this patient at 2 chronologically distinct times. The second sequence (lower row) demonstrates a decrease in ventricular caliber compared with the prior sequence (upper row; standard CT performed 11 months earlier). Both sequences provided adequate visualization of the ventricular catheter, as determined by all reviewers.

  • View in gallery

    Case 23. Example of limited-sequence head CT. This compressed view displays the limited sequence of 7 axial head CT slices for this patient at 2 chronologically distinct times. The second sequence (lower row) demonstrates an increase in ventricular caliber compared with the prior sequence (upper row; standard CT performed 3 months earlier). Although providing less visualization of the catheter than in the patient in Case 8, both limited sequences display the proximal tip within the lateral ventricles, as determined by all reviewers.

  • View in gallery

    Bar graph showing frequency of head CT scanning in study subjects, 2008–2012. This graph shows the number of study subjects (y axis) within each specified group of head CT numbers (x axis) at our institution from 2008 to 2012. The percentages of subjects within each group are indicated above the respective bars.

  • View in gallery

    Limited-sequence shunt CT, example from prospective study. This compressed view displays an example of the limited-sequence shunt CT implemented in a current prospective study at our institution. The limited sequence of 7 axial slices starts above the EAM and terminates below the vertex, displaying the ventricular system and proximal ventricular catheter.

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