Evaluation of Lower-Dose Spiral Head CT for Detection of Intracranial Findings Causing Neurologic Deficits

Fellows’ Journal Club

Projection data from 83 patients undergoing unenhanced spiral head CT for suspected neurologic deficits were collected. A routine dose was obtained using 250 effective mAs and iterative reconstruction. Lower-dose configurations were reconstructed (25-effective mAs iterative reconstruction, 50-effective mAs filtered back-projection and iterative reconstruction, 100-effective mAs filtered back-projection and iterative reconstruction, 200-effective mAs filtered back-projection). Three neuroradiologists circled findings, indicating diagnosis, confidence, and image quality. The routine-dose jackknife alternative free-response receiver operating characteristic figure of merit was 0.87. Noninferiority was shown for 100-effective mAs iterative reconstruction and 200-effective mAs filtered back-projection, but not for100-effective mAs filtered back-projection. The authors conclude that substantial opportunity exists for dose reduction using spiral nonenhanced head CT and that the dose level might potentially be reduced to 40% of routine dose levels or a volume CT dose index of approximately 15mGy if slight decreases in performance are acceptable. The beneficial effect of iterative reconstrution was most pronounced at this 15-mGy dose level.

Abstract

Acute left lentiform nucleus infarct (circle indicates reference neuroradiologist markings at routine dose) with corresponding lower-dose FBP CT images along with reader results. The imaging finding on this CT examination evolved with time, with corresponding clinical confirmation of corresponding neurologic deficit by a staff neurologist, and the final diagnosis was recorded as acute left striatal infarct.

BACKGROUND AND PURPOSE

Despite the frequent use of unenhanced head CT for the detection of acute neurologic deficit, the radiation dose for this exam varies widely. Our aim was to evaluate the performance of lower-dose head CT for detection of intracranial findings resulting in acute neurologic deficit.

MATERIALS AND METHODS

Projection data from 83 patients undergoing unenhanced spiral head CT for suspected neurologic deficits were collected. Cases positive for infarction, intra-axial hemorrhage, mass, or extra-axial hemorrhage required confirmation by histopathology, surgery, progression of findings, or corresponding neurologic deficit; cases negative for these target diagnoses required negative assessments by two neuroradiologists and a clinical neurologist. A routine dose head CT was obtained using 250 effective mAs and iterative reconstruction. Lower-dose configurations were reconstructed (25-effective mAs iterative reconstruction, 50-effective mAs filtered back-projection and iterative reconstruction, 100-effective mAs filtered back-projection and iterative reconstruction, 200-effective mAs filtered back-projection). Three neuroradiologists circled findings, indicating diagnosis, confidence (0–100), and image quality. The difference between the jackknife alternative free-response receiver operating characteristic figure of merit at routine and lower-dose configurations was estimated. A lower 95% CI estimate of the difference greater than −0.10 indicated noninferiority.

RESULTS

Forty-two of 83 patients had 70 intracranial findings (29 infarcts, 25 masses, 10 extra- and 6 intra-axial hemorrhages) at routine head CT (CT dose index = 38.3 mGy). The routine-dose jackknife alternative free-response receiver operating characteristic figure of merit was 0.87 (95% CI, 0.81–0.93). Noninferiority was shown for 100-effective mAs iterative reconstruction (figure of merit difference, −0.04; 95% CI, −0.08 to 0.004) and 200-effective mAs filtered back-projection (−0.02; 95% CI, −0.06 to 0.02) but not for 100-effective mAs filtered back-projection (−0.06; 95% CI, −0.10 to −0.02) or lower-dose levels. Image quality was better at higher-dose levels and with iterative reconstruction (P < .05).

CONCLUSIONS

Observer performance for dose levels using 100–200 eff mAs was noninferior to that observed at 250 effective mAs with iterative reconstruction, with iterative reconstruction preserving noninferiority at a mean CT dose index of 15.2 mGy.

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