Temporal Bone CT: Improved Image Quality and Potential for Decreased Radiation Dose Using an Ultra-High-Resolution Scan Mode with an Iterative Reconstruction Algorithm.

S Leng,K.K Koeller,R.E Carter,J.I Lane,F.E Diehn,C.H Mccollough,R.J Witte

doi:10.3174/ajnr.a4338

Abstract

Radiation dose in temporal bone CT imaging can be high due to the requirement of high spatial resolution. In this study, we assessed whether CT imaging of the temporal bone by using an ultra-high-resolution scan mode combined with iterative reconstruction provides higher spatial resolution and lower image noise than a z-axis ultra-high-resolution mode. Patients with baseline temporal bone CT scans acquired by using a z-axis ultra-high-resolution protocol and a follow-up scan by using the ultra-high-resolution-iterative reconstruction technique were identified. Images of left and right temporal bones were reconstructed in the axial, coronal, and Poschl planes. Three neuroradiologists assessed the spatial resolution of the following structures: round and oval windows, incudomallear and incudostapedial joints, basal turn spiral lamina, and scutum. The paired z-axis ultra-high-resolution and ultra-high-resolution-iterative reconstruction images were displayed side by side in random order, with readers blinded to the imaging protocol. Image noise was compared in ROIs over the posterior fossa. We identified 8 patients, yielding 16 sets of temporal bone images (left and right). Three sets were excluded because the patient underwent surgery between the 2 examinations. Spatial resolution was comparable (Poschl) or slightly better (axial and coronal planes) with ultra-high-resolution-iterative reconstruction than with z-axis ultra-high-resolution. A paired t test indicated that noise was significantly lower with ultra-high-resolution-iterative reconstruction than with z-axis ultra-high-resolution (P < .001), with a mean noise reduction of 37% (range, 18%-49%). The ultra-high-resolution-iterative reconstruction scan mode has similar or slightly better resolution relative to the z-axis ultra-high-resolution mode for CT of the temporal bone but significantly (P < .01) lower image noise, which may enable the dose to be reduced by approximately 50%.

Highlights

BACKGROUND AND PURPOSERadiation dose in temporal bone CT imaging can be high due to the requirement of high spatial resolution
Since the introduction of multidetector techniques, CT has become a major diagnostic technique for temporal bone imaging because its high spatial resolution is well-suited to the task of visualizing the fine anatomic structures of the middle and inner ear.[1,2,3,4,5]
Different approaches have been introduced. One of these is the use of an attenuating comb filter to reduce the detector aperture in both fan and cone angle directions, which is referred to as the z-axis ultra-high-resolution

Summary

MATERIALS AND METHODS

Patient Enrollment and CT Scans This retrospective study was approved by our institutional review board and was Health Insurance Portability and Accountability Act– compliant. Patients with temporal bone CT scans acquired by using a zUHR protocol who underwent a follow-up scan by using the UHR-IR technique were identified by searching the electronic medical records. The thinnest available image section thickness (0.5 mm) was used, with an increment of 0.3 mm For both original and follow-up examinations, images of the left and right temporal bones were reconstructed in the axial, coronal, and Poschl planes, as per our routine clinical protocol. Statistical Analysis The Wilcoxon signed rank test was performed to compare the scores of image resolution between UHR-IR and zUHR for the aforementioned individual structures (round window, incudomallear joint, oval window, incudostapedial joint, spiral lamina in the basal turn, and scutum) and reconstruction planes (axial, coronal, and Poschl). The difference in image noise between the 2 techniques was calculated, and dose reduction was estimated on the basis of the relationship between image noise and radiation dose (ie, radiation dose is inversely proportional to the square of image noise in CT)

RESULTS

CONCLUSIONS