Comparison of 128-Section Single-Shot Technique with Conventional Spiral Multisection CT for Imaging of the Temporal Bone

Discussion

High-resolution MR imaging and MSCT are the preferred tools for imaging the temporal bone. While high-resolution MR imaging has the advantage of excellent soft-tissue resolution, high-spatial-resolution MSCT is better at visualizing the delicate osseous structures and, thus, is the basis for the diagnosis of developmental anomalies, traumas, acute or chronic infections, and masses.^1⇓⇓–4 Although the scanning time with modern MSCT scanners is short, especially in contrast to MR imaging, the patient must still remain motionless during the image acquisition, which can be a problem in uncooperative patients or children. Another disadvantage of MSCT is that it exposes patients to rather high levels of ionizing radiation.

With the introduction of wider detector arrays, the SST (in which only 1 rotation of the tube-detector system is performed for data acquisition) became feasible for imaging whole-organ systems (eg, the heart or brain) or anatomic regions such as the temporal bone. The SST data in this study were acquired by using a 128-section MSCT system with a total volume coverage of 38.4 mm. Because the whole temporal bone is scanned with a single gantry rotation, imaging times of ≤1 second can be achieved. Therefore, even moderately cooperative patients could be examined with SST. Moreover, with our SST protocol, a dramatic decrease in radiation exposure could be achieved. As a negative effect, the higher section thickness of our SST resulted in an inferior resolution of the sections compared with MSCT, which was performed in the ultra-high-resolution mode using a special grid in front of the detector array to reduce section thicknesses to 0.4 mm.

The aim of this study was to compare SST with MSCT to determine whether a compromise in the resolution of the SST would result in a reduced diagnostic value.

Although SST subjectively resulted in examinations that had less “sharp” images than MSCT using the ultra high resolution grid, the statistical comparison revealed no significant difference in the overall judgment of diagnostic image quality, whereas the separate evaluation showed a significant difference in 2 of 38 structures (5%).

The bony septum between the apical and middle turn of the cochlea could be judged significantly better with SST. The integrity of the latter together with, for example, the entire cochlear morphology is of special importance in the preoperative assessment of the inner ear before cochlear implantation^8⇓–10 or for the diagnosis of labyrinthitis ossificans.¹¹ Using SST could be an advantage in these patients.

The second anatomic structure that showed a significant difference between SST and MSCT was the lateral mallear ligament, which was imaged more clearly with MSCT. Because either fixation or detachment of the mallear ligaments may lead to impaired transduction of acoustic stimuli, a proper visualization of these delicate anatomic structures is necessary¹² and can be achieved with high-resolution CT.¹³ Therefore, on the basis of our data, we recommend imaging of the mallear ligaments with MSCT rather than SST.

One major advantage of SST is its significantly shorter time of image acquisition. Our SST protocol took only 1 second compared with a duration of 12.29 seconds in our MSCT protocol. This decrease in imaging time of 92% could be of particular use in uncooperative persons and children, who sometimes have to undergo sedation or general anesthesia with all its drawbacks and risks.^14,15 On the other hand, moving artifacts may theoretically lead to repetitive scans, which means additional radiation exposure.

The evaluation of our examinations showed no moving artifacts in the SST scans, whereas the image quality of MSCT was decreased due to patients' moving in 2 cases (Fig 6). Nevertheless, in both cases, image quality was sufficient to establish a diagnosis; thus, the examinations did not need to be repeated.

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Fig 6.

MSCT of the temporal bone, coronal (A) and axial (B) MPR, section thickness 0.8 mm. No moving artifacts are found in the SST scans, whereas the image quality of MSCT is decreased due to the patients moving in 2 cases (A and B). In 1 of those 2 individuals, the mastoid segment of the facial nerve canal (arrow) can barely be seen on the left side due to motion artifacts (B). However, image quality was sufficient to establish a diagnosis in both patients; thus, the examinations did not need to be repeated.

CT plays a major role in overall radiation exposure of patients. Therefore, it would be desirable to find radiation-saving techniques for CT. In this study, SST led to a reduction of the CTDI_vol and DLP of 67% and 79%, respectively, compared with MSCT with a comparable diagnostic quality and, thus, could be an interesting alternative.

A limitation of this study is that the highest possible spatial resolution of the MSCT protocol was not assessed. Whereas SST used a collimation of 0.75 mm, collimation was 0.4 mm with MSCT. However, image evaluation was performed with a constant section thickness of 0.8 mm for both SST and MSCT by using MPRs; thus, the 2 series were made comparable.

Moreover, a possible influence of the slightly different milliampere-second (250 in SST versus 230 in MSCT) and FOV (200 in SST versus 141 in MSCT) on image quality requires discussion. Subsequent studies should be performed with image acquisition at constant section thicknesses, FOVs, and exposure parameters, to confirm our findings.

The evaluation was performed with a windowing of 4000 HU at a center of 700 HU; therefore, we do not think the contrast that was given for SST had any influence on the results of our study.

Because subjective image quality was different in SST and MSCT, a possible influence of this nonintended “unblinding” of the studies on the evaluation must be borne in mind. Although all examinations were performed to examine pathologies at the temporal bone, the visualization of these pathologies was not evaluated in this study. It is not clear whether using this new technique would be equally informative compared with MSCT in a specific disease process or patient population. This question has to be addressed in further investigations.