Pointwise Encoding Time reduction with Radial Acquisition (PETRA) is the ultrashort TE sequence and the shortest TE will be given by transmit (TX) / receive (RX) switching time and gradient performance ^{1, 2}. Ultrashort echo time (TE) sequence allows MRI to be applied to short T2 tissues such as musculoskeletal ³ and lung ⁴. This PETRA sequence with ultrashort TE contrast can be utilized for direct visualization of short T2 tissue contrast as well as denoised MR imaging 2. MR cisternography is often used for evaluation of inner ear structures, however, evaluation of bony structures in the inner ear on MRI is still challenging. The visualization of the inner ear and facial nerve canal was compared between PETRA and CT in this study.

This study was approved by local IRB. The total 24 patients who underwent MRI including PETRA and whole brain CT were enrolled in this study (average 61 ± 10 years).

MR imaging

All MR image scans were performed at a 3T scanner with 32-channel head coild (Magnetom Skyra, Siemens, Erlangen, Germany). The imaging parameter of PETRA is as follows: TR 4 msec, TE 0.07 msec, Slice thickness 0.67 mm, Flip angle 4 degree, FOV 213 × 213mm, Matrix 320 × 320, Resolution 0.67 × 0.67 mm.

CT imaging

CT scan were performed at multi-detector row CT with 320 rows or 64 rows (Acquillion One or Acquillion 64, Toshiba Medical Systems Corporations, Otawara, Japan). Temporal bone CT images were created with bone window. The imaging parameter of CT imaging is as follows: FOV 210 × 210 mm, Matrix 512 × 512, Exposure Time 750 msec, X-ray Tube Current: 250mA, Exposure 187mAs, Beam Pitch 0.641.

Evaluation

CT images were registered to PETRA by using SPM8 (www.fil.ion.ucl.ac.uk/spm/software). Visualization of auditory ossicles (maleus, incus, stapes), semicircular canals, and facial nerve canal are evaluated with consensus reading by two neuroradiologists. Scores were as follows: grade 3= excellently visible, grade 2= partially visible, grade 1=scarcely visible.

Example images of inner ear structure, auditory ossicles, and the facial nerve canal are shown in Figure 1 and Figure 2. All of auditory ossicles, semicircular canals, and facial nerve canal were more visible on CT than PETRA (Figure 3). The sum of grades are shown. It is evident that CT is superior to PETRA in visualization of inner ear structures, however, facial nerve canal and semicircular canals were commonly recognized, and auditory ossicles were occasionally visualized on PETRA.The evaluation auditory ossicles and facial nerve canals are usually performed on the temporal CT. With the recent advance of reduction technique of irradiation, CT has become a convenient patients-friendly modality. In spite of disadvantageous results in the current study, some improvement will be expected with the higher resolution UTE imaging in near future. It is important for us to explore imaging techniques so as to eliminate unnecessary irradiation in medical imaging.The temporal CT was superior to PETRA in visualization of inner ear structures, however, facial nerve canal and semicircular canals are commonly recognized, and ossicles are occasionally visualized on PETRA.