Zero TE MRI for Craniofacial Bone Imaging

Technical Considerations

In ZTE, readout gradients are turned on before the RF pulse so that data acquisition can begin immediately after excitation. This requires high-performance coils with rapid transmit/receive switching capabilities and uses center-out k-space encoding. With ZTE, the readout gradient is already on during the RF pulse, precluding conventional section/slab selection. This necessitates a high-bandwidth excitation with a short-duration hard pulse to ensure that the excitation is as homogeneous and as consistent as possible across the entire FOV from repetition to repetition. The RF excitation is immediately followed by a 3D data acquisition. Gradients are used continuously and modulated between repetitions to sample data along radial trajectories in k-space, rather than being rapidly turned on and off. This process minimizes mechanical vibration and enables a virtually silent technique. Fully sampling the k-space with radial trajectories requires significantly more repetitions (TRs) than an equivalent 3D Cartesian acquisition. Reduction of total scan time is achieved with the use of short TRs and small flip angles for quasi-steady-state magnetization. Most scan time is targeted toward data acquisition, with minimal dead time occurring during the RF pulse and a single gradient step (On-line Fig 1).^3-6 This feature permits rapid isotropic scanning with high resistance to artifacts, including susceptibility and motion. The technique uses lower changes in amplitude over time (dB/dt) than con-ventional sequences; low flip angles and built-in scanner safety measures counter the short RF pulse duration that would otherwise raise concern for a high specific absorption rate (SAR).⁹

Patient Indications

Imaging examinations (CT and MR imaging) were ordered by referring physicians for various diagnostic indications, including craniosynostosis, trauma, head and neck cancer, radiation therapy, and surgical planning and follow-up. Sedation or anesthesia or both were used for imaging only when deemed medically necessary by the referring clinician. In patients undergoing MR imaging, ZTE images of the whole head were acquired using FDA-approved product hardware (8-channel brain coil array; GE Healthcare, Milwaukee, Wisconsin) and software (Silenz; GE Healthcare).^7,8 Imaging was preferentially performed at 3T (Discovery MR750w, DV 26.0 software; GE Healthcare), except for patients with MR imaging–conditional devices necessitating imaging at 1.5T (Optima MR450w, DV 26.0 software; GE Healthcare). The ZTE sequence had the following parameters: 3D imaging mode with sagittal acquisition; TE = 0.016 ms, FOV = 18 × 18 cm, readout matrix = 180 × 180 × 180, section thickness = 1 mm, NEX = 1, receiver bandwidth = 31.25 kHz, flip angle = 2°. These yielded whole-head coverage at a 1-mm isotropic voxel size. Sequence scan time was 2 minutes at 3T and 2 minutes 30 seconds at 1.5T. Two separate data acquisitions were obtained in each patient, for a total imaging time of 4–5 minutes. We used 2 relatively short acquisitions rather than a single acquisition with a higher number of averages to reduce the overall impact of patient motion. All SAR measurements were confirmed to be within FDA safety limits based on predetermined scanner settings. The institutional review board at Mayo Clinic approved retrospective review and analysis of CT and MR images.

Image Postprocessing

By means of a novel technique developed at our institution, source images underwent postprocessing using an off-line Matlab code (MathWorks, Natick, Massachusetts) to improve bone-air differentiation and generate “bright-bone” reconstructions visually analogous to CT.²⁰ For most patients, both ZTE acquisitions were averaged to achieve better SNR after rigid body registration. In cases where 1 ZTE acquisition was visibly distorted by motion and the other was not, only the acquisition with less motion was used for postprocessing, albeit with a lower total SNR. Raw magnitude images underwent signal intensity correction on a section-by-section basis, with normalization to mean soft-tissue signal within each section. Multiresolution ROI-based intensity correction was then applied to account for residual signal intensity variations and thus reduce overall image noise.²¹ Logarithmic inversion of the intensity-corrected data yielded preliminary bright-bone images, but without optimal bone-air delineation. An intensity histogram was generated and an intensity mask was selected to remove the peak signal of air, generating the final pseudoCT image and enabling generation of 3D volume-renderings at a separate workstation (Fig 1).

Case Series

Fourteen patients (2 months to 17 years of age) were successfully imaged. The following examples demonstrate that ZTE was qualitatively comparable with CT for pre- and postoperative assessment of bony abnormalities of the skull, face, and craniocervical junction. The additional value of MR imaging was provided for brain parenchymal and soft-tissue characterization in multiple instances.

Patient 1 was a 3-month-old boy with left anterior plagiocephaly. CT and ZTE comparably demonstrated premature fusion of the left coronal suture with associated calvarial flattening and harlequin eye deformity. The remaining cranial sutures were confirmed to be patent (On-line Fig 2).

Patient 2 was a 7-month-old girl with Apert syndrome. CT and ZTE both demonstrated multisuture craniosynostosis with turribrachycephaly. Gyral remodeling of the inner table (“copper beaten” appearance) was appreciated on both modalities and reflective of increased intracranial pressure. Intracranial malformations were more fully depicted on MR imaging (On-line Fig 3).

Patient 3 was a 2-year-old girl with achondroplasia. CT and ZTE both demonstrated the characteristic J-shaped sella and foramen magnum stenosis. Intracranial abnormalities were more completely imaged on MR imaging (On-line Fig 4).

Patient 4 was a 6-year-old boy with Saethre-Chotzen syndrome. MR imaging was degraded by severe patient motion. Nevertheless, CT and ZTE both demonstrated craniocervical segmentation abnormality with anterior homeotic transformation and basilar impression (On-line Fig 5).

Patient 5 was a 7-month-old boy with hydrocephalus postshunting with worsening posterior plagiocephaly. MR imaging was performed at 1.5T for the patient’s MR imaging–compatible Strata shunt (Medtronic, Minneapolis, Minnesota). CT and ZTE both demonstrated the course of the shunt catheter. There was secondary flattening of the right parietal calvaria subjacent to the shunt valve, without synostosis. No appreciable difference in diagnostic image quality was observed on the 1.5T scan. Brain abnormalities were more completely characterized on MR imaging than on CT (On-line Fig 6).

Patient 6 was a 15-month-old girl with Shprintzen-Goldberg syndrome postshunting. MR imaging was performed at 1.5T for the patient’s MR imaging–compatible Strata shunt. CT and ZTE both imaged the entirety of the shunt catheter. Multiple craniofacial anomalies were imaged, including pancraniosynostosis, severe turribrachycephaly, copper beaten skull, midface hypoplasia, exorbitism, and basilar impression. No appreciable difference in diagnostic image quality was observed on the 1.5T scan. Intracranial malformations were better evaluated on MR imaging than on CT (On-line Fig 7).

Patient 7 was a 7-month-old boy with atraumatic head deformity. CT and ZTE revealed a Galassi type III arachnoid cyst with associated calvarial remodeling and intracranial midline shift (On-line Fig 8).

Patient 8 was a 1-year-old boy with a remote history of a fall and worsening head deformity. CT and ZTE equivalently demonstrated a leptomeningeal cyst or growing fracture of the right parietal calvaria with underlying parenchymal encephalomalacia. This presumably represented expansion of a remote skull fracture with dural injury and chronic CSF pulsations (On-line Fig 9).

Patient 9 was an 8-year-old boy with chronic epilepsy on anticonvulsants. CT and ZTE equivalently demonstrated diffuse hyperostosis with normal sutural, sinonasal, and temporal bone anatomy for his age. Brain abnormalities were better depicted on MR imaging (On-line Fig 10).

Patient 10 was a 2-month-old girl with breathing difficulties. CT and ZTE both showed bilateral choanal stenosis with inward bowing of the posterior nasal walls. Brain abnormalities were more readily evaluated on MR imaging (On-line Fig 11).

Patient 11 was a 17-year-old girl with neurofibromatosis type 1. MR imaging demonstrated orbitotemporal neurofibromatosis with left proptosis and transspatial plexiform neurofibroma. ZTE confirmed characteristic bone remodeling with expansion of the left bony orbit and sphenoid wing dysplasia. Soft-tissue findings were better depicted on MR imaging (On-line Fig 12).

Patient 12 was an 11-year-old girl with right facial pain and swelling. CT and ZTE both depicted erosion of the right pterygoid plate secondary to a right masticator space mass, biopsied as alveolar rhabdomyosarcoma. Additional soft-tissue detail was obtained using MR imaging (On-line Fig 13).

Patient 13 was a 6-year-old boy post-calvarial reconstruction with a polyether etherketone implant. CT and ZTE comparably showed the morphology of the cranioplasty and fixating hardware, with minimal image artifacts (On-line Fig 14).

Patient 14 was a 9-year-old girl with facial trauma due to a motor vehicle collision. Preoperative CT demonstrated a mildly displaced left zygomaticomaxillary fracture. Following surgical fixation, ZTE was performed and confirmed anatomic realignment of fracture fragments by fixating hardware, with minimal image artifacts (On-line Fig 15).