2118 Solid-state MRI as a nonradiative alternative to computed
tomography for craniofacial imaging
Authors: Zhang Rosaline , Lee Hyunyeol , Zhao Xia , Song Hee K. , Wehrli Felix W. , Bartlett Scott P. 
Published: November 2018

OBJECTIVES/SPECIFIC AIMS: Computed tomography (CT) enables
3-dimensional (3D) visualization of cortical bone structures with high spatial
resolution, and thus has been the gold-standard method for evaluation and
diagnosis of craniofacial skeletal pathologies. However, ionizing radiation and,
in particular, repeated scanning for presurgery and postsurgery assessments, is
of concern when applied to infants and young children. Recent advances in
solid-state MRI allow the capture of the short-T2 signals in cortical bone while
suppressing the signal from soft-tissue protons having T2 relaxation time
1–2 orders of magnitude longer (50–100 ms). One approach,
a dual-radiofrequency (RF) pulse and ultrashort echo time (UTE) imaging based
method, exploits different sensitivities of bone and soft tissue to different RF
pulse widths and TEs. This study aims to demonstrate the feasibility of
producing 3D renderings of the human skull and visualization of cranial sutures
using the bone-selective MRI technique in comparison to CT.
METHODS/STUDY POPULATION: Imaging technique: Two RF pulses differing
in duration and amplitude are alternately applied in successive repetition time
(TR) along the pulse train. Within each TR, 2 echoes are acquired. Acquisition
of the first echo starts at the ramp-up of the encoding gradient (TE1), allowing
for capture of signals with very short lifetimes (bone), while that of the
second starts after a longer delay (TE2). In total, 4 echoes are obtained:
ECHO11 (RF1TE1), ECHO12 (RF1TE2), ECHO21 (RF2TE1), and ECHO22 (RF2TE2). During
reconstruction, ECHO11 is combined with ECHO21 and ECHO12 is combined with
ECHO22, resulting in 2 images. The subtraction of these 2 images yields an
enhanced bone contrast. Data acquisition/processing: The pulse
sequence described above was applied for MR imaging of a human cadaveric skull
and 2 adult human subjects in vivo, at 3T field strength (Siemens Prisma,
Erlangen, Germany). Imaging parameters:
TR/TE1/TE2=7/0.06/2.46
ms, RF1/RF2 durations=40/520 μs,
flip angle=12°, matrix size=2563, field of
view=2803 mm3, voxel size=1.1 mm isotropic,
number of radial spokes=25,000, and scan time=6 minutes.
Segmentation of bone voxels was performed using ITK-SNAP in a semi-automatic
fashion, leading to 3D renderings of the skull. For comparison, a CT scan was
also performed in the human cadaveric skull with 1 mm isotropic resolution.
Validation: The biometric accuracy was assessed by measuring eight anatomic
distances: (1) Maximum craniocaudal aperture of the right orbit. (2) Maximum
craniocaudal aperture of the left orbit. (3) Maximum height of the mandible from
chin point in the midline. (4) Maximum cranial length (5) Maximum cranial width.
(6) Maximum height of piriform aperture. (7) Distance between lateral most
aspect of mandibular condyles. (8) Distance between lateral most aspect of
posterior hard palate in both CT- and MRI-based 3D renderings of the human
cadaveric skull using Mimics software (Materialise®,
Ghent, Belgium). These distances were compared with those directly measured on
the cadaveric skull. RESULTS/ANTICIPATED RESULTS: Compares CT with
the proposed MRI method on cadaveric human skull images, along with
corresponding 3D renderings. Compared with CT, the 3D rendered images maintain
most features over the entire head (e.g., zygomatic arch), except for appearance
of some artifacts in the mandibular region. In vivo head images in 2 adult
subjects: axial magnitude images and 3D rendering. In the axial images, bone
voxels as well as the inner table of the cranium are clearly visualized, and
cranial and spinal bone structures are well depicted in the 3D renderings. Some
voxels were erroneously included or excluded in the renderings. The mean
difference in measurements of the 8 anatomic distances was 6, 4, and 2 mm when
comparing MRI Versus CT, MRI Versus in situ, and CT Versus in situ,
respectively. DISCUSSION/SIGNIFICANCE OF IMPACT: Bone proton
magnetization exhibits a substantial level of signal decay during the relatively
long duration of RF2 due to its very short T2 relaxation time. In contrast,
soft-tissue retains nearly the same level of signal intensities over all echoes.
Thus, subtraction of ECHO22 from ECHO11, when compared with the difference
between ECHO11 and ECHO12, enhances bone contrast from soft tissue. The
proposed, dual-RF dual-echo 3D UTE imaging technique produces isotropic
high-resolution bone-specified images in the whole head within a clinically
feasible imaging time (6 min), leading to clear visualization of craniofacial
skeletal structures. These are key components necessary for translation to the
clinical setting. Optimization of postprocessing for more realistic 3D
renderings and thus accurate anatomic measurements is currently being
implemented. The proposed method’s potential as a nonradiative
alternative to CT will then be thoroughly evaluated in pediatric patients.