Ultrashort Echo Time and Fast Field Echo Imaging for Spine Bone Imaging with Application in Spondylolysis Evaluation

Abstract

Isthmic spondylolysis is characterized by a stress injury to the pars interarticularis bones of the lumbar spines, often missed by conventional magnetic resonance imaging (MRI) necessitating a computed tomography (CT) for accurate diagnosis. We compare MRI techniques suitable for producing CT-like images. Lumbar spines of asymptomatic and low back pain (LBP) subjects were imaged at 3-Tesla with multi-echo ultrashort echo time (UTE) and field echo (FE) sequences followed by simple post-processing of averaging and inverting to depict spinal bone with CT-like appearance. Contrast-to-noise ratio (CNR) for bone was determined to compare UTE vs. FE and single-echo vs. multi-echo images. Visually, both sequences depicted cortical bone with good contrast; UTE-processed provided a flatter contrast for soft tissues that made it easy to distinguish from bone, while FE-processed images had better resolution and bone-muscle contrast, important for fracture detection. Additionally, multiecho images provided significantly (p=0.03) greater CNR compared single-echo. Using these techniques, a progressive spondylolysis was detected in a LBP subject. This study has demonstrated the feasibility of spine bone MRI to yield CT-like contrast. Through the employment of multiecho UTE and FE sequences combined with simple processing, we have observed enhancements in image quality and contrast, sufficient to detect pars fracture.

Keywords: Bone Fracture; Low Back Pain; MRI; Pars Interarticularis.

Figure 1.

Raw MRI images of a lumbar spine acquired with (A) fast spin echo T2 weighted fat suppressed (FSE T2 FS), (B, C) 3D ultrashort echo time (UTE) at varying echo times (TE), and (D, E) 3D field echo (FE) at varying TE.

Figure 2.

Processed images to create CT-like contrast from (A) UTE 1^st echo image, (B) UTE multiecho images, and (C) FE 1^st echo image, and (D) FE multiecho images. Differences between sequences (UTE vs. FE) and improvements in contrast and image quality from multiecho processing are apparent.

Figure 3.

Regions of interest including bone of the pars interarticularis, paraspinal muscles, and air, analyzed to determine SNR and CNR.

Figure 4.

Detection of a moderately-sized pars defect (arrows) in an adolescent athlete with persistent low back pain using FSE T2 FS (A), UTE multiecho processing (B) and FE multiecho processing (C). The processed images may enable easier and more confident diagnosis of isthmic spondylolysis.