New-Generation Low-Field Magnetic Resonance Imaging of Hip Arthroplasty Implants Using Slice Encoding for Metal Artifact Correction

Abstract

ObjectivesDespite significant progress, artifact-free visualization of the bone and soft tissues around hip arthroplasty implants remains an unmet clinical need. New-generation low-field magnetic resonance imaging (MRI) systems now include slice encoding for metal artifact correction (SEMAC), which may result in smaller metallic artifacts and better image quality than standard-of-care 1.5 T MRI. This study aims to assess the feasibility of SEMAC on a new-generation 0.55 T system, optimize the pulse protocol parameters, and compare the results with those of a standard-of-care 1.5 T MRI.Materials and MethodsTitanium (Ti) and cobalt-chromium total hip arthroplasty implants embedded in a tissue-mimicking American Society for Testing and Materials gel phantom were evaluated using turbo spin echo, view angle tilting (VAT), and combined VAT and SEMAC (VAT + SEMAC) pulse sequences. To refine an MRI protocol at 0.55 T, the type of metal artifact reduction techniques and the effect of various pulse sequence parameters on metal artifacts were assessed through qualitative ranking of the images by 3 expert readers while taking measured spatial resolution, signal-to-noise ratios, and acquisition times into consideration. Signal-to-noise ratio efficiency and artifact size of the optimized 0.55 T protocols were compared with the 1.5 T standard and compressed-sensing SEMAC sequences.ResultsOverall, the VAT + SEMAC sequence with at least 6 SEMAC encoding steps for Ti and 9 for cobalt-chromium implants was ranked higher than other sequences for metal reduction (P < 0.05). Additional SEMAC encoding partitions did not result in further metal artifact reductions. Permitting minimal residual artifacts, low magnetic susceptibility Ti constructs may be sufficiently imaged with optimized turbo spin echo sequences obviating the need for SEMAC. In cross-platform comparison, 0.55 T acquisitions using the optimized protocols are associated with 45% to 64% smaller artifacts than 1.5 T VAT + SEMAC and VAT + compressed-sensing/SEMAC protocols at the expense of a 17% to 28% reduction in signal-to-noise ratio efficiency. B1-related artifacts are invariably smaller at 0.55 T than 1.5 T; however, artifacts related to B0 distortion, although frequently smaller, may appear as signal pileups at 0.55 T.ConclusionsOur results suggest that new-generation low-field SEMAC MRI reduces metal artifacts around hip arthroplasty implants to better advantage than current 1.5 T MRI standard of care. While the appearance of B0-related artifacts changes, reduction in B1-related artifacts plays a major role in the overall benefit of 0.55 T.

Visual and quantitative assessment of hip implant-related metal artifacts at low field MRI: a phantom study comparing a 0.55-T system with 1.5-T and 3-T systems

“…During quantitative artifact assessment, the lowest artifact intensities were observed for the Ti implant at 1.5 T. This contrasts previous investigations, e.g., the study of Matsuura et al, who demonstrated less artifacts for neurosurgical biomaterials at 0.5 T compared to 1.5 T and 3 T [20,21]. This discrepancy might be explained by substantial differences in technical specifications of the scanner systems used.…”

Low‐field MRI: Clinical promise and challenges

“…In the mid‐field range, Khodarahmi et al compared artifacts in 0.55T and 1.5T imaging of patients with hip implants. They found a 45%–64% reduction in image artifact when using 0.55T compared to 1.5T, even when 1.5T scanners used a slice encoding for metal artifact correction protocol, though there was a modest 17%–28% reduction in SNR at 0.55T 167 …”

“…They found a 45%-64% reduction in image artifact when using 0.55T compared to 1.5T, even when 1.5T scanners used a slice encoding for metal artifact correction protocol, though there was a modest 17%-28% reduction in SNR at 0.55T. 167 Another challenge at low-field is fat suppression, which increases contrast when evaluating cartilage, menisci, or bone marrow. 168 Many fat suppression techniques rely on the chemical shift between lipids and water, which is proportional to magnetic field strength.…”

A flexible MRI coil based on a cable conductor and applied to knee imaging

“…We applied the cable coil concept to knee MRI, which is the most widely accepted imaging tool for diagnosing various knee injuries 32 . Amidst the pursuit of faster scan time and better SNR, many advantages of low-field MRI systems (< 1.0 T) for musculoskeletal radiology, such as reduced metal artifacts 33 are often overlooked. Recently, a new-generation whole-body 0.55 T MRI system has been cleared by the FDA for clinical imaging 34 , 35 .…”

“…Recently, a new-generation whole-body 0.55 T MRI system has been cleared by the FDA for clinical imaging 34 , 35 . The system can offer some of the advantages of dedicated extremity scanners such as greater accessibility due to lower cost compared to high-field systems, improved patient comfort given its 80-cm wide bore, as well as reduced susceptibility artifacts and specific absorption rate 33 .…”