Authors (including presenting author) :
Xiao L(1), Chan OL(2), Lee YS(2), Siu J(2), Li A(2), Sin O(2), Lee D(2)
Affiliation :
(1)Department of Oncology, Tuen Mun Hospital, NTWC, (2)Department of Radiology, Tuen Mun Hosptial, NTWC
Keyword 1: :
Interventional MRI, Introperative MRI, Magentic Resonance Imaging
Introduction :
In interventional magnetic resonance (iMRI) or intraoperative MR (IoMRI) practice, patient anatomy and procedural access often require imaging with off-center positioning. This deviation may degrade image quality due to increased susceptibility to system nonlinearities, main field (B₀) inhomogeneity, and gradient imperfections. A comprehensive, quantitative assessment of this effect is essential for establishing quality assurance protocols and ensuring diagnostic confidence during procedures.
Objectives :
To systematically evaluate the degradation of key image quality parameters as a function of off-center displacement in a clinical iMR environment using the American College of Radiology (ACR) MRI accreditation phantom.
Methodology :
The large ACR MRI phantom was imaged on a clinical Siemens Sola 1.5T MRI system (Siemens, Germany). The phantom was sequentially positioned at isocenter and at systematic table offsets (5 cm, 7 cm, 8 cm, 10cm) in the superior-inferior (S/I) directions. Standard ACR T1 spin echo sequences were performed at each position. The signal to noise ratio of the homogeneity slice was calculated based on the method provided by National Manufacturers Electrical Association (NEMA)1. Quantitative metrics per ACR guidelines were measured: geometric accuracy, high-contrast spatial resolution, slice thickness accuracy, slice position accuracy, percent signal ghosting, low-contrast object detectability, and image intensity uniformity 2.
Result & Outcome :
Measurable degradation in image quality parameters was observed with increasing off-center distance. The most pronounced effects were a significant decrease in image intensity uniformity (increased percentage integral uniformity) and SNR which decreased around 0.54% and 1.72% and at offset (8cm) respectively. Geometric distortion and slice position errors also increased beyond ACR tolerance limits at maximum offsets (15cm). Low-contrast detectability and spatial resolution showed minimal change within 10cm but deteriorated at the largest displacement. Off-center positioning in iMRI or IoMRI, while often necessary, leads to quantifiable degradation in specific image quality metrics, primarily SNR and uniformity. The ACR phantom provides a robust framework for this assessment. For high-quality iMRI or IoMRI guidance, protocols should minimize off-center distances where possible, and site-specific tolerances (e.g., ≤10 cm offset) should be established through routine QA. This study provides a baseline for protocol optimization and quality control in interventional MRI practice.
