Sunday Case of the Day Category Physics Question:

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Presentation transcript:

Sunday Case of the Day Category Physics Question: Authors: Miguel Flores MD, Steven Messina MD, John Murray MD, William F Sensakovic PhD Florida Hospital Department of Radiology Question: Increasing which parameter will reduce the demonstrated MRI artifact (ignore motion)? TR Receiver bandwidth Frequency encode matrix size Static field strength (B0) Figure 1: T2-weighted MR image through the level of the orbits.

Diagnosis: B. Receiver Bandwidth

Correct Answer: B. Receiver bandwidth: Patient has a remote history of left retinal detachment with subsequent silicone oil retinopexy. Silicone oil protons resonate at a lower frequency than water protons. Since spatial positioning of tissue (mapping) on the final image depends on resonant frequency during readout, the lower resonant frequency of silicone will cause it to be artificially displaced from its true position along the frequency encoding direction. This results in a dark rim along one side of the silicone oil, and a light rim on the other. Other answers: Incorrect. TR will not impact chemical shift artifact Correct. Increasing the receiver bandwidth increases “bandwidth per pixel,” thus reducing the magnitude of the chemical shift artifact Incorrect. Considering bandwidth per pixel is calculated by dividing [total receiver bandwidth] / [frequency encode matrix size (pixel)], increasing the frequency encode matrix size will decrease “bandwidth per pixel” Incorrect. Increasing the B0 field will increase magnitude of the chemical shift artifact

Figure 2: T2-weighted MR images demonstrate left intraocular, left para-chiasmatic, and left intraventricular (frontal horn of the left lateral ventricle) silicone oil with associated chemical shift artifact along the frequency-encoding direction (arrows). Ocular motion artifact along the phase-encoding direction is also noted.

Correct Answer: 61 kHz/px 122 kHz/px In vitro preparation of silicone oil on water. At the interface between silicone oil and water (arrow) results in a large band when scanned at a bandwidth of 61 kHz/pixel (left) the defect is roughly halved when scanned at 122 kHz (right). 61 kHz/px 122 kHz/px Increasing receiver bandwidth can reduce artifacts and scan time, but at the expense of signal to noise ratio as demonstrated in the above images.

Intraocular Silicone Oil Correct Answer: How big is the chemical shift? The difference in resonant frequencies between silicone oil and water protons is ~4.4 ppm. Consequently, silicone will have a frequency that is ~280 Hz* slower than water at 1.5T and ~560 Hz slower at 3T. If a sample protocol results in a bandwidth per pixel of 140 Hz/pixel, then a 2 pixel chemical shift can be anticipated at 1.5T (280 Hz / 140 Hz/pixel = 2 pixels). Doubling the receiver bandwidth with an unchanged matrix will double the bandwidth per pixel and decrease chemical shift by a factor of 2 (280 Hz / 280 Hz/pixel = 1 pixel). Intraocular Silicone Oil Vitreous Silicone 140 Hz/pixel 1 Pix 2 Pix 280 Hz/pixel Figure 3: Schematic demonstration of chemical shift magnitude in response to changes in receiver bandwidth * 4.4 ppm*42.6 MHz/T*1.5 T = 281 Hz

References/Bibliography: Acknowledgments We would like to thank ophthalmologist Dr. My Hanh Nguyen from Winter Park, FL for her assistance in obtaining a sample of silicone oil used in practice. Additionally, we would like to thank Florida Hospital Radiology Specialists of Florida (RSF) for the scanner time utilized in the acquisition of included images. References/Bibliography: Elster AD. Chemical Shift Artifact. 2017 [07/24/2017]; Available from: http://mri-q.com/chemical-shift-artifact.html. Mathews VP, Elster AD, Barker PB, et al. Intraocular silicone oil: in vitro and in vivo MR and CT characteristics. AJNR Am J Neuroradiol 1994;15:343-7. Williams RL, Beatty RL, Kanal E, Weissman JL. MR imaging of intraventricular silicone: case report. Radiology. 1999;212(1):151-4.