Hybrid MRI‐Ultrasound acquisitions, and scannerless real‐time imaging

Abstract

Purpose To combine MRI, ultrasound and computer science methodologies toward generating MRI contrast at the high frame rates of ultrasound, inside and even outside the MRI bore. Methods A small transducer, held onto the abdomen with an adhesive bandage, collected ultrasound signals during MRI. Based on these ultrasound signals and their correlations with MRI, a machine-learning algorithm created synthetic MR images at frame rates up to 100 per second. In one particular implementation volunteers were taken out of the MRI bore with ultrasound sensor still in place, and MR images were generated on the basis of ultrasound signal and learned correlations alone in a ‘scannerless’ manner. Results Hybrid ultrasound-MRI data were acquired in eight separate imaging sessions. Locations of liver features, in synthetic images, were compared to those from acquired images: the mean error was 1.0 pixel (2.1mm), with best case 0.4 and worst case 4.1 pixels (in the presence of heavy coughing). For results from outside the bore, qualitative validation involved optically tracked ultrasound imaging with/without coughing. Conclusion The proposed setup can generate an accurate stream of high-speed MR images, up to 100 frames per second, inside or even outside the MR bore.

Respiratory motion estimation of the liver with abdominal motion as a surrogate

“…The results also illustrated the accuracy of the estimated SI liver motion; the MAE ranged between 0.8 mm and 1.9 mm for the 3 regression models. The results demonstrated performances consistent with previous research studies on the liver using external markers, ultrasound and accelerometers as a surrogate . Additionally, Lasso generally outperformed MVR and Ridge in the overall estimation accuracy.…”

“…However, the two requirements are usually contradictory. For instance, MRI offers high image quality (contrast and spatial resolution) at a low update rate, whereas ultrasound offers the opposite . For liver interventions, the soft tissue contrast in MRI makes it the preferable modality .…”

“…For instance, MRI offers high image quality (contrast and spatial resolution) at a low update rate, whereas ultrasound offers the opposite . For liver interventions, the soft tissue contrast in MRI makes it the preferable modality . Apart from the imaging modality used, accurate targeting of hepatic lesions is vital to successfully complete the interventions .…”

“…The navigators are convenient for MR applications since they do not require extra equipment or installation. However, the main disadvantages of an MR navigator are that it is a 1‐dimensional signal and that it might slightly increase the acquisition time . Spirometery is another example of a surrogate signal that relies on measuring the volume of the air flow in and out of the lungs .…”

“…Surrogates could also rely on other medical imaging modalities. For example, high‐resolution US has been studied to measure the diaphragm motion while using low‐resolution MRI as motion data . However, US is still not MRI‐compatible unless the US is wired with optical fibres, which makes it expensive and fragile.…”