Kiaran P. McGee

Image metric-based correction (Autocorrection) of motion effects: Analysis of image metrics

Magnetic resonance (MR) imaging of the shoulder necessitates high spatial and contrast resolution resulting in long acquisition times, predisposing these images to degradation due to motion. Autocorrection is a new motion correction algorithm that attempts to deduce motion during imaging by calculating a metric that reflects image quality and searching for motion values that optimize this metric. The purpose of this work is to report on the evaluation of 24 metrics for use in autocorrection of MR images of the rotator cuff. Raw data from 164 clinical coronal rotator cuff exams acquired with interleaved navigator echoes were used. Four observers then scored the original and corrected images based on the presence of any motion‐induced artifacts. Changes in metric values before and after navigator‐based adaptive motion correction were correlated with changes in observer score using a least‐squares linear regression model. Based on this analysis, the metric that exhibited the strongest relationship with observer ratings of MR shoulder images was the entropy of the one‐dimensional gradient along the phase‐encoding direction. We speculate (and show preliminary evidence) that this metric will be useful not only for autocorrection of shoulder MR images but also for autocorrection of other MR exams. J. Magn. Reson. Imaging 2000;11:174–181.

3 Tesla MR imaging provides improved contrast in first-pass myocardial perfusion imaging over a range of gadolinium doses

PURPOSE To compare myocardial enhancement during first-pass myocardial perfusion imaging at 3.0 Tesla (T) and 1.5T. MATERIALS AND METHODS First-pass myocardial perfusion imaging was performed on twelve normal subjects at 3T and 1.5T using an interleaved notched saturation recovery gradient echo pulse sequence. Subjects received either 0.10 mmol/kg for both scans (group 1), 0.075 mmol/kg for both scans (group 2), or 0.075 mmol/kg for the 3T scan and 0.10 mmol/kg for the 1.5T scan (group 3). RESULTS Contrast enhancement was significantly greater at 3T than at 1.5T for the 12 subjects whether enhancement was normalized to baseline signal intensity (2.58 +/- 0.76 vs. 1.52 +/- 0.37, p < 0.0001) or to noise (57.6 +/- 19.7 vs. 14.7 +/- 7.8, p < 0001). For each of the three groups, contrast enhancement was significantly greater at 3T versus 1.5T (p < 0.0001, p < 0.001, p < 0.008 when normalized to baseline signal; p < 0.0001 for all groups when normalized to noise). CONCLUSION 3T improves contrast in first-pass myocardial perfusion imaging at either 0.10 mmol/kg or 0.075 mmol/kg.

Magnetic resonance elastography as a method for the assessment of effective myocardial stiffness throughout the cardiac cycle.

MR elastography (MRE) is a noninvasive technique in which images of externally generated waves propagating in tissue are used to measure stiffness. The first aim is to determine, from a range of driver configurations, the optimal driver for the purpose of generating waves within the heart in vivo. The second aim is to quantify the shear stiffness of normal myocardium throughout the cardiac cycle using MRE and to compare MRE stiffness to left ventricular chamber pressure in an in vivo pig model. MRE was performed in six pigs with six different driver setups, including no motion, three noninvasive drivers, and two invasive drivers. MRE wave displacement amplitudes were calculated for each driver. During the same MRI examination, left ventricular pressure and MRI‐measured left ventricular volume were obtained, and MRE myocardial stiffness was calculated for 20 phases of the cardiac cycle. No discernible waves were imaged when no external motion was applied, and a single pneumatic drum driver produced higher amplitude waves than the other noninvasive drivers (P < 0.05). Pressure–volume loops overlaid onto stiffness–volume loops showed good visual agreement. Pressure and MRE‐measured effective stiffness showed good correlation (R2 = 0.84). MRE shows potential as a noninvasive method for estimating effective myocardial stiffness throughout the cardiac cycle. Magn Reson Med, 2010.

MR elastography as a method for the assessment of myocardial stiffness: Comparison with an established pressure–volume model in a left ventricular model of the heart

Magnetic resonance elastography (MRE) measurements of shear stiffness (μ) in a spherical phantom experiencing both static and cyclic pressure variations were compared to those derived from an established pressure–volume (P‐V)‐based model. A spherical phantom was constructed using a silicone rubber composite of 10 cm inner diameter and 1.3 cm thickness. A gradient echo MRE sequence was used to determine μ within the phantom at static and cyclic pressures ranging from 55 to 90 mmHg. Average values of μ using MRE were obtained within a region of interest and were compared to the P‐V‐derived estimates. Under both static and cyclic pressure conditions, the P‐V‐ and MRE‐based estimates of μ ranged from 98.2 to 155.1 kPa and 96.2 to 150.8 kPa, respectively. Correlation coefficients (R2) of 0.98 and 0.97 between the P‐V and MRE‐based estimates of shear stiffness measurements were obtained. For both static and cyclic pressures, MRE‐based measures of μ agree with those derived from a P‐V model, suggesting that MRE can be used as a new, noninvasive method of assessing μ in sphere‐like fluid‐filled organs such as the heart. Magn Reson Med, 2009.

Magnetic resonance elastography of the lung: technical feasibility.

Magnetic resonance elastography (MRE) is a phase‐contrast technique that can spatially map shear stiffness within tissue‐like materials. To date, however, MRE of the lung has been too technically challenging—primarily because of signal‐to‐noise ratio (SNR) limitations and phase instability. We describe an approach in which shear wave propagation is not encoded into the phase of the MR signal of a material, but rather from the signal arising from a polarized noble gas encapsulated within. To determine the feasibility of the approach, three experiments were performed. First, to establish whether shear wave propagation within lung parenchyma can be visualized with phase‐contrast MR techniques, MRE was performed on excised porcine lungs inflated with room air. Second, a phantom consisting of open‐cell foam filled with thermally polarized 3He gas was imaged with MRE to determine whether shear wave propagation can be encoded by the gas. Third, preliminary evidence of the feasibility of MRE in vivo was obtained by using a longitudinal driver on the chest of a normal volunteer to generate shear waves in the lung. The results suggest that MRE in combination with hyperpolarized noble gases is potentially useful for noninvasively assessing the regional elastic properties of lung parenchyma, and merits further investigation. Magn Reson Med, 2006.

Myocardial tagging and strain analysis at 3 Tesla: Comparison with 1.5 Tesla imaging

To determine whether imaging at 3 T could improve and prolong the tag contrast compared to images acquired at 1.5 T in normal volunteers, and whether such improvement would translate into the ability to perform strain measurements in diastole.

Three-dimensional Physical Modeling: Applications and Experience at Mayo Clinic

Radiologists will be at the center of the rapid technologic expansion of three-dimensional (3D) printing of medical models, as accurate models depend on well-planned, high-quality imaging studies. This article outlines the available technology and the processes necessary to create 3D models from the radiologists perspective. We review the published medical literature regarding the use of 3D models in various surgical practices and share our experience in creating a hospital-based three-dimensional printing laboratory to aid in the planning of complex surgeries.

MR elastography of the lung with hyperpolarized 3He

MR elastography (MRE) is a phase contrast–based technique for spatially mapping the mechanical properties of tissue‐like materials. While hyperpolarized noble gases such as helium‐3 (3He) have proven to be an ideal contrast mechanism for imaging of the lung using conventional MR techniques, their applicability for lung MRE is unknown, due to the fact that gases do not support shear. In this study, we report on the application of MRE to an ex vivo porcine lung specimen inflated with a hyperpolarized noble gas. Unlike proton MRE, shear wave propagation is encoded into the gas entrapped within the alveolar spaces rather than the parenchyma itself. These data provide first evidence of the technical feasibility of MRE of the lung using hyperpolarized noble gases. Magn Reson Med, 2007.

Cardiac magnetic resonance parallel imaging at 3.0 Tesla: Technical feasibility and advantages

To quantify changes in signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), specific absorption rate (SAR), RF power deposition, and imaging time in cardiac magnetic resonance imaging with and without the application of parallel imaging at 1.5 T and 3.0 T.

Magnetic Resonance Imaging With Cochlear Implant Magnet in Place: Safety and Imaging Quality.

Objective To evaluate the safety and image quality of 1.5-T MRI in patients with cochlear implants and retained internal magnets. Study Design Retrospective case series from 2012 to 2014. Setting Single tertiary academic referral center. Patients All cochlear implant recipients undergoing 1.5-T MRI without internal magnet removal. Intervention(s) MRI after tight headwrap application. Main Outcome Measures Patient tolerance, complications, and characteristics of imaging artifact. Results Nineteen ears underwent a total of 34 MRI scans. Two patients did not tolerate imaging with the headwrap in place and required magnet removal before rescanning. One subject experienced two separate episodes of polarity reversal in the same device from physical realignment (i.e., flipping) of the internal magnet requiring surgical repositioning. Three patients were discovered to have canting of the internal magnet after imaging. In all three cases, the magnet could be reseated by applying gentle firm pressure to the scalp until the magnet “popped” back into place. These patients continue to use their device without difficulty and have not required surgical replacement. In patients receiving head MRI, the ipsilateral internal auditory canal and cerebellopontine angle could be visualized without difficulty in 94% of cases. There were no episodes of cochlear implant device failure or soft tissue complications. Conclusion Under controlled conditions, 1.5-T MRI can be successfully performed in most patients without the need for cochlear implant magnet removal. In nearly all cases, imaging artifact does not impede evaluation of the ipsilateral skull base. Patients should be counseled regarding the risk of internal magnet movement that may occur in up to 15% of cases, even with tight headwrap application. If internal magnet polarity reversal occurs, a trial of reversing the external magnet can be considered. If canting or mild displacement of the internal magnet occurs, an attempt at reseating can be made by applying gentle firm pressure to the scalp over the internal magnet. If conservative measures fail, the magnet should be surgically repositioned to minimize interruption of device use and to prevent scalp complications.