Purpose The aim of the study was to investigate whether incorrectly compensated eddy currents are the source of persistent X‐nuclear spectroscopy and imaging artifacts, as well as methods to correct this. Methods Pulse‐acquire spectra were collected for 1 H and X‐nuclei ( 23 Na or 31 P) using the minimum TR permitted on a 3T clinical MRI system. Data were collected in 3 orientations (axial, sagittal, and coronal) with the spoiler gradient at the end of the TR applied along the slice direction for each. Modifications to system calibration files to tailor eddy current compensation for each X‐nucleus were developed and applied, and data were compared with and without these corrections for: slice‐selective MRS (for 23 Na and 31 P), 2D spiral trajectories (for 13 C), and 3D cones trajectories (for 23 Na). Results Line‐shape distortions characteristic of eddy currents were demonstrated for X‐nuclei, which were not seen for 1 H. The severity of these correlated with the amplitude of the eddy current frequency compensation term applied by the system along the axis of the applied spoiler gradient. A proposed correction to eddy current compensation, taking account of the gyromagnetic ratio, was shown to dramatically reduce these distortions. The same correction was also shown to improve data quality of non‐Cartesian imaging (2D spiral and 3D cones trajectories). Conclusion A simple adaptation of the default compensation for eddy currents was shown to eliminate a range of artifacts detected on X‐nuclear spectroscopy and imaging.
This chapter summarizes the current state of carotid magnetic resonance angiography (MRA) and deals with the technical aspects of the various types of MRA, including time-of-flight (TOF), phase-contrast (PC) and contrast-enhanced MRA. TOF MRA can be performed as either multiple 2D single slice acquisitions or as a 3D volumetric flow compensated acquisition. Multiple overlapping thin slab acquisition (MOTSA) method aims to reduce the saturation effect by reducing the thickness of the 3D slabs, but maintains the volume coverage by using multiple slabs. Phase-contrast MRA relies on detecting changes in the phase of blood's transverse magnetization as it moves along a magnetic field gradient. Gadolinium-based contrast agents have been used with TOF MRA in order to improve the signal-to-noise ratio (SNR) of carotid angiograms. Amongst the various types of MRA available, 2D TOF and PC techniques are suitable for screening but they suffer from a number of technical limitations.
Abstract The main objective of this study was to assess the long‐term cost‐effectiveness of five alternative diagnostic strategies for identification of severe carotid stenosis in recently symptomatic patients. A decision‐analytical model with Markov transition states was constructed. Data sources included a prospective study involving 167 patients who had screening Doppler ultrasound (DUS), confirmatory contrast‐enhanced magnetic resonance angiography (CEMRA) and confirmatory digital subtraction angiography (DSA), individual patient data from the European Carotid Surgery Trial and other published clinical and cost data. A “selective” strategy, whereby all patients receive DUS and CEMRA (only proceeding to DSA if the CEMRA is positive and the DUS is negative), was most cost‐effective. This was both the cheapest imaging and treatment strategy ($35,205 per patient) and yielded 6.1590 quality‐adjusted life years (QALYs), higher than three alternative imaging strategies. Probabilistic sensitivity analysis demonstrated that there was less than a 10% probability that imaging with either DUS or DSA alone are cost‐effective at the conventional $50,000/QALY threshold. In conclusion, DSA is not cost‐effective in the routine diagnostic workup of most patients. DUS, with additional imaging in the form of CEMRA, is recommended, with a strategy of “CEMRA and selective DUS review” being shown to be the optimal imaging strategy. Ann Neurol 2005;58:506–515
In Chapter 1, we discussed the fundamentals of NMR. In this chapter, we discuss how to obtain data for the formation of images with spatial encoding performed using magnetic field gradients.
This study investigated the effect of temporal resolution on the dual-input pharmacokinetic (PK) modelling of dynamic contrast-enhanced MRI (DCE-MRI) data from normal volunteer livers and from patients with hepatocellular carcinoma. Eleven volunteers and five patients were examined at 3 T. Two sections, one optimized for the vascular input functions (VIF) and one for the tissue, were imaged within a single heart-beat (HB) using a saturation-recovery fast gradient echo sequence. The data was analysed using a dual-input single-compartment PK model. The VIFs and/or uptake curves were then temporally sub-sampled (at interval ▵t = [2-20] s) before being subject to the same PK analysis. Statistical comparisons of tumour and normal tissue PK parameter values using a 5% significance level gave rise to the same study results when temporally sub-sampling the VIFs to HB < ▵t <4 s. However, sub-sampling to ▵t > 4 s did adversely affect the statistical comparisons. Temporal sub-sampling of just the liver/tumour tissue uptake curves at ▵t ≤ 20 s, whilst using high temporal resolution VIFs, did not substantially affect PK parameter statistical comparisons. In conclusion, there is no practical advantage to be gained from acquiring very high temporal resolution hepatic DCE-MRI data. Instead the high temporal resolution could be usefully traded for increased spatial resolution or SNR.
In this chapter, we discuss the methods for parameter estimation by decoding the images acquired from the time-series data. In Chapter 3, we discussed the fundamentals of Bloch simulations and methods whereby multiple image contrasts could be obtained. Here, we focus our investigation on methods that can decode the contrast information in the steady-state or transient-state data, resulting in quantitative multi-parametric maps. We begin our formalism with the least squares approach, afterwards deriving a "pattern matching" algorithm based on a maximum inner product search used in magnetic resonance fingerprinting, thereafter exploring developments into compressed sensing anti-aliasing and machine learning routines.
The paper reports a free-breathing black-blood CINE fast-spin echo (FSE) technique for measuring abdominal aortic wall motion. The free-breathing CINE FSE includes the following MR techniques: (1) variable-density sampling with fast iterative reconstruction; (2) inner-volume imaging; and (3) a blood-suppression preparation pulse. The proposed technique was evaluated in eight healthy subjects. The inner-volume imaging significantly reduced the intraluminal artifacts of respiratory motion (p = 0.015). The quantitative measurements were a diameter of 16.3 ± 2.8 mm and wall distensibility of 2.0 ± 0.4 mm (12.5 ± 3.4%) and 0.7 ± 0.3 mm (4.1 ± 1.0%) for the anterior and posterior walls, respectively. The cyclic cross-sectional distensibility was 35 ± 15% greater in the systolic phase than in the diastolic phase. In conclusion, we developed a feasible CINE FSE method to measure the motion of the abdominal aortic wall, which will enable clinical scientists to study the elasticity of the abdominal aorta.
This work addresses a central topic in Magnetic Resonance Imaging (MRI) which is the motion-correction problem in a joint reconstruction and registration framework. From a set of multiple MR acquisitions corrupted by motion, we aim at - jointly - reconstructing a single motion-free corrected image and retrieving the physiological dynamics through the deformation maps. To this purpose, we propose a novel variational model. First, we introduce an $L^2$ fidelity term, which intertwines reconstruction and registration along with the weighted total variation. Second, we introduce an additional regulariser which is based on the hyperelasticity principles to allow large and smooth deformations. We demonstrate through numerical results that this combination creates synergies in our complex variational approach resulting in higher quality reconstructions and a good estimate of the breathing dynamics. We also show that our joint model outperforms in terms of contrast, detail and blurring artefacts, a sequential approach.
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