Regional phase correction of inversion-recovery MR images

Fast simultaneous noncontrast angiography and intraplaque hemorrhage (fSNAP) sequence for carotid artery imaging

PURPOSE

To propose a fast simultaneous noncontrast angiography and intraplaque hemorrhage (fSNAP) sequence for carotid artery imaging.

METHODS

The proposed fSNAP sequence uses a low-resolution reference acquisition for phase-sensitive reconstruction to speed up the scan, and an inversion recovery acquisition with arbitrary k-space filling order to generate similar contrast to conventional SNAP. Four healthy volunteers and eight patients were recruited to test the performance of fSNAP in vivo. The lumen area quantification, muscle-blood CNR, IPH-blood CNR, lumen SNR, and standard deviation and intraplaque hemorrhage (IPH) detection accuracy were compared between fSNAP and SNAP.

RESULTS

By using a low-resolution reference acquisition with 1/4 matrix size of the full-resolution reference scan, the scan time of fSNAP was 37.5% less than that of SNAP. A high agreement of lumen area measurement (ICC = 0.97, 95% CI: 0.96-0.99) and IPH detection (Kappa = 1) were found between fSNAP and SNAP. Also, no significant difference was found for muscle-blood CNR (P = 0.25), IPH-blood CNR (P = 0.35), lumen SNR (P = 0.60), and standard deviation (P = 0.46) between the two techniques.

CONCLUSION

The feasibility of fSNAP was validated. fSNAP can improve the imaging efficiency with similar performance to SNAP on carotid artery imaging. Magn Reson Med 77:753-758, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

2D phase-sensitive inversion recovery imaging to measure in vivo spinal cord gray and white matter areas in clinically feasible acquisition times

PURPOSE

To present and assess a procedure for measurement of spinal cord total cross-sectional areas (TCA) and gray matter (GM) areas based on phase-sensitive inversion recovery imaging (PSIR). In vivo assessment of spinal cord GM and white matter (WM) could become pivotal to study various neurological diseases, but it is challenging because of insufficient GM/WM contrast provided by conventional magnetic resonance imaging (MRI).

MATERIALS AND METHODS

We acquired 2D PSIR images at 3T at each disc level of the spinal axis in 10 healthy subjects and measured TCA, cord diameters, WM and GM areas, and GM area/TCA ratios. Second, we investigated 32 healthy subjects at four selected levels (C2-C3, C3-C4, T8-T9, T9-T10, total acquisition time <8 min) and generated normative reference values of TCA and GM areas. We assessed test-retest, intra- and interoperator reliability of the acquisition strategy, and measurement steps.

RESULTS

The measurement procedure based on 2D PSIR imaging allowed TCA and GM area assessments along the entire spinal cord axis. The tests we performed revealed high test-retest/intraoperator reliability (mean coefficient of variation [COV] at C2-C3: TCA = 0.41%, GM area = 2.75%) and interoperator reliability of the measurements (mean COV on the 4 levels: TCA = 0.44%, GM area = 4.20%; mean intraclass correlation coefficient: TCA = 0.998, GM area = 0.906).

CONCLUSION

2D PSIR allows reliable in vivo assessment of spinal cord TCA, GM, and WM areas in clinically feasible acquisition times. The area measurements presented here are in agreement with previous MRI and postmortem studies.

Referenceless acquisition of phase-sensitive inversion-recovery with decisive reconstruction (RAPID) imaging

PURPOSE

To describe a referenceless reconstruction approach to generate phase-sensitive inversion recovery images without relying on the extra acquisition of reference images.

METHODS

The basic idea of the Referenceless Acquisition of Phase-sensitive Inversion-recovery with Decisive reconstruction, or RAPID, algorithm is to retrieve the magnetization polarity by estimating the background phase variation of a complex valued image. The theory and algorithm of RAPID is described in detail. To evaluate the performance of RAPID, seven patients were recruited then scanned in different anatomical regions (cardiac, brain, and vascular). Standard Phase Sensitive Inversion Recovery (PSIR) reconstructions using reference image information were compared with RAPID reconstructions using the same source data.

RESULTS

RAPID reconstructed images were found to provide very good agreement with PSIR reconstructed images on all cases, although no reference image info was used in the RAPID algorithm. For neuroimaging applications, it was found that RAPID reconstruction is more robust compared with the PSIR algorithm as RAPID can avoid potential errors introduced by the reference acquisition.

CONCLUSION

The RAPID technique for phase-sensitive reconstruction is promising and can improve the imaging efficiency by a factor of 2 compared with PSIR. RAPID was also shown to provide more robust reconstruction by avoiding errors caused by the reference acquisition.

Phase-sensitive black-blood coronary vessel wall imaging

Black-blood MR coronary vessel wall imaging may become a powerful tool for the quantitative and noninvasive assessment of atherosclerosis and positive arterial remodeling. Although dual-inversion recovery is currently the gold standard, optimal lumen-to-vessel wall contrast is sometimes difficult to obtain, and the time window available for imaging is limited due to competing requirements between blood signal nulling time and period of minimal myocardial motion. Further, atherosclerosis is a spatially heterogeneous disease, and imaging at multiple anatomic levels of the coronary circulation is mandatory. However, this requirement of enhanced volumetric coverage comes at the expense of scanning time. Phase-sensitive inversion recovery has shown to be very valuable for enhancing tissue-tissue contrast and for making inversion recovery imaging less sensitive to tissue signal nulling time. This work enables multislice black-blood coronary vessel wall imaging in a single breath hold by extending phase-sensitive inversion recovery to phase-sensitive dual-inversion recovery, by combining it with spiral imaging and yet relaxing constraints related to blood signal nulling time and period of minimal myocardial motion.

Robust phase sensitive inversion recovery imaging using a Markov random field model

This paper presents a novel method for phase sensitive inversion recovery (PSIR) imaging for improved T/sub 1/ contrast. This method models the phase of the complex magnetic resonance image using a statistical model based on Markov random fields. A computationally efficient optimization method is developed. Computer simulations and in-vivo brain imaging experiments show that the proposed method can produce PSIR images with enhanced T/sub 1/ contrast and it is robust against high levels of data noise even when rapid phase variations are presented.

Multislice and multicoil phase-sensitive inversion-recovery imaging

Phase-sensitive inversion-recovery (PSIR) imaging may provide enhanced T(1) contrast. However, clinical implementation of PSIR imaging is hindered because image reconstruction with this method often lacks robustness and requires manual intervention, particularly for data acquired in multiple slices and with phased-array coils. In this paper, a new algorithm suitable for automatic PSIR image reconstruction of multislice and multicoil data is presented. This algorithm phase corrects by region-growing, employing both the magnitude and the phase information of image pixels. Specifically, phase gradients of the original complex image are first calculated and then used to determine the sequence of the region-growing. The signal direction relating to the phase error for each pixel is then determined during the region-growing using both the magnitude and the phase of the previously determined pixels that are located within a boxcar neighborhood of the pixel. Finally, the intrinsic intercoil and interslice correlation is exploited to ensure consistency in the global polarity of all of the PSIR images. The results are demonstrated with in vivo human brain images acquired at 3 Tesla with an eight-channel phased-array coil.

Phase-sensitive inversion recovery for detecting myocardial infarction using gadolinium-delayed hyperenhancement

After administration of gadolinium, infarcted myocardium exhibits delayed hyperenhancement and can be imaged using an inversion recovery (IR) sequence. The performance of such a method when using magnitude-reconstructed images is highly sensitive to the inversion recovery time (TI) selected. Using phase-sensitive reconstruction, it is possible to use a nominal value of TI, eliminate several breath-holds otherwise needed to find the precise null time for normal myocardium, and achieve a consistent contrast. Phase-sensitive detection is used to remove the background phase while preserving the sign of the desired magnetization during IR. Experimental results are presented which demonstrate the benefits of both phase-sensitive IR image reconstruction and surface coil intensity normalization for detecting myocardial infarction (MI). The phase-sensitive reconstruction method reduces the variation in apparent infarct size that is observed in the magnitude images as TI is changed. Phase-sensitive detection also has the advantage of decreasing the sensitivity to changes in tissue T(1) with increasing delay from contrast agent injection.

Inversion recovery image reconstruction with multiseed region-growing spin reversal

A new algorithm is introduced for inversion recovery (IR) image reconstruction. The original complex image is modeled as a product of three factors: magnitude, polarity, and a smoothly changing phase factor. The simple binary polarity factor is first unified by a region-growing spin reversal (RGSR) operation, allowing the phase factor to be extracted. Multiplying the complex conjugate of the phase factor with the original complex data yields the desired IR contrast. The RGSR process is repeated with multiple seeds distributed in the field of view (FOV), and the results are added together, enabling disconnected tissues in the FOV to be handled. The extracted phase factor is filtered to reduce noise and artifacts, without losing useful information. The method is fully automatic and has been used practically in a large number of clinical examinations. The algorithm may also be useful for phase correction in simple proton spectroscopic imaging.

Homodyne detection in magnetic resonance imaging

Magnetic detection of complex images in magnetic resonance imaging (MRI) is immune to the effects of incidental phase variations, although in some applications information is lost or images are degraded. It is suggested that synchronous detection or demodulation can be used in MRI systems in place of magnitude detection to provide complete suppression of undesired quadrature components, to preserve polarity and phase information, and to eliminate the biases and reduction in signal-to-noise ratio (SNR) and contrast in low SNR images. The incidental phase variations in an image are removed through the use of a homodyne demodulation reference, which is derived from the image or the object itself. Synchronous homodyne detection has been applied to the detection of low SNR images, the reconstruction of partial k-space images, the simultaneous detection of water and lipid signals in quadrature, and the preservation of polarity in inversion-recovery images.