Man-Woo Lee

A simple auto prescan calibration method for multislice fast spin echo MRI

The image quality of fast spin echo (FSE) is more sensitive than the typical spin echo pulse sequence caused by the eddy current effect. Microsecond‐scale misalignment of primary spin echoes produces a large spatial variation in image signal intensity. In this study, we describe an auto prescan calibration method that can improve the FSE image quality and minimize the eddy current effect on the image. We used a 0.32 T MRI system and obtained phantom and lumbar images. For FSE image correction, the optimal ranges and steps were determined to find the appropriate values, which were added to or subtracted from the gradient area values for each slice. The appropriate value of each slice could be found using the maximum signal intensity when the refocusing gradient area was changed by a number of steps in the optimal range. The determined value of each slice was applied before each slice image acquisition. The determined optimal step numbers and ranges were applied to in vivo image acquisition, and confirmed the reconstructed image quality. Based on our results, the obtained phantom and lumbar images were shown to be well corrected. The corrected images represented the image quality improvement and elimination of ghosting and blurring artifacts. In conclusion, we have proposed an FSE correction technique that automatically adjusts slice selection for the refocusing gradient balance during prescan, and confirmed that the calibration technique is very reliable even within complex in vivo images. We believe that our proposed technique will greatly benefit in MRI systems.

Sparse sampling MR image reconstruction using bregman iteration: A feasibility study at low tesla MRI system

MR images reconstruction need many samples that are sequentially sampled by phase encoding gradients in MRI system. MRI takes long scan time, therefore, many researchers have been studied to reduce scan time. Especially, the Compressed Sensing (CS) that is used sparse images and reconstruction from fewer sampling data which the k-space is not fully sampled. Recently, an iterative technique based on Bregman method is developed for denoising. The Bregman iteration method improves on the Total Variation (TV) regularization by gradually recovering the fine scale structures that are usually lost in the TV regularization. In this study, we studied sparse sampling image reconstruction using Bregman iteration at low tesla MRI system for improving the temporal resolution and validated the usefulness. The image was obtained at 0.32T MRI scanner (Magfinder II, Genpia, Korea) using 2D T1-weighed spin-echo pulse sequence with phantom and in-vivo human brain in the head coil. We applied the random k-space sampling and sampling ratios are determined by half of fully sampled k-space. The Bregman iteration was used to generate the final images based on the reduced data. The number of Bregman iterations used for the reconstruction was minimum 1 to maximum 100. We also calculated Root Mean Square Error (RMSE) values from error images that were performed according to number of bregman iterations. The results which are reconstructed images using the bregman iteration to sparse sampling image shown well reconstruction images compared with original images. Moreover, the RMSE values can be seen that sparse reconstructed phantom image and human images are converge to the original image. We confirmed the feasibility of sparse sampling image reconstruction methods using Bregman iteration at low tesla MRI system and obtained good results. Although our results used half of sampling ratio, this method will helpful to increase the temporal resolution at low tesla MRI system.

Crosstalk and scatter correction in simultaneous dual isotope SPECT imaging using four energy windows method

One of the limitations of the simultaneous dual isotope SPECT with Tc-99m and I-123 is the crosstalk of the photons from one radionuclide to the other radionuclides photopeak, which is serious since the difference between the photopeak of Tc-99m and I-123 is small (140 keV, 159 keV). We designed a method for correcting this crosstalk effect using four energy windows, by which we could reduce the effect of scatter as well as crosstalk. We also validated our method by comparing the corrected images to the single-isotope image acquired with phantom study. After defining the energy windows for each isotope, crosstalk fractions from the photopeak of one isotope to the photopeak/scatter windows of the other isotope were determined. Using these fractions, the effect of scatter and crosstalk was compensated. Relative errors were estimated between the single and the crosstalk/scatter corrected dual isotope images. Measured crosstalk fractions did not change regardless of the activity of each isotope. Contamination of activity was reduced as a result of crosstalk correction (CC), and image contrast was improved by scatter correction (SC). Relative errors decreased further after SC on CC images. Using four energy windows method, we could obtain a more accurate result in simultaneous dual isotope SPECT.

Signal intensity correction for multichannel MR images using radon transformation

The purpose of this study is to correct signal intensity at low‐field MRI system with multichannel receiver coils using Radon transformation and filtered backprojection (FBP) method. An open‐type 0.32 T MRI system and a body size phantom were used to acquire the MR images. We used various types of coils from 2‐channels to 4‐channels, which minimized the loss of signal. In the intensity correction process, Radon transform was used for the images of each channel and low‐pass filtering was applied to reduce noise. After that FBP was used for the space transform again from the Radon space to the image space. We also made changes to the projection ranges and their intervals, and then confirmed them to evaluate the optimal parameters. All the intensity corrected results were compared with its original sum‐of‐square (SOS) images, and the corrected images showed more uniform and homogeneous intensities than the images without correction. In addition, these results were also shown in the quantitative values through the signal intensity variations according to the cut view along the horizontal lines of the images. The feasibility of our approach and results for signal intensity correction may be useful and helpful for the researchers of low‐field system with multichannel coils.

Keyhole-3D phase contrast magnetic resonance angiography: A time-resolved reconstruction method

In this study, we studied the keyhole imaging technique to 3D Phase‐contrast magnetic resonance angiography (PC MRA) to improve its temporal resolution. Previously, our research group has already studied the 2D PC MRA combined with keyhole technique, and evaluated the applicability. For keyhole‐3D PC MRA, the keyhole factor was used from 12.5% to 50% of the full k‐space. With keyhole factors above 50%, the images were similar to the original image and the vessels in the brain were well observed. We believe the keyhole‐3D PC MRA will give some advantages for improving the temporal resolution of MR systems.

A feasibility study for compressed sensing combined phase contrast MR angiography reconstruction

Phase contrast magnetic resonance angiography (PC MRA) is a technique for flow velocity measurement and vessels visualization, simultaneously. The PC MRA takes long scan time because each flow encoding gradients which are composed bipolar gradient type need to reconstruct the angiography image. Moreover, it takes more image acquisition time when we use the PC MRA at the low-tesla MRI system. In this study, we studied and evaluation of feasibility for CS MRI reconstruction combined PC MRA which data acquired by low-tesla MRI system. We used non-linear reconstruction algorithm which named Bregman iteration for CS image reconstruction and validate the usefulness of CS combined PC MRA reconstruction technique. The results of CS reconstructed PC MRA images provide similar level of image quality between fully sampled reconstruction data and sparse sampled reconstruction using CS technique. Although our results used half of sampling ratio and do not used specification hardware device or performance which are improving the temporal resolution of MR image acquisition such as parallel imaging reconstruction using phased array coil or non-cartesian trajectory, we think that CS combined PC MRA technique will be helpful to increase the temporal resolution and at low-tesla MRI system.

Improving the temporal resolution of 3D phase contrast MR angiography using keyhole technique at low tesla open-MRI system

Phase Contrast MR Angiography (PC MRA) takes longer scan time than other MRA technique. Especially, the 3D PC MRA images obtaining at low tesla MRI system takes more long scan time because the lower characteristic of hardware system than high tesla MRI system. The keyhole imaging was introduced that is a simple way to increase the temporal resolution of dynamic imaging studies. In this study, we applied the keyhole imaging technique to 3D PC MRA for improving the temporal resolution at low tesla MRI system. We also validated image quality and usefulness of combination of two techniques. All image data were obtained on a MagfinderII (Genpia, Korea) 0.32T MRI system. Using the 3D PC MRA pulse sequence, the vascular images for a human brain targeted on the Superior Sagittal Sinus (SSS) and Circle of Willis were obtained. The reference image was acquired fully sampled k-space data without velocity encoding gradients. Three flow sensitive images were obtained low frequency parts of k-space. The keyhole factor was used from 12.5% to 50% of the full k-space. We also used tukey window function to minimize the erroneous effects induced from frequency discontinuous. Moreover, the artifact power (AP) value was measured to validate the reconstructed image quality. Based on the our results, image artifacts that were considered as frequency discontinuous using the keyhole reconstruction were shown until use the 12.5% and 25% keyhole factors. Using above 50% keyhole factors, the images are similar to original image and the vessels in the brain are well observed. In conclusion, reconstructed images using keyhole technique combined 3D PC MRA data are enough to application at low tesla MRI system. Although our results are in the early stage of keyhole imaging and draw a conclusion from restrictive number of subjects, we considered that keyhole technique combined 3D PC MRA will give some advantages for improving the temporal resolution at low tesla MRI system.

Evaluation of velocity measurements for keyhole imaging combined phase contrast MR angiography

Phase Contrast MR Angiography (PC MRA) is excellent MRA technique for measuring the velocity of vessels in the human body. PC MRA need to at least 4 images for angiogram reconstruction and it caused longer scan time. Therefore, we used keyhole imaging combined PC MRA to reduce the scan time. However, keyhole imaging can lead the erroneous effects as loss of phase information or frequency discontinuous. In this study, we applied the keyhole imaging combined 2D PC MRA for improving the temporal resolution and also measured the velocity to evaluate the accuracy of phase information. We used 0.32T MRI scanner (MagfinderII, Genpia, Korea). Using the 2D PC MRA pulse sequence, the vascular images for a human brain targeted on the Superior Sagittal Sinus (SSS) were obtained. We applied tukey window function for keyhole images to minimize the ringing artifact and erroneous factors that are induced frequency discontinuous and phase information loss. We also applied zero-padded algorithm to peripheral missing k-space lines to compare keyhole imaging results and the artifact power (AP) value was measured on the complex difference images to validate the image quality. Consider as based on our results, heavy image distortions and artifacts were shown until using at least 50% keyhole factor. Using above the 50% keyhole factors are shown well reconstructed and matched for magnitude images and velocity information measurements. In conclusion, we confirmed the image quality and velocity information of keyhole technique combined 2D PC MRA. Especially, velocity information were measured similar to the full sampled k-space image despite of frequency discontinuous and phase information loss in the keyhole imaging reconstruction process. Consequently, the keyhole imaging combined 2D PC MRA will give some clinical usefulness and advantages as improving the temporal resolution and measuring the velocity information via selecting the appropriate keyhole factor at low tesla MRI system.

Reproducibility and feasibility study for Phase Contrast MR Angiography at low tesla Open-MRI system

Phase Contrast MR Angiography (PC MRA) was used very powerful tools for diagnosis of blood vessel disease such as hemadostenosis, aneurism or flow velocity measurement. Many studies have been researched PC MRA application at high tesla MRI system because of main magnet field inhomogeneity and low signal to ratio (SNR) at low tesla MRI system. The purpose of this study is to develop the PC MRA pulse sequence for low tesla MRI system and assessed its clinical usefulness and reproducibility at low tesla system. We also evaluated the flow velocity using self-made phantom to accuracy of velocity measurement. The 0.32T MRI scanner (Magfinder II, AILab, Korea) was used in this study. For developed the pulse sequence, bipolar gradients are added to three orthogonal directions and flow compensation gradients were applied to slice selection direction and frequency encoding direction. Reference images were scanned without applied flow encoding gradients. Using flow encoded images and references images, magnitude and phase difference images were calculated. We also used zero-filling technique and Maximum Intensity Projection (MIP) for 3D PC MRA to improvement of image quality. Especially, using the self-made flow phantom, we measured the flow velocity and assessed its accuracy. Consider as based on our results, 2D/3D PC MRA images provide excellent visualization of vasculature of blood vessels and velocity information. Particularly, velocity measurement using phase images were nearly accuracy information about blood vessels velocity compared with doppler ultrasound. In conclusion, Using PC MRA technique displays both anatomic and flow information of blood vessels on images. Although, the image quality and SNR were lower than high tesla MRI system and the scan time was longer than high tesla MRI system, our result sufficient for explanation of flow direction and flow velocity changes in vivo studies during clinical application using at low tesla MRI system.