Percent Image Uniformity Research Articles

Development of a Proton-Frequency-Transparent Birdcage Radiofrequency Coil for In Vivo 13C MRS/MRSI Study in a 3.0 T MRI System

A proton-frequency-transparent (PFT) birdcage RF coil that contains carbon-proton switching circuits (CPSCs) is presented to acquire 13C MR signals, which, in turn, enable 1H imaging with existing 1H RF coils without being affected by a transparent 13C birdcage RF coil. CPSCs were installed in the PFT 13C birdcage RF coil to cut the RF coil circuits during 1H MR imaging. Finite-difference time-domain (FDTD) electromagnetic (EM) simulations were performed to verify the performance of the proposed CPSCs. The performance of the PFT 13C birdcage RF coil with CPSCs was verified via phantom and in vivo MR studies. In the phantom MR studies, 1H MR images and 13C MR spectra were acquired and compared with each other using the 13C birdcage RF coil with and without the CPSCs. For the in vivo MR studies, hyperpolarized 13C cardiac MRS and MRSI of swine were performed. The proposed PFT 13C birdcage RF coil with CPSCs led to a percent image uniformity (PIU) reduction of 1.53% in the proton MR images when compared with the case without it. FDTD EM simulations revealed PIU reduction of 0.06% under the same conditions as the phantom MR studies. Furthermore, an SNR reduction of 5.5% was observed at 13C MR spectra of corn-oil phantom using the PFT 13C birdcage RF coil with CPSCs compared with that of the 13C birdcage RF coil without CPSCs. Utilizing the PFT 13C birdcage RF coil, 13C-enriched compounds were successfully acquired via in vivo hyperpolarized 13C MRS/MRSI experiments. In conclusion, the applicability and utility of the proposed 16-leg low-pass PFT 13C birdcage RF coil with CPSCs were verified via 1H MR imaging and hyperpolarized 13C MRS/MRSI studies using a 3.0 T MRI system.

Fabrication and evaluation of bilateral Helmholtz radiofrequency coil for thermo-stable breast image with reduced artifacts.

PurposeThe positron emission tomography (PET)‐magnetic resonance (MR) system is a newly emerging technique that yields hybrid images with high‐resolution anatomical and metabolic information. With PET‐MR imaging, a definitive diagnosis of breast abnormalities will be possible with high spatial accuracy and images will be acquired for the optimal fusion of anatomic locations. Therefore, we propose a PET‐compatible two‐channel breast MR coil with minimal disturbance to image acquisition which can be used for simultaneous PET‐MR imaging in patients with breast cancer.Materials and MethodsFor coil design and construction, the conductor loops of the Helmholtz coil were tuned, matched, and subdivided with nonmagnetic components. Element values were optimized with an electromagnetic field simulation. Images were acquired on a GE 600 PET‐computed tomography (CT) and GE 3.0 T MR system. For this study, we used the T1‐weighted image (volunteer; repetition time (TR), 694 ms; echo time (TE), 9.6 ms) and T2‐weighted image (phantom; TR, 8742 ms; TE, 104 ms) with the fast spin‐echo sequence.ResultsThe results of measuring image factors with the proposed radiofrequency (RF) coil and standard conventional RF coil were as follows: signal‐to‐noise ratio (breast; 207.7 vs. 175.2), percent image uniformity (phantom; 89.22%–91.27% vs. 94.63%–94.77%), and Hounsfield units (phantom; ‐4.51 vs. 2.38).ConclusionsOur study focused on the feasibility of proposed two‐channel Helmholtz loops (by minimizing metallic components and soldering) for PET‐MR imaging and found the comparable image quality to the standard conventional coil. We believe our work will help significantly to improve image quality with the development of a less metallic breast MR coil.

Evaluation of cine imaging during multileaf collimator and gantry motion for real-time magnetic resonance guided radiation therapy.

PurposeReal‐time magnetic resonance guided radiation therapy (MRgRT) uses 2D cine imaging for target tracking. This work evaluates the percent image uniformity (PIU) and spatial integrity of cine images in the presence of multileaf collimator (MLC) and gantry motion in order to simulate sliding window and volumetric modulated arc therapy (VMAT) conditions.MethodsPercent image uniformity and spatial integrity of cine images were measured (1) during MLC motion, (2) as a function of static gantry position, and (3) during gantry rotation. PIU was calculated according to the ACR MRI Quality Control Manual. Spatial integrity was evaluated by measuring the geometric distortion of 16 measured marker positions (10 cm or 15.225 cm from isocenter).ResultsThe PIU of cine images did not vary by more than 1% from static linac conditions during MLC motion and did not vary by more than 3% during gantry rotation. Banding artifacts were present during gantry rotation. The geometric distortion in the cine images was less than 0.88 mm for all points measured throughout MLC motion. For all static gantry positions, the geometric distortion was less than 0.88 mm at 10 cm from isocenter and less than 1.4 mm at 15.225 cm from isocenter. During gantry rotation, the geometric distortion remained less than 0.92 mm at 10 cm from isocenter and less than 1.60 mm at 15.225 cm from isocenter.ConclusionDuring MLC motion, cine images maintained adequate PIU, and the geometric distortion of points within 15.225 cm from isocenter was less than the 1 mm threshold necessary for real‐time target tracking and gating. During gantry rotation, PIU was negatively affected by banding artifacts, and spatial integrity was only maintained within 10 cm from isocenter. Future work should investigate the effects imaging artifacts have on real‐time target tracking during MRgRT.

An Automated Method for Quality Control in MRI Systems: Methods and Considerations

Objective: The purpose of this study was to develop an automated method for performing quality control (QC) tests in magnetic resonance imaging (MRI) systems, investigate the effect of different definitions of QC parameters and its sensitivity with respect to variations in regions of interest (ROI) positioning, and validate the reliability of the automated method by comparison with results from manual evaluations. Materials and Methods: Magnetic Resonance imaging MRI used for acceptance and routine QC tests from five MRI systems were selected. All QC tests were performed using the American College of Radiology (ACR) MRI accreditation phantom. The only selection criterion was that in the same QC test, images from two identical sequential sequences should be available. The study was focused on four QC parameters: percent signal ghosting (PSG), percent image uniformity (PIU), signal-to-noise ratio (SNR), and SNR uniformity (SNRU), whose values are calculated using the mean signal and the standard deviation of ROIs defined within the phantom image or in the background. The variability of manual ROIs placement was emulated by the software using random variables that follow appropriate normal distributions. Results: Twenty-one paired sequences were employed. The automated test results for PIU were in good agreement with manual results. However, the PSG values were found to vary depending on the selection of ROIs with respect to the phantom. The values of SNR and SNRU also vary significantly, depending on the combination of the two out of the four standard rectangular ROIs. Furthermore, the methodology used for SNR and SNRU calculation also had significant effect on the results. Conclusions: The automated method standardizes the position of ROIs with respect to the ACR phantom image and allows for reproducible QC results.

Development of a hybrid magnetic resonance/computed tomography-compatible phantom for magnetic resonance guided radiotherapy.

ABSTRACTThe purpose of the present study was to develop a hybrid magnetic resonance/computed tomography (MR/CT)-compatible phantom and tissue-equivalent materials for each MR and CT image. Therefore, the essential requirements necessary for the development of a hybrid MR/CT-compatible phantom were determined and the development process is described. A total of 12 different tissue-equivalent materials for each MR and CT image were developed from chemical components. The uniformity of each sample was calculated. The developed phantom was designed to use 14 plugs that contained various tissue-equivalent materials. Measurement using the developed phantom was performed using a 3.0-T scanner with 32 channels and a Somatom Sensation 64. The maximum percentage difference of the signal intensity (SI) value on MR images after adding K2CO3 was 3.31%. Additionally, the uniformity of each tissue was evaluated by calculating the percent image uniformity (%PIU) of the MR image, which was 82.18 ±1.87% with 83% acceptance, and the average circular-shaped regions of interest (ROIs) on CT images for all samples were within ±5 Hounsfield units (HU). Also, dosimetric evaluation was performed. The percentage differences of each tissue-equivalent sample for average dose ranged from −0.76 to 0.21%. A hybrid MR/CT-compatible phantom for MR and CT was investigated as the first trial in this field of radiation oncology and medical physics.

Comparison of PET image quality using simultaneous PET/MR by attenuation correction with various MR pulse sequences

Abstract Positron emission tomography (PET)/magnetic resonance (MR) scanning has the advantage of less additional exposure to radiation than does PET/computed tomography (CT). In particular, MR based attenuation correction (MR AC) can greatly affect the image quality of PET and is frequently obtained using various MR sequences. Thus, the purpose of the current study was to quantitatively compare the image quality between MR non-AC (MR NAC) and MR AC in PET images with three MR sequences. Percent image uniformity (PIU), percent contrast recovery (PCR), and percent background variability (PBV) were estimated to evaluate the quality of PET images with MR AC. Based on the results of PIU, 15.2% increase in the average quality was observed for PET images with MR AC than for PET images with MR NAC. In addition, 28.6% and 71.1% improvement in the average results of PCR and PBV respectively, was observed for PET images with MR AC compared with that with MR NAC. Moreover, no significant difference was observed among the average values using three MR sequences. In conclusion, the current study demonstrated that PET with MR AC improved the image quality and can be help diagnosis in all MR sequence cases.

Development of a bias field-based uniformity correction in magnetic resonance imaging with various standard pulse sequences

To avoid artifacts and interpretation mistakes, improvements in the uniformity of magnetic resonance imaging (MRI) is very important. The aim of this study is to develop a bias field-based uniformity correction algorithm and to confirm the feasibility of its application with an integrated positron emission tomography (PET)/MRI system using standard MRI pulse sequences (T2-weighted, half-Fourier, single-shot turbo spin echo, T2-weighted turbo spin echo with fat suppression, and three-dimensional, volumetric, T1-weighted gradient recalled echo interpolated breath-hold examination using controlled aliasing in parallel imaging that results in increased acceleration). We used phantoms containing NaCl and NaCl + NiSO4 solutions, and considered the percent image uniformity (PIU) as the evaluation parameter. The elicited PIU results using the correction algorithm were distinct for all sequences and phantoms. The results of this study suggested that the bias field-based algorithm allows uniformity improvements in MRI in terms of PIU.

SU‐E‐J‐211: Assessing the Consistency of the ViewRay 0.35 T MRI System

Purpose:ViewRay is a novel image‐guided radiotherapy approach with an integrated 0.35 T MR unit and three Cobalt‐60 heads. In order to continuously ensure the high resolution and high soft tissue contrast available in MR images, we quantified a multitude of relevant imaging parameters over a period of six months to establish its stability and imaging quality. In the assessment process, consideration is also given to the need to establish the number of tests required to have confidence in the performance of the system for radiation therapy planning applications.Methods:Daily, weekly, monthly and annual imaging tests were performed over a period of six months using standardized phantoms (a 24 cm diameter sphere and the ACR phantom) to quantify the performance of the system. In addition to the ACR and NEMA recommended tests, we also included element testing, spatial integrity, Eddy current and magnetic field inhomogeneity measurements. The ACR test is used for assessing the following parameters for T1 and T2: geometric accuracy, high contrast spatial resolution, slice thickness accuracy, slice position accuracy, percent signal ghosting, low contrast object detectability. It also includes percent image uniformity (PIU) for T1. The NEMA test is primarily designed to check SNR and PIU.Results:Over the period of six months, all the parameters were maintained within the recommendations provided in the ACR and NEMA standards. PIU and SNR were found to be sensitive to malfunctions in the components of the multileaf collimators. Details of the findings will be presented.Conclusion:The data suggests that ViewRay imaging system has functioned in a consistent and reliable manner. MR imaging from ViewRay 0.35T system complies with the ACR and NEMA recommended acceptance standards.

SU‐FF‐I‐78: Evaluating the Effect of Dielectric Resonance On Image Uniformity at 1.5T and 3T in An ACR MRI Phantom Filled with Mineral Oil and Silicone Fluid

Purpose: The use of oil‐based phantoms instead of water‐based ones is known to be a more suitable method of assessing image uniformity of high field magnets. This study aims to investigate the appropriateness of using oil or silicone in the ACR phantom for routine quality control for both 1.5T and 3T MRI systems paying particular attention to the dielectric resonance effect. Methods and Materials: A prototype ACR phantom which was used to help develop the existing ACR MRI phantom was filled with three different materials and scanned in both a 1.5T and 3T magnets using the recommended ACR T1 sequence. The first material was the standard phantom water‐based liquid, the second material was mineral oil and the third was a silicone fluid. Percent image uniformity (PIU) as well as low and high contrast resolutions were assessed for all three filling liquids at both fields. Results: For the standard MRI phantom solution, the PIU was calculated to be 95% at 1.5 T and 80% at 3T. The same phantom filled with mineral oil produced a PIU of 99% on the 1.5 T and 95% on the 3T magnet. There was a marked decrease in SNR in going from standard aqueous solution to mineral oil, but this did not seem to affect the high contrast resolution or the low contrast detectability at either field. Results for the silicone fluid will also be presented. Conclusions: Central brightening artifacts are often seen as high signal intensity in the center of the imaged object and have caused problems with the PIU analysis of the water‐based ACR phantom on 3T systems. Filling the phantom with a material that has a lower dielectric value than water such as mineral oil or silicone provides images which are more uniform and tends to improve the artifactual lower PIU measurements.

TU-EE-A4-02: Some Issues With Image Intensity Uniformity Test Performed On 3T MRI Scanners

Purpose: To investigate why Image Intensity Uniformity (IIU) tests performed on commercial 3T MRI systems according to the ACR MR Accreditation Program (MRAP) instructions are subject to frequent failures. Method and Materials: The phantom scanning instructions for ACR MRAP tests specify that a dedicated phantom (ACR phantom) must be used to assess the MRI scanners performance. As a part of required test, an assessment of IIU is performed by measuring the Percent Image Uniformity (PIU) according to the specified formula. The data used in calculation of PIU must be acquired for a rigidly defined scanning protocols, and the process of data collection is also precisely specified. For the 3T MRI scanner to pass the test, the measured PIU must be equal or greater than 82%. PIU tests were performed using the same ACR phantom on five different 3T MR scanners from two different vendors, following the ACR MRAP acquisition and measurement protocol exactly. The scanners tested were installed in routine clinical settings and all head coils, available on site, were tested independently. In addition, if scanner offered multiple gradient performance modes, all modes were tested. Finally, effects of image post-processing routines, designed to improve the image uniformity and installed on the scanners were investigated as well. Results: A vast majority of PIU tests using non-postprocessed images failed (more than 95%). All tests that used postprocessed images passed. Upon closer investigation, the likely cause of failures was linked to RF properties of the ACR phantom. Conclusion: The IIU test needs to be carefully reassessed to ensure that it provides a meaningful characterization of MRI scanner's performance, and is not hampered by the limitations imposed by the electromagnetic physics of scanning at 127 MHz, the resonant RF frequency of 3T MRI systems.

SU‐FF‐I‐57: MR Image Quality Testing of K‐Space‐ and Image‐Based Parallel Imaging Techniques Using the ACR Phantom

Purpose: Using ACR MRI phantom testing to evaluate the change of image quality caused by k‐space‐ and image‐based parallel imaging techniques in a 3‐Tesla MRI scanner. Method and Materials: All phantom images are acquired from a 3 Tesla Siemens Tim‐Trio MRI scanner. Four sequences, namely ACR T1, ACR T2, Site T1 IR‐TSE, TR/TE/TI=2200 ms/11 ms/900 ms, ETL=5 and Site T2 TSE, TR/TE=4280 ms/88 ms, ETL=15, are scanned with each of the conventional, GRAPPA (iPAT=2) and modified SENSE (mSENSE, iPAT=2) modes. Images are analyzed according to standard ACR MRI QC procedures on a PC‐based workstation. Results: The results derived from the phantom images indicating that both of the tested parallel imaging techniques make no significant changes in the degree of the geometric distortion, the high contrast spatial resolution, the measured slice thickness and position accuracy, and the percent image uniformity. The signal‐to‐noise ratios (SNRs) obtained from all scans are lower while using parallel imaging techniques (SNR_conventional / SNR_GRAPPA = 1.18 +/− 0.08, SNR_conventional / SNR_mSENSE = 1.11 +/− 0.19). The averaged scores of low contrast object detecting test in conventional, GRAPPA, and mSENSE group are 39.3 +/− 0.5, 35.0 +/− 4.4, and 34.3 +/− 3.9, respectively. The percent signal ghosting (PSG) of GRAPPA and mSENSE images are 1.57 +/− 0.72 and 2.26 +/− 1.47 times of that measured in the conventional images. In addition, there is an obvious aliasing artifact observed in the mSENSE groups that cannot be demonstrated by the above data analysis. Conclusion: The ACR QC test is able to detect the lower SNR, lower contrast detectability, and higher PSG caused by both k‐space‐ and image‐based parallel imaging techniques. More accurate measurement of SNR reduction and improved artifact detection require repeated phantom imaging and additional image analysis.