Fundamentals of Camera-PPG Based Magnetic Resonance Imaging.

In Magnetic Resonance Imaging (MRI), cardiac triggering that synchronizes data acquisition with cardiac contractions is an essential technique for acquiring high-quality images. Triggering is typically based on the Electrocardiogram (ECG) signal (e.g. R-peak). Since ECG acquisition involves extra workflow steps like electrode placement and ECG signals are usually disturbed by magnetic fields in high Magnetic Resonance (MR) systems, we explored camera-based photoplethysmography (PPG) as an alternative. We used the in-bore camera of a clinical MR system to investigate the feasibility and challenges of camera-based cardiac triggering. Data from ECG, finger oximeter and camera were synchronously collected. Compared to finger-PPG, camera-based PPG provides a higher availability of the signal and the PPG marker delay relative to the ECG R-peak is considerably less with a camera monitoring the forehead. The insights obtained in this study provide a basis for an envisioned system-design phase.

Camera-based respiratory triggering improves the image quality of 3D magnetic resonance cholangiopancreatography.

PURPOSE: To evaluate the performance of a novel camera-based breathing navigation system in respiratory-triggered (CRT) 3D-magnetic resonance cholangiopancreatography (MRCP) at 3T MRI. METHODS: Two 3D-MRCP data sets were acquired subsequently within one imaging session with traditional respiratory belt- (BRT) or camera- (CRT) based triggering in 28 patients. Overall image quality, blurring, motion artifacts and discernibility of the pancreaticobiliary tree (PBT) structures were scored on a 4-point scale retrospectively by 2 radiologists. The contrast ratio between the common bile duct and its adjacent tissue was measured by region-of-interest (ROI) analysis. The signal intensity increase at the duct boundaries was quantified by line profiles to objectify blurring and motion artifacts. The extracted respiratory signal curves were analyzed for signal quality and trigger timing. RESULTS: Total scan time was 72 s for both acquisitions. CRT yielded significantly better ratings in image quality, background suppression, blurring and discernibility of PBT structures compared to BRT. Contrast ratios were significantly higher in CRT (0.94 ±â€¯0.03) than in BRT (0.93 ±â€¯0.03) exams; paired t test P = 0.0017. Line profile slopes through the common bile duct revealed significantly higher values in CRT (42.23 ±â€¯8.74% of maximum intensity/mm) compared to BRT (36.06 ±â€¯8.96% of maximum intensity/mm; paired t test P < 0.0001). Camera-derived respiratory signal curves showed a higher SNR, lower standard deviation of the signal amplitude and less incorrect triggering than the respiratory belt-derived respiratory signal curves. CONCLUSION: Camera-based respiratory triggering significantly improves image quality of 3D-MRCP compared to conventional respiratory belt triggering.

Optimal high b-value for diffusion weighted MRI in diagnosing high risk prostate cancers in the peripheral zone.

PURPOSE: To retrospectively determine the optimal b-value(s) of diffusion-weighted imaging (DWI) associated with intermediate-high risk cancer in the peripheral zone (PZ) of the prostate. MATERIALS AND METHODS: Forty-two consecutive patients underwent multi b-value (16 evenly spaced b-values between 0 and 2000 s/mm2 ) DWI along with multi-parametric MRI (MP-MRI) of the prostate at 3 Tesla followed by trans-rectal ultrasound/MRI fusion guided targeted biopsy of suspicious lesions detected at MP-MRI. Computed DWI images up to a simulated b-value of 4000 s/mm2 were also obtained using a pair of b-values (b = 133 and 400 or 667 or 933 s/mm2 ) from the multi b-value DWI. The contrast ratio of average intensity of the targeted lesions and the background PZ was determined. Receiver operator characteristic curves and the area under the curve (AUCs) were obtained for separating patients eligible for active surveillance with low risk prostate cancers from intermediate-high risk prostate cancers as per the cancer of the prostate risk assessment (CAPRA) scoring system. RESULTS: The AUC first increased then decreased with the increase in b-values reaching maximum at b = 1600 s/mm2 (0.74) with no statistically significant different AUC of DWI with b-values 1067-2000 s/mm2 . The AUC of computed DWI increased then decreased with the increase in b-values reaching a maximum of 0.75 around b = 2000 s/mm2 . There was no statistically significant difference between the AUC of optimal acquired DWI and either of optimal computed DWI. CONCLUSION: The optimal b-value for acquired DWI in differentiating intermediate-high from low risk prostate cancers in the PZ is b = 1600 s/mm2 . The computed DWI has similar performance as that of acquired DWI with the optimal performance around b = 2000 s/mm2 . LEVEL OF EVIDENCE: 4 J. Magn. Reson. Imaging 2017;45:125-131.

Quantitative diffusion-weighted imaging and dynamic contrast-enhanced characterization of the index lesion with multiparametric MRI in prostate cancer patients.

PURPOSE: To compare a simplified intravoxel incoherent motion (sIVIM) model to commonly used monoexponential and biexponential models in the characterization of prostate cancer (PCa) and noncancerous prostate tissues, and to investigate combinations of diffusion-weighted imaging (DWI) measures with dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI)-derived parameters in MRI-visible index lesions, to facilitate PCa risk stratification. MATERIALS AND METHODS: In this retrospective, Institutional Review Board (IRB)-approved study, 43 consecutive patients with PCa who had 3T MRI exams followed by radical prostatectomy were included. DWI and DCE parameters were measured from one index lesion per patient, and noncancerous central gland and peripheral zone. Logistic regression modeling was performed to select the optimal combination of DWI and DCE measurements for tumor risk assessment. RESULTS: All diffusion models showed the lowest diffusion coefficients in tumors, intermediate values in noncancerous central gland, and highest values in noncancerous peripheral zone (all P < 0.001). Ktrans and kep were higher in tumors compared to central gland (P < 0.005) and peripheral zone (P < 0.001). The initial area under the contrast concentration curve was higher in tumor than the peripheral zone (P < 0.001). The area under the receiver operating characteristic curve of the combined DWI and DCE parameters (0.78) was higher than its individual components (0.73 and 0.63, respectively) for discriminating low- and intermediate-to-high-risk tumors. CONCLUSION: The sIVIM model provided comparable results with fewer b-values and shorter image acquisition time. The combination of DWI and DCE measurements of MRI-visible index lesions improved the preoperative prostate cancer risk characterization compared to the individual parameters from either technique alone. LEVEL OF EVIDENCE: 3 J. Magn. Reson. Imaging 2017;45:908-916.

Quantitative R2* MRI of the liver with rician noise models for evaluation of hepatic iron overload: Simulation, phantom, and early clinical experience.

PURPOSE: To compare Rician and non-Rician noise models for quantitative R2 * magnetic resonance imaging (MRI), in a simulation, phantom, and human study. MATERIALS AND METHODS: Synthetic 12-echo spoiled GRE (SGRE) datasets were generated with various R2 * rates (0-2000 sec(-1) ) at a signal-to-noise ratio (SNR) of 50, 20, 10, and 5. Phantoms of different MnCl2 concentrations (0-25 mM) were constructed and imaged using a 12-echo 3D SGRE sequence at 1.5T. Increasing levels of synthetic noise was added to the original data to simulate sequentially lower SNR conditions. Sixteen patients with suspected or known iron overload were imaged using 12-echo 3D SGRE at 1.5T. Various R2 * quantification methods, based on Rician and non-Rician noise models, were compared in the simulation, phantom, and human datasets. RESULTS: Non-Rician R2 * estimates were variably inaccurate in the high R2 * range (>500 sec(-1) ), with SNR-dependent linear goodness-of-fit statistic (R(2) ) of 0.373-0.999. Rician R2 * estimates were accurate even in the high R2 * range, with high R(2) of 0.940-0.999 regardless of SNR. Non-Rician R2 * estimates were variably nonlinear at high MnCl2 concentrations, with SNR-dependent R(2) of 0.345-0.994. Rician R2 * estimates were linear even at high MnCl2 concentrations, with high R(2) of 0.923-0.994 regardless of SNR. Between-method agreement of the R2 * estimates was excellent in patients with low ferritin but poor in patients with high ferritin. Rician R2 * estimates had excellent correlation with ferritin (r = 0.966 P < 0.001). CONCLUSION: Rician R2 * estimates were most consistent in the high R2 * conditions and under varying SNR, and may be more reliable when high iron load is suspected.

Correction of Gradient Nonlinearity Bias in Quantitative Diffusion Parameters of Renal Tissue with Intra Voxel Incoherent Motion.

Spatially non-uniform diffusion weighting bias due to gradient nonlinearity (GNL) causes substantial errors in apparent diffusion coefficient (ADC) maps for anatomical regions imaged distant from magnet isocenter. Our previously-described approach allowed effective removal of spatial ADC bias from three orthogonal DWI measurements for mono-exponential media of arbitrary anisotropy. The present work evaluates correction feasibility and performance for quantitative diffusion parameters of the two-component IVIM model for well-perfused and nearly isotropic renal tissue. Sagittal kidney DWI scans of a volunteer were performed on a clinical 3T MRI scanner near isocenter and offset superiorly. Spatially non-uniform diffusion weighting due to GNL resulted both in shift and broadening of perfusion-suppressed ADC histograms for off-center DWI relative to unbiased measurements close to isocenter. Direction-average DW-bias correctors were computed based on the known gradient design provided by vendor. The computed bias maps were empirically confirmed by coronal DWI measurements for an isotropic gel-flood phantom. Both phantom and renal tissue ADC bias for off-center measurements was effectively removed by applying pre-computed 3D correction maps. Comparable ADC accuracy was achieved for corrections of both b-maps and DWI intensities in presence of IVIM perfusion. No significant bias impact was observed for IVIM perfusion fraction.

Dynamic and simultaneous MR measurement of R1 and R2* changes during respiratory challenges for the assessment of blood and tissue oxygenation.

This work presents a novel method for the rapid and simultaneous measurement of R1 and R2* relaxation rates. It is based on a dynamic short repetition time steady-state spoiled multigradient-echo sequence and baseline R1 and B1 measurements. The accuracy of the approach was evaluated in simulations and a phantom experiment. The sensitivity and specificity of the method were demonstrated in one volunteer and in four patients with intracranial tumors during carbogen inhalation. We utilized (ΔR2*, ΔR1) scatter plots to analyze the multiparametric response amplitude of each voxel within an area of interest. In normal tissue R2* decreased and R1 increased moderately in response to the elevated blood and tissue oxygenation. A strong negative ΔR2* and ΔR1 response was observed in veins and some tumor areas. Moderate positive ΔR2* and ΔR1 response amplitudes were found in fluid-rich tissue as in cerebrospinal fluid, peritumoral edema, and necrotic areas. The multiparametric approach was shown to increase the specificity and sensitivity of oxygen-enhanced MRI compared to measuring ΔR2* or ΔR1 alone. It is thus expected to provide an optimal tool for the identification of tissue areas with low oxygenation, e.g., in tumors with compromised oxygen supply.

Positive visualization of implanted devices with susceptibility gradient mapping using the original resolution.

In magnetic resonance imaging, implantable devices are usually visualized with a negative contrast. Recently, positive contrast techniques have been proposed, such as susceptibility gradient mapping (SGM). However, SGM reduces the spatial resolution making positive visualization of small structures difficult. Here, a development of SGM using the original resolution (SUMO) is presented. For this, a filter is applied in k-space and the signal amplitude is analyzed in the image domain to determine quantitatively the susceptibility gradient for each pixel. It is shown in simulations and experiments that SUMO results in a better visualization of small structures in comparison to SGM. SUMO is applied to patient datasets for visualization of stent and prostate brachytherapy seeds. In addition, SUMO also provides quantitative information about the number of prostate brachytherapy seeds. The method might be extended to application for visualization of other interventional devices, and, like SGM, it might also be used to visualize magnetically labelled cells.

Accelerated T2 mapping for characterization of prostate cancer.

Prostate T(2) mapping was performed in 34 consecutive patients using an accelerated multiecho spin-echo sequence with 4-fold k-space undersampling leading to a net acceleration factor of 3.3 on a 3T scanner. The mean T(2) values from the accelerated and conventional, unaccelerated sequences demonstrated a very high correlation (r = 0.99). Different prostate segments demonstrated similarly good interscan reproducibility (p = not significant) with slightly larger difference at base: 2.0% ± 1.6% for left base and 2.1% ± 1.1% for right base. In patients with subsequent targeted biopsy, T(2) values of histologically proven malignant tumor areas were significantly lower than the suspicious looking but nonmalignant lesions (p < 0.05) and normal areas (p < 0.001): 100 ± 10 ms for malignant tumors, 114 ± 23 ms for suspicious lesions and 149 ± 32 ms for normal tissues. The proposed method can provide an effective approach for accelerated T(2) quantification for prostate patients.

Concurrent MR blood volume and vessel size estimation in tumors by robust and simultaneous ΔR2 and ΔR2* quantification.

This work presents a novel method for concurrent estimation of the fractional blood volume and the mean vessel size of tumors based on a multi-gradient-echo-multi-spin-echo sequence and the injection of a super-paramagnetic blood-pool agent. The approach further comprises a post-processing technique for simultaneous estimation of changes in the transverse relaxation rates R(2) and R(2)*, which is robust against global B(0) and B(1) field inhomogeneities and slice imperfections. The accuracy of the simultaneous ΔR(2) and ΔR(2)* quantification approach is evaluated in a phantom. The simultaneous blood volume and vessel size estimates, obtained with MR, compare well to the immunohistological findings in a preclinical experiment (HT1080 cells, implanted in nude mice). Clinical translation is achieved in a patient with a pleomorphic sarcoma in the left pubic bone. The latter demonstrates the robustness of the technique against changes in the contrast agent concentration in blood during washout.

Fast T(2) relaxometry with an accelerated multi-echo spin-echo sequence.

A new method has been developed to reduce the number of phase-encoding steps in a multi-echo spin-echo imaging sequence allowing fast T(2) mapping without loss of spatial resolution. In the proposed approach, the k-space data at each echo time were undersampled and a reconstruction algorithm that exploited the temporal correlation of the MR signal in k-space was used to reconstruct alias-free images. A specific application of this algorithm with multiple-receiver acquisition, offering an alternative to existing parallel imaging methods, has also been introduced. The fast T(2) mapping method has been validated in human brain T(2) measurements in a group of nine volunteers with acceleration factors up to 3.4. The results demonstrated that the proposed method exhibited excellent linear correlation with the regular T(2) mapping with full sampling and achieved better image reconstruction and T(2) mapping with respect to SNR and reconstruction artifacts than the selected reference acceleration techniques. The new method has also been applied for quantitative tracking of injected magnetically labeled breast cancer cells in the rat brain with acceleration factors of 1.8 and 3.0. The proposed technique can provide an effective approach for accelerated T(2) quantification, especially for experiments with single-channel coil when parallel imaging is not applicable.

Compressed sensing reconstruction for magnetic resonance parameter mapping.

Compressed sensing (CS) holds considerable promise to accelerate the data acquisition in magnetic resonance imaging by exploiting signal sparsity. Prior knowledge about the signal can be exploited in some applications to choose an appropriate sparsifying transform. This work presents a CS reconstruction for magnetic resonance (MR) parameter mapping, which applies an overcomplete dictionary, learned from the data model to sparsify the signal. The approach is presented and evaluated in simulations and in in vivo T(1) and T(2) mapping experiments in the brain. Accurate T(1) and T(2) maps are obtained from highly reduced data. This model-based reconstruction could also be applied to other MR parameter mapping applications like diffusion and perfusion imaging.

A concept for magnetic resonance visualization of surgical textile implants.

PURPOSE: To develop a method for visualizing surgical textile implant (STI) with superparamagnetic iron oxides (SPIO), using magnetic resonance imaging (MRI). Therefore, positive-contrast inversion-recovery with on-resonant water suppression (IRON) was applied and its properties were evaluated in vitro. MATERIALS AND METHODS: STI with different concentrations of SPIO integrated into the base material were produced. Imaging was performed on a clinical 1.5 Tesla scanner, using conventional balanced gradient echo sequences (SSFP), T2*-weighted sequences, and IRON-imaging. In vitro experiments were conducted in an agarose phantom. On MR-images, contrast-to-noise-ratios, and the dimensions of the implant were assessed. RESULTS: Conventional MRI exhibited SPIO-loaded STI as signal voids. Using IRON, the mesh was clearly exhibited hyperintensely with suppression of on-resonant background signals with a distinct differentiation to other sources of off-resonances. Concentrations of approximately 9 mg/g led to best positive contrast and highest contrast-to-noise-ratios using IRON. Depending on B0-orientation, phase encoding direction and the STI's SPIO-load, the IRON-signal showed a characteristic pattern and an overestimation of STI size up to 4.6 mm. CONCLUSION: The integration of SPIOs into the base material combined with IRON is a feasible approach to visualize STI with MRI. This method could help to identify mesh-related problems in time and to reduce the need for surgical revision.

Iterative off-resonance and signal decay estimation and correction for multi-echo MRI.

Signal dephasing due to field inhomogeneity and signal decay due to transverse relaxation lead to perturbations of the Fourier encoding commonly applied in magnetic resonance imaging. Hence, images acquired with long readouts suffer from artifacts such as blurring, distortion, and intensity variation. These artifacts can be removed in reconstruction, usually based on separately collected information in form of field and relaxation maps. In this work, a recently proposed gridding-based algorithm for off-resonance correction is extended to also address signal decay. It is integrated into a new fixed-point iteration, which permits the joint estimation of an image and field and relaxation maps from multi-echo acquisitions. This approach is then applied in simulations and in vivo experiments and demonstrated to improve both images and maps. The rapid convergence of the fixed-point iteration in combination with the efficient gridding-based correction promises to render the running time of such a joint estimation acceptable.