Breathheld autocalibrated phase‐contrast imaging

Purpose:

To compare generalized autocalibrating partially parallel acquisitions (GRAPPA), modified sensitivity encoding (mSENSE), and SENSE in phase‐contrast magnetic resonance imaging (PC‐MRI) applications.

Materials and Methods:

Aliasing of the torso can occur in PC‐MRI applications. If the data are further undersampled for parallel imaging, SENSE can be problematic in correctly unaliasing signals due to coil sensitivity maps that do not match that of the aliased volume. Here, a method for estimating coil sensitivities in flow applications is described. Normal volunteers (n = 5) were scanned on a 1.5 T MRI scanner and underwent PC‐MRI scans using GRAPPA, mSENSE, SENSE, and conventional PC‐MRI acquisitions. Peak velocity and flow through the aorta and pulmonary artery were evaluated.

Results:

Bland–Altman statistics for flow in the aorta and pulmonary artery acquired with mSENSE and GRAPPA methods (R = 2 and R = 3 cases) have comparable mean differences to flow acquired with conventional PC‐MRI. GRAPPA and mSENSE PC‐MRI have more robust measurements than SENSE when there is aliasing artifact caused by insufficient coil sensitivity maps. For peak velocity, there are no considerable differences among the mSENSE, GRAPPA, and SENSE reconstructions and are comparable to conventional PC‐MRI.

Conclusion:

Flow measurements of images reconstructed with autocalibration techniques have comparable agreement with conventional PC‐MRI and provide robust measurements in the presence of wraparound. J. Magn. Reson. Imaging 2010;31:1004–1014. ©2010 Wiley‐Liss, Inc.     

Mixed-bandwidth acquisitions: signal-to-noise ratio and signal-to-noise efficiency

Purpose:

To measure the hemodynamic response to exercise using real‐time velocity mapping magnetic resonance imaging (MRI), incorporating a high temporal resolution spiral phase contrast (PC) sequence accelerated with sensitivity encoding (SENSE).

Materials and Methods:

Twenty healthy adults underwent MRI at rest and during supine exercise at two different exercise levels. Flow volumes were assessed in the ascending aorta using a spiral SENSE real‐time PC sequence. The sequence was validated at rest against a vendor supplied gated PC sequence, and also at rest and during exercise against left ventricular volumes assessed using a radial k‐t SENSE real‐time sequence. Combining the measured flow volumes with simultaneous oscillometric blood pressure measurements, enabled the noninvasive calculations of systemic vascular resistance (SVR) and arterial compliance (C).

Results:

Measured flow volumes correlated very well between the sequences at rest and during exercise. Cardiac output (CO) and heart rate were found to significantly increase during exercise, while SVR and C were found to decrease significantly.

Conclusion:

Hemodynamic response to exercise can be accurately quantified using a high temporal resolution spiral SENSE real‐time flow imaging. This may allow early detection of hypertension and a greater understanding of the early changes in this condition. J. Magn. Reson. Imaging 2010;31:997–1003. ©2010 Wiley‐Liss, Inc.     

Computational simulations and experimental studies of 3D phase‐contrast imaging of fluid flow in carotid bifurcation geometries

Purpose:

To evaluate the use of computational fluid dynamic (CFD)‐based magnetic resonance imaging (MRI) simulations to predict the image appearance and velocity measurement of fluid flow in human carotid bifurcation geometries, and to compare the results with images from experimental MRI studies.

Materials and Methods:

Simulated particle paths were calculated from available CFD datasets of normal and moderately stenosed carotid bifurcation geometries. An MRI simulator based on the spin isochromat method was used to generate images corresponding to a 3D phase‐contrast sequence with velocity encoding in three orthogonal directions. The resulting images were compared qualitatively with experimental MRI scans of the corresponding physical models.

Results:

The simulations predicted the main features observed in experimental studies, such as the low image intensity in regions of complex flow and the position and bright appearance of the jet in the stenosed bifurcation. Simulated velocity images also agreed well with experimental results. The effects of sequence parameters such as repetition time (TR) and echo time (TE) were readily demonstrated by the simulations.

Conclusion:

CFD‐based MRI simulations can be used to predict the appearance of MRI images of regions of physiological flow, and may be useful in the development of improved pulse sequences for flow measurement. J. Magn. Reson. Imaging 2010;31:928–934. ©2010 Wiley‐Liss, Inc.     

Added value of breathhold diffusion‐weighted MRI in detection of small hepatocellular carcinoma lesions compared with dynamic contrast‐enhanced MRI alone using receiver operating characteristic curve analysis

Purpose

To evaluate the added value of single‐breathhold diffusion‐weighted MRI (DWI) in detection of small hepatocellular carcinoma (HCC) lesions (≤2 cm) in patients with chronic liver disease, by comparing the detection sensitivity of combined DWI/conventional dynamic contrast‐enhanced (DCE)‐MRI to that of conventional DCE‐MRI alone.

Materials and Methods

A total of 37 patients with chronic liver diseases underwent abdominal MRI at 1.5T, including T1‐weighted imaging (T1WI), T2‐weighted imaging (T2WI), and 2D conventional DCE. For each patient study, axial DWI was performed with a single‐shot echo‐planar imaging (EPI) sequence using a modified sensitivity‐encoding (mSENSE) technique with b‐value of 500 seconds/mm². A total of 20–24 slices were obtained during a 15–17‐second breathhold. Two observers independently interpreted the combined DWI/conventional DCE‐MRI images and the conventional DCE‐MRI images alone in random order. For all small HCC lesions, the diagnostic performance using each imaging set was evaluated by receiver operating characteristic (ROC) curve analysis. Sensitivity and positive predictive values were also calculated and analyzed.

Results

A total of 47 small HCCs were confirmed as final result. The area under the ROC curve (Az) of combined DWI/conventional DCE‐MRI images (observer 1, 0.922; observer 2, 0.918) were statistically higher than those of conventional DCE‐MRI alone (observer 1, 0.809; observer 2, 0.778) for all small HCC lesions (P < 0.01). The lesion detection sensitivities using the combined technique for both observers were significantly higher than those using conventional DCE‐MRI alone (P < 0.01). The sensitivity values for two observers using the combined technique were 97.87% and those using conventional DCE‐MRI alone were 85.11% to 82.98%. The positive predictive values for two observers using the combined imaging technique (97.87%) were slightly higher than those using conventional DCE‐MRI alone (92.86–93.02%), but there was no significant difference between the two imaging sets.

Conclusion

Combined use of breathhold DWI with conventional DCE‐MRI helped to provide higher sensitivities than conventional DCE‐MRI alone in the detection of small HCC lesions in patients with chronic liver disease. J. Magn. Reson. Imaging 2009;29:341–349. © 2009 Wiley‐Liss, Inc.     

Quantitative determination of magnetic force on a coronary stent in MRI

Purpose:

To introduce an analytical method for a quantitative determination of magnetic force on a coronary stent in the magnetic resonance imaging (MRI) magnet that is generally applicable to metallic implants. Magnetic forces on metallic implants in the MRI magnets are traditionally determined empirically by measuring deflection from the vertical plane at the central axis of the magnet and at the point of the largest force along the longitudinal axis of the magnet.

Materials and Methods:

Magnetic and chemical characterization of the stents was performed by a commercial magnetometer and energy‐dispersive X‐ray spectroscopy. Magnetic force on the stents fabricated of paramagnetic alloys (surgical stainless steel and cobalt–chromium) was determined by measuring the stent's magnetic dipole moment and employing the on‐axis magnetic field profile of an MRI magnet.

Results:

The maximum force on the stainless steel stent was found to be F_S,max = 0.18 mN, whereas on the cobalt–chromium stent it was F_C,max = 0.06 mN.

Conclusion:

The magnetic force on the investigated paramagnetic stents is even smaller than the gravitational force acting on the stents in the Earth's gravity field, so that it has no physiological impact on the stented vessels. J. Magn. Reson. Imaging 2013;37:391–397. © 2012 Wiley Periodicals, Inc.     

Venous and arterial flow quantification are equally accurate and precise with parallel imaging compressed sensing 4D phase contrast MRI

Purpose:

To evaluate the precision and accuracy of parallel‐imaging compressed‐sensing 4D phase contrast (PICS‐4DPC) magnetic resonance imaging (MRI) venous flow quantification in children with patients referred for cardiac MRI at our children's hospital.

Materials and Methods:

With Institutional Review Board (IRB) approval and Health Insurance Portability and Accountability Act (HIPAA) compliance, 22 consecutive patients without shunts underwent 4DPC as part of clinical cardiac MRI examinations. Flow measurements were obtained in the superior and inferior vena cava, ascending and descending aorta, and the pulmonary trunk. Conservation of flow to the upper, lower, and whole body was used as an internal physiologic control. The arterial and venous flow rates at each location were compared with paired t‐tests and F‐tests to assess relative accuracy and precision.

Results:

Arterial and venous flow measurements were strongly correlated with the upper (ρ = 0.89), lower (ρ = 0.96), and whole body (ρ = 0.97); net aortic and pulmonary trunk flow rates were also tightly correlated (ρ = 0.97). There was no significant difference in the value or precision of arterial and venous flow measurements in upper, lower, or whole body, although there was a trend toward improved precision with lower velocity‐encoding settings.

Conclusion:

With PICS‐4DPC MRI, the accuracy and precision of venous flow quantification are comparable to that of arterial flow quantification at velocity‐encodings appropriate for arterial vessels. J. Magn. Reson. Imaging 2013;37:1419–1426. © 2012 Wiley Periodicals, Inc.     

Variable velocity encoding in a three‐dimensional,three‐directional phase contrast sequence: Evaluation in phantom and volunteers

Purpose:

To evaluate accuracy and noise properties of a novel time‐resolved, three‐dimensional, three‐directional phase contrast sequence with variable velocity encoding (denoted 4D‐vPC) on a 3 Tesla MR system, and to investigate potential benefits and limitations of variable velocity encoding with respect to depicting blood flow patterns.

Materials and Methods:

A 4D PC‐MRI sequence was modified to allow variable velocity encoding (VENC) over the cardiac cycle in all three velocity directions independently. 4D‐PC sequences with constant and variable VENC were compared in a rotating phantom with respect to measured velocities and noise levels. Additionally, comparison of flow patterns in the ascending aorta was performed in six healthy volunteers.

Results:

Phantom measurements showed a linear relationship between velocity noise and velocity encoding. 4D‐vPC MRI presented lower noise levels than 4D‐PC both in phantom and in volunteer measurements, in agreement with theory. Volunteer comparisons revealed more consistent and detailed flow patterns in early diastole for the variable VENC sequences.

Conclusion:

Variable velocity encoding offers reduced noise levels compared with sequences with constant velocity encoding by optimizing the velocity‐to‐noise ratio (VNR) to the hemodynamic properties of the imaged area. Increased VNR ratios could be beneficial for blood flow visualizations of pathology in the cardiac cycle. J. Magn. Reson. Imaging 2012; 36:1450–1459. © 2012 Wiley Periodicals, Inc.     

Artifact reduction from metallic dental materials in T1‐weighted spin‐echo imaging at 3.0 tesla

Purpose:

To investigate and propose a method of artifact reduction arising from metallic dental materials by applying a slice‐encoding for metal artifact correction (SEMAC) technique on T1‐weighted spin‐echo (SE) imaging at 3 Tesla.

Materials and Methods:

The view angle tilting (VAT) technique was adapted to conventional T1‐weighted spin‐echo (SE) sequence to correct the in‐plane distortion, and the SEMAC technique was used for correcting the remaining through‐plane distortions. Fourier transform based B0 field simulations were performed to estimate the amount of field perturbation and a scout imaging method was developed which guide in selecting the number of slice‐encodings needed in SEMAC sequences. Phantoms of six different dental materials with various shapes and sizes that are used in practice (amalgam; titanium implant; gold and Ni‐Cr crowns; Ni‐Ti and stainless steel orthodontic wires) were imaged. In vivo images of two subjects were also acquired. The amounts of artifact reduction were quantified in phantom studies.

Results:

Compared with conventional SE imaging in phantom studies, in‐plane artifacts were reduced by up to 43% in the VAT SE images and 80% in the SEMAC images. Through‐plane artifacts were reduced by up to 65% in SEMAC images. In vivo SEMAC images also showed reduced artifacts.

Conclusion:

The SEMAC technique can mitigate artifact caused by metallic dental materials for T1w‐SE imaging. J. Magn. Reson. Imaging 2013;37:471–478. © 2012 Wiley Periodicals, Inc.     

Free‐breathing high‐spatial‐resolution delayed contrast‐enhanced three‐dimensional viability MR imaging of the myocardium at 3.0T: A feasibility study

Purpose

To assess the feasibility of free‐breathing high‐spatial‐resolution delayed contrast‐enhanced three‐dimensional (3D) viability magnetic resonance imaging (MRI) at 3.0T for the detection of myocardial damages.

Materials and Methods

Twenty‐five patients with myocardial diseases, including myocardial infarction and cardiomyopathies, were enrolled after informed consent was given. Free‐breathing 3D viability MRI with high spatial resolution (1.5 × 1.25 × 2.5 mm) at 3.0T, for which cardiac and navigator gating techniques were employed, was compared with breath‐hold two‐dimensional (2D) viability imaging (1.77 × 1.18 × 10 mm) for assessment of contrast‐to‐noise ratio (CNR) and myocardial damage.

Results

Free‐breathing 3D viability imaging was achieved successfully in 21 of the 25 patients. This imaging technique depicted 84.6% of hyperenhancing myocardium with a higher CNR between hyperenhancing myocardium and blood and with excellent agreement for the transmural extension of myocardial damage (k = 0.91). In particular, the 3D viability images delineated the myocardial infarction and linear hyperenhancing myocardium, comparable to the 2D viability images.

Conclusion

Free‐breathing high‐spatial‐resolution delayed contrast‐enhanced 3D viability MRI using 3.0T was feasible for the evaluation of hyperenhancing myocardium, as seen with myocardial infarction and cardiomyopathies. J. Magn. Reson. Imaging 2008;28:1361–1367. © 2008 Wiley‐Liss, Inc.     

Accelerated bilateral dynamic contrast‐enhanced 3D spiral breast MRI using TSENSE

Purpose

To assess the ability of adaptive sensitivity encoding incorporating temporal filtering (TSENSE) to accelerate bilateral dynamic contrast‐enhanced (DCE) 3D breast MRI.

Materials and Methods

Bilateral DCE breast magnetic resonance imaging (MRI) exams were performed using a dual‐band water‐only excitation and a “stack‐of‐spirals” imaging trajectory. TSENSE was applied in the slab direction with an acceleration factor of 2. Four different techniques for sensitivity map calculation were compared by analyzing resultant contrast uptake curves qualitatively and quantitatively for 10 patient datasets. In addition, image quality and temporal resolution were compared between unaccelerated and TSENSE images.

Results

TSENSE can increase temporal resolution by a factor of 2 in DCE imaging, providing better depiction of contrast uptake curves and good image quality. Of the different methods tested, calculation of static sensitivity maps by averaging late postcontrast frames yields the lowest aliasing artifact level based on ROI analysis.

Conclusion

TSENSE acceleration combined with 3D spiral imaging is very time‐efficient, providing 11‐second temporal resolution and 1.1 × 1.1 × 3 mm³ spatial resolution over a 20 × 20 × 10 cm³ field of view for each breast. J. Magn. Reson. Imaging 2008;28:1425–1434. © 2008 Wiley‐Liss, Inc.     

Model-based nonlinear inverse reconstruction for T2 mapping using highly undersampled spin-echo MRI

Purpose:

To develop a model‐based reconstruction technique for T2 mapping based on multi‐echo spin‐echo MRI sequences with highly undersampled Cartesian data encoding.

Materials and Methods:

The proposed technique relies on a nonlinear inverse reconstruction algorithm which directly estimates a T2 and spin‐density map from a train of undersampled spin echoes. The method is applicable to acquisitions with single receiver coils but benefits from multi‐element coil arrays. The algorithm is validated for trains of 16 spin echoes with a spacing of 10 to 12 ms using numerical simulations as well as human brain MRI at 3 Tesla (T).

Results:

When compared with a standard T2 fitting procedure using fully sampled T2‐weighted images, and depending on the available signal‐to‐noise ratio and number of coil elements, model‐based nonlinear inverse reconstructions for both simulated and in vivo MRI data yield accurate T2 estimates for undersampling factors of 5 to 10.

Conclusion:

This work describes a promising strategy for T2‐weighted MRI that simultaneously offers accurate T2 relaxation times and properly T2‐weighted images at arbitrary echo times. For a standard spin‐echo MRI sequence with Cartesian encoding, the method allows for a much higher degree of undersampling than obtainable by conventional parallel imaging. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Gadofluorine M enhanced MRI in experimental glioma: superior and persistent intracellular tumor enhancement compared with conventional MRI

Purpose:

To compare conventional magnetic resonance imaging (MRI) techniques (T2‐w and Gadolinium‐DTPA‐enhanced T1‐w images) and Gadofluorine‐M (GfM), a novel contrast agent in MRI, in murine gliomas.

Materials and Methods:

Growth monitoring of murine gliomas (induced in mice) was performed on a 2.3 Tesla Bruker Biospec MRI unit. First all animals were investigated with conventional MRI techniques. In group I GfM was applied at an early stage of disease, in group II at a later stage. After injection of GfM follow‐up MRI was performed without further injection of contrast agent. On MR images tumor size and signal intensities were assessed. Animals were killed for histological evaluation.

Results:

In both groups GfM delineated tumor extents larger and more precisely than conventional MRI techniques. The difference between GfM and conventional MRI techniques reached level of significance at both tumor stages. Follow‐up MRI after singular injection of GfM showed persistence of GfM in tumor tissue. On tissue sections GfM‐enhancing areas corresponded closely to vital tumor tissue. GfM showed a mainly intracellular accumulation.

Conclusion:

Application of GfM resulted in superior delineation of experimental glioma compared with conventional MRI techniques. Thus, GfM bears a high potential in clinical application. J. Magn. Reson. Imaging 2012;35:551‐560. © 2011 Wiley Periodicals, Inc.     

Ultrahigh-field DCE-MRI of angiogenesis in a novel angiogenesis mouse model

Purpose:

To be able to screen and identify potential candidate agents for noninvasive imaging of diseases involving angiogenesis, a standardized in vivo angiogenesis model is needed. Angiogenesis is a common feature of many pathological conditions and has become an important target for diagnosis and treatment, with many noninvasive imaging agents emerging.

Materials and Methods:

Uniform scaffolds consisting of porous and flexible polycaprolactone were implanted subcutaneously in mice and studied after 1 to 6 weeks to describe the time course of angiogenesis. The model was characterized by histology and dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI).

Results:

Microscopic examination revealed progressive ingrowth of new vessels from the periphery, leading to a fully vascularized scaffold within 6 weeks. Blood flow through the new vessels, assessed by DCE‐MRI, revealed peripheral vascularization corresponding to 12.3% (SD 6.1%) of scaffold area at week 1 and a more uniform and complete distribution of vessels corresponding to 84.1% (SD 16.2%) of scaffold area at week 4.

Conclusion:

In agreement with microscopic examination, noninvasive DCE‐MRI visualized progressive development of new vessels in a novel and standardized murine angiogenesis model, making this model suitable for screening angiogenesis‐related drugs and contrast agents. J. Magn. Reson. Imaging 2012;35:703‐710. © 2011 Wiley Periodicals, Inc.     

Performance of a fully automatic lesion detection system for breast DCE-MRI

Purpose:

To describe and test a new fully automatic lesion detection system for breast DCE‐MRI.

Materials and Methods:

Studies were collected from two institutions adopting different DCE‐MRI sequences, one with and the other one without fat‐saturation. The detection pipeline consists of (i) breast segmentation, to identify breast size and location; (ii) registration, to correct for patient movements; (iii) lesion detection, to extract contrast‐enhanced regions using a new normalization technique based on the contrast‐uptake of mammary vessels; (iv) false positive (FP) reduction, to exclude contrast‐enhanced regions other than lesions. Detection rate (number of system‐detected malignant and benign lesions over the total number of lesions) and sensitivity (system‐detected malignant lesions over the total number of malignant lesions) were assessed. The number of FPs was also assessed.

Results:

Forty‐eight studies with 12 benign and 53 malignant lesions were evaluated. Median lesion diameter was 6 mm (range, 5–15 mm) for benign and 26 mm (range, 5–75 mm) for malignant lesions. Detection rate was 58/65 (89%; 95% confidence interval [CI] 79%–95%) and sensitivity was 52/53 (98%; 95% CI 90%–99%). Mammary median FPs per breast was 4 (1st–3rd quartiles 3–7.25).

Conclusion:

The system showed promising results on MR datasets obtained from different scanners producing fat‐sat or non–fat‐sat images with variable temporal and spatial resolution and could potentially be used for early diagnosis and staging of breast cancer to reduce reading time and to improve lesion detection. Further evaluation is needed before it may be used in clinical practice. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.     

Limitations of dynamic contrast-enhanced MRI in monitoring radiation-induced changes in the fraction of radiobiologically hypoxic cells in human melanoma xenografts

Purpose

To investigate the potential of gadopentetate dimeglumine (Gd‐DTPA)‐based dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) in detecting radiation‐induced changes in the fraction of radiobiologically hypoxic cells in A‐07 human melanoma xenografts.

Materials and Methods

A‐07 tumors were randomly assigned to an unirradiated control group or a group given a single radiation dose of 20 Gy. DCE‐MRI and measurement of fraction of hypoxic cells were performed immediately before and 24 h after the radiation exposure. Tumor images of E · F (E is the initial extraction fraction of Gd‐DTPA and F is blood perfusion) and λ (λ is proportional to extracellular volume fraction) were produced by subjecting DCE‐MRI series to Kety analysis. Fraction of hypoxic cells was measured by using a radiobiological assay based on the paired survival curve method.

Results

Fraction of radiobiologically hypoxic cells was higher in irradiated tumors (26.2 ± 5.8%) than in unirradiated tumors (7.5 ± 2.7%) by a factor of 3.5 ± 1.5 (P = 0.0093), whereas only minor radiation‐induced changes in E · F and λ could be detected.

Conclusion

DCE‐MRI does not seem to offer insight into the changes in fraction of radiobiologically hypoxic cells occurring in A‐07 tumors within 24 h after irradiation with 20 Gy. J. Magn. Reson. Imaging 2008;28:1209–1218. © 2008 Wiley‐Liss, Inc.     

Hepatic vascular flow measurements by phase contrast MRI and doppler echography: A comparative and reproducibility study

Purpose:

To directly compare and study the variability of parameters related to hepatic blood flow measurements using 3 T phase‐contrast magnetic resonance imaging (PC‐MRI) and Doppler ultrasound (US).

Materials and Methods:

Nine healthy subjects were studied. Blood velocities and flow rate measurements were performed in the portal vein and the proper hepatic artery. MR studies were performed using a 3 T imager. Gradient‐echo fast phase contrast sequences were used with both cardiac and respiratory gating. MR and Doppler flow parameters were extracted and compared. Two methods of calculation were used for Doppler flow rate analysis.

Results:

Compared to Doppler US, PC‐MRI largely underestimated hepatic flow data with lower variability and higher reproducibility. This reproducibility was more pronounced in the portal vein than in the proper hepatic artery associated with poorer velocity correlations. Total hepatic flow values were 1239 ± 223 mL/min and 1595 ± 521 mL/min for PC‐MRI and Doppler US, respectively.

Conclusion:

Free‐breathing PC‐MRI can provide reliable noninvasive measurement of hepatic flow parameters compared to Doppler US. The MR technique could help to improve Doppler flow calculations, thereby allowing standardization of protocols, particularly for applications in disease. J. Magn. Reson. Imaging 2010;31:579–588. © 2010 Wiley‐Liss, Inc.     

Computer‐aided detection of brain tumor invasion using multiparametric MRI

Purpose

To determine the potential of using a computer‐aided detection method to intelligently distinguish peritumoral edema alone from peritumor edema consisting of tumor using a combination of high‐resolution morphological and physiological magnetic resonance imaging (MRI) techniques available on most clinical MRI scanners.

Materials and Methods

This retrospective study consisted of patients with two types of primary brain tumors: meningiomas (n = 7) and glioblastomas (n = 11). Meningiomas are typically benign and have a clear delineation of tumor and edema. Glioblastomas are known to invade outside the contrast‐enhancing area. Four classifiers of differing designs were trained using morphological, diffusion‐weighted, and perfusion‐weighted features derived from MRI to discriminate tumor and edema, tested on edematous regions surrounding tumors, and assessed for their ability to detect nonenhancing tumor invasion.

Results

The four classifiers provided similar measures of accuracy when applied to the training and testing data. Each classifier was able to identify areas of nonenhancing tumor invasion supported with adjunct images or follow‐up studies.

Conclusion

The combination of features derived from morphological and physiological imaging techniques contains the information necessary for computer‐aided detection of tumor invasion and allows for the identification of tumor invasion not previously visualized on morphological, diffusion‐weighted, and perfusion‐weighted images and maps. Further validation of this approach requires obtaining spatially coregistered tissue samples in a study with a larger sample size. J. Magn. Reson. Imaging 2009;30:481–489. © 2009 Wiley‐Liss, Inc.     

Consideration of physiological response in numerical models of temperature during MRI of the human head

Purpose

To examine the thermal effects of the physiological response to heating during exposure to radiofrequency (RF) electromagnetic fields in magnetic resonance imaging (MRI) with a head‐specific volume coil.

Materials and Methods

Numerical methods were used to calculate the temperature elevation in MRI of the human head within volume coils from 64–400 MHz at different power levels both with and without consideration of temperature‐induced changes in rates of metabolism, perspiration, radiation, and perfusion.

Results

At the highest power levels currently allowed in MRI for head volume coils, there is little effect from the physiological response as predicted with existing methods. This study does not rule out the possibility that at higher power levels or in different types of coils (such as extremity or whole‐body coils) the physiological response may have more significant effects.

Conclusion

In modeling temperature increase during MRI of the human head in a head‐sized volume coil at up to 3.0 W/kg head‐average specific energy absorption rates, it may not be necessary to consider thermally induced changes in rates of metabolism, perfusion, perspiration, and radiation. J. Magn. Reson. Imaging 2008;28:1303–1308. © 2008 Wiley‐Liss, Inc.     

Flow‐sensitive 4D MRI of the thoracic aorta: Comparison of image quality,quantitative flow,and wall parameters at 1.5 T and 3 T

Purpose:

To evaluate the effect of field strength on flow‐sensitive 4D magnetic resonance imaging (MRI) of the thoracic aorta. A volunteer study at 1.5 T and 3 T was conducted to compare phase‐contrast MR angiography (MRA) and 3D flow visualization quality as well as quantification of aortic hemodynamics.

Materials and Methods:

Ten healthy volunteers were examined by flow‐sensitive 4D MRI at both 1.5 T and 3 T MRI with identical imaging parameters (TE/TR = 6/5.1 msec, spatial/temporal resolution ≈2 mm/40.8 msec). Analysis included assessment of image quality of derived aortic 3D phase contrast (PC) angiography and 3D flow visualization (semiquantitative grading on a 0–2 scale, two blinded observers) and quantification of blood flow velocities, net flow per cardiac cycle, wall shear stress (WSS), and velocity noise.

Results:

Quality of 3D blood flow visualization (average grading = 1.8 ± 0.4 at 3 T vs. 1.1 ± 0.7 at 1.5 T) and the depiction of aortic lumen geometry by 3D PC‐MRA (1.7 ± 0.5 vs. 1.2 ± 0.6) were significantly (P < 0.01) improved at 3 T while velocity noise was significantly higher (P < 0.01) at 1.5 T. Velocity quantification resulted in minimally altered (0.05 m/s, 3 mL/cycle and 0.01 N/m²) but not statistically different (P = 0.40, P = 0.39, and P = 0.82) systolic peak velocities, net flow, and WSS for 1.5 T compared to 3 T.

Conclusion:

Flow‐sensitive 4D MRI at 3 T provided improved image quality without additional artifacts related to higher fields. Imaging at 1.5 T MRI, which is more widely available, was also feasible and provided information on aortic 3D hemodynamics of moderate quality with identical performance regarding quantitative analysis. J. Magn. Reson. Imaging 2012;36:1097–1103. © 2012 Wiley Periodicals, Inc.