Determination of whole‐brain oxygen extraction fractions by fast measurement of blood T2 in the jugular vein

The oxygen extraction fraction of the brain reports on the balance between oxygen delivery and consumption and can be used to assess deviations in physiological homeostasis. This is relevant clinically as well as for calibrating blood oxygen level–dependent functional MRI responses. Oxygen extraction fraction is reflected in the arteriovenous difference in oxygen saturation fraction (Y_v ? Y_a), which can be determined from venous T₂ values when arterial oxygenation is known. A pulse sequence is presented that allows rapid measurement (<1 min) of blood T₂s in the internal jugular vein. The technique combines slice‐saturation and blood inflow to attain high signal‐to‐noise ratio in blood and minimal contamination from tissue. The sequence is sensitized to T₂ using a nonselective Carr‐Purcell‐Meiboom‐Gill T₂ preparation directly after slice saturation. Fast scanning (pulse repetition time of about 2 sec) is possible by using a nonselective saturation directly after acquisition to rapidly achieve steady‐state longitudinal magnetization. The venous T₂ (for 10 msec Carr‐Purcell‐Meiboom‐Gill interecho time) for normal volunteers was 62.4 ± 6.1 msec (n = 20). A calibration curve relating T₂ to blood oxygenation was established using a blood perfusion phantom. Using this calibration, a whole‐brain oxygen extraction fraction of 0.37 ± 0.04 was determined (n = 20), in excellent agreement with literature values. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Diffusion characteristics associated with neuronal injury and glial activation following hypoxia‐ischemia in the immature brain

To identify quantitative MRI indices of injury in the brain following neonatal hypoxic‐ischemic brain injury, we subjected mouse pups to hypoxia‐ischemia on postnatal day 7 and obtained conventional and diffusion‐weighted in vivo images of the brain 24 h later followed by histological assessment. T₂‐weighted images showed increased signal intensity in the CA1 and CA2 regions of the hippocampus ipsilateral to the injury and adjacent white matter. In contrast, diffusion imaging showed reduced apparent diffusion coefficient (ADC) values in CA1 and CA2, but increased values in the adjacent white matter. Histological analysis showed widespread gliosis with degenerating oligodendrocytes in the ipsilateral hippocampus. In addition, white matter areas that were abnormal by MRI showed an increase in the number of activated microglia (CD45 positive cells). Activated caspase‐3 immunostaining showed a marked increase in neurons in the hippocampal regions corresponding to those with reduced ADC, and a quantitative measure of staining showed a statistically significant correlation with the ADC. In contrast, ADC was higher in adjacent white matter, where histology showed activation of microglia and reactive oligodendrocytes but not caspase‐3 activation. These results suggest that the ADC response differs between areas of neuronal injury as compared with those showing glial changes without marked cell death. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Contrast‐enhanced whole‐heart coronary MRI with bolus infusion of gadobenate dimeglumine at 1.5 T

We sought to investigate the T₁ kinetics of blood and myocardium after three infusion schemes of gadobenate dimeglumine (Gd‐BOPTA) and subsequently compared contrast‐enhanced whole‐heart coronary MRI after a bolus Gd‐BOPTA infusion with nonenhanced coronary MRI at 1.5 T. Blood and myocardium T₁ was measured in seven healthy adults, after each underwent three Gd‐BOPTA infusion schemes (bolus: 0.2 mmol/kg at 2 mL/sec, hybrid: 0.1 mmol/kg at 2 mL/sec followed by 0.1 mmol/kg at 0.1 mL/sec, and slow: 0.2 mmol/kg at 0.3 mL/sec). Fourteen additional subjects underwent contrast‐enhanced coronary MRI with an inversion‐recovery steady‐state free precession sequence after bolus Gd‐BOPTA infusion. Images were compared with nonenhanced T₂‐prepared steady‐state free precision whole‐heart coronary MRI in signal‐to‐noise ratio, contrast‐to‐noise ratio, depicted vessel length, vessel sharpness, and subjective image quality. Bolus and slow infusion schemes resulted in similar T₁ during coronary MRI, whereas the hybrid infusion method yielded higher T₁ values. A bolus infusion of Gd‐BOPTA significantly improved signal‐to‐noise ratio, contrast‐to‐noise ratio, depicted coronary artery length, and subjective image quality, when all segments were collectively compared but not when compared segment by segment. In conclusion, whole‐heart steady‐state free precision coronary MRI at 1.5 T can benefit from a bolus infusion of 0.2 mmol/kg Gd‐BOPTA. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Multiple‐bolus dynamic contrast‐enhanced MRI in the pancreas during a glucose challenge

Purpose:

To assess the feasibility of multiple‐bolus dynamic contrast‐enhanced (DCE) magnetic resonance imaging (MRI) in the pancreas; to optimize the analysis; and to investigate application of the method to a glucose challenge in type 2 diabetes.

Materials and Methods:

A 4‐bolus DCE‐MRI protocol was performed on five patients with type 2 diabetes and 11 healthy volunteers during free‐breathing. Motion during the dynamic time series was corrected for using a model‐driven nonlinear registration. A glucose challenge was administered intravenously between the first and second DCE‐MRI acquisition in all patients and in seven of the healthy controls.

Results:

Image registration improved the reproducibility of the DCE‐MRI model parameters across the repeated bolus‐acquisitions in the healthy controls with no glucose challenge (eg, coefficient of variation for K^trans improved from 38% to 28%). Native tissue T₁ was significantly lower in patients (374 ± 68 msec) compared with volunteers (519 ± 41 msec) but there was no significant difference in any of the baseline DCE‐MRI parameters. No effect of glucose challenge was observed in either the patients or healthy volunteers.

Conclusion:

Multiple bolus DCE‐MRI is feasible in the pancreas and is improved by nonlinear image registration but is not sensitive to the effects of an intravenous glucose challenge. J. Magn. Reson. Imaging 2010;32:622–628. © 2010 Wiley‐Liss, Inc.     

Mouse myocardial first‐pass perfusion MR imaging

A first‐pass myocardial perfusion sequence for mouse cardiac MRI is presented. A segmented ECG‐triggered acquisition combined with parallel imaging acceleration was used to capture the first pass of a Gd‐DTPA bolus through the mouse heart with a temporal resolution of 300–400 msec. The method was applied in healthy mice (N = 5) and in mice with permanent occlusion of the left coronary artery (N = 6). Baseline semiquantitative perfusion values of healthy myocardium showed excellent reproducibility. Infarct regions revealed a significant decrease in the semiquantitative myocardial perfusion values (0.05 ± 0.02) compared to remote myocardium (0.20 ± 0.04). Myocardial areas of decreased perfusion correlated well to infarct areas identified on the delayed‐enhancement scans. This protocol is a valuable addition to the mouse cardiac MRI toolbox for preclinical studies of ischemic heart disease. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Combined use of T2‐weighted MRI and T1‐weighted dynamic contrast–enhanced MRI in the automated analysis of breast lesions

A multiparametric computer‐aided diagnosis scheme that combines information from T₁‐weighted dynamic contrast–enhanced (DCE)‐MRI and T₂‐weighted MRI was investigated using a database of 110 malignant and 86 benign breast lesions. Automatic lesion segmentation was performed, and three categories of lesion features (geometric, T₁‐weighted DCE, and T₂‐weighted) were automatically extracted. Stepwise feature selection was performed considering only geometric features, only T₁‐weighted DCE features, only T₂‐weighted features, and all features. Features were merged with Bayesian artificial neural networks, and diagnostic performance was evaluated by ROC analysis. With leave‐one‐lesion‐out cross‐validation, an area under the ROC curve value of 0.77 ± 0.03 was achieved with T₂‐weighted‐only features, indicating high diagnostic value of information in T₂‐weighted images. Area under the ROC curve values of 0.79 ± 0.03 and 0.80 ± 0.03 were obtained for geometric‐only features and T₁‐weighted DCE‐only features, respectively. When all features were considered, an area under the ROC curve value of 0.85 ± 0.03 was achieved. We observed P values of 0.006, 0.023, and 0.0014 between the geometric‐only, T₁‐weighted DCE‐only, and T₂‐weighted‐only features and all features conditions, respectively. When ranked, the P values satisfied the Holm–Bonferroni multiple‐comparison test; thus, the improvement of multiparametric computer‐aided diagnosis was statistically significant. A computer‐aided diagnosis scheme that combines information from T₁‐weighted DCE and T₂‐weighted MRI may be advantageous over conventional T₁‐weighted DCE‐MRI computer‐aided diagnosis. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Assessment of liver function in thioacetamide‐induced rat acute liver injury using an empirical mathematical model and dynamic contrast‐enhanced MRI with Gd‐EOB‐DTPA

Purpose:

To evaluate thioacetamide (TAA)‐induced acute liver injury in rats using an empirical mathematical model (EMM) and dynamic contrast‐enhanced magnetic resonance imaging (DCE‐MRI) with gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid (Gd‐EOB‐DTPA).

Materials and Methods:

Eighteen rats were divided into three groups (normal control [n = 6], TAA [140] [n = 6], and TAA [280] groups [n = 6]). The rats of the TAA (140) and TAA (280) groups were intravenously injected with 140 and 280 mg/kg body weight (BW) of TAA, respectively, while those of the normal control group were intravenously injected with the same volume of saline. DCE‐MRI studies were performed using Gd‐EOB‐DTPA (0.025 mmol Gd/kg; 0.1 mL/kg BW) as the contrast agent 48 hours after TAA or saline injection. After the DCE‐MRI study, blood was sampled and serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were measured. We calculated the rate of contrast uptake (α), the rate of contrast washout (β), the elimination half‐life of relative enhancement (RE) (T_1/2), the maximum RE (RE_max), and the time to (RE_max) (T_max) from time‐signal intensity curves using EMM.

Results:

The RE_max values in the TAA (140) groups and TAA (280) groups were significantly smaller than that in the normal control group. The T_max value in the TAA (280) group was significantly greater than that in the normal control group. The β value in the TAA (280) group was significantly smaller than those in the normal control and TAA (140) groups, whereas there were no significant differences in β among groups. The T_1/2 value in the TAA (280) group was significantly greater than those in the normal control and TAA (140) groups. The RE_max, T_max, β, and T_1/2 values significantly correlated with AST and ALT.

Conclusion:

The EMM is useful for evaluating TAA‐induced acute liver injury using DCE‐MRI with Gd‐EOB‐DTPA. J. Magn. Reson. Imaging 2012; 36:1483–1489. © 2012 Wiley Periodicals, Inc.     

Four‐dimensional transcatheter intraarterial perfusion (TRIP)‐MRI for monitoring liver tumor embolization in VX2 rabbits

Transcatheter intraarterial perfusion (TRIP)‐MRI is an intraprocedural technique to iteratively monitor liver tumor perfusion changes during transcatheter arterial embolization (TAE) and chemoembolization (TACE). However, previous TRIP‐MRI approaches using two‐dimensional (2D) T₁‐weighted saturation‐recovery gradient‐recalled echo (GRE) sequences provided only limited spatial coverage and limited capacity for accurate perfusion quantification. In this preclinical study, a quantitative 4D TRIP‐MRI technique (serial iterative 3D volumetric perfusion imaging) with rigorous radiofrequency (RF) B₁ field calibration and dynamic tissue longitudinal relaxation rate R₁ measurement is presented for monitoring intraprocedural liver tumor perfusion during TAE. 4D TRIP‐MRI and TAE were performed in five rabbits with eight VX2 liver tumors (N = 8). After B₁ calibrated baseline and dynamic R₁ quantification, subsequent tissue contrast agent concentration time curves were derived. A single‐input flow‐limited pharmacokinetic model and peak gradient method were applied for perfusion analysis. The perfusion Fρ reduced significantly from pre‐TAE 0.477 (95% confidence interval [CI]: 0.384–0.570) to post‐TAE 0.131 (95% CI: 0.080–0.183 ml/min/ml, P < 0.001). Magn Reson Med 60:970–975, 2008. © 2008 Wiley‐Liss, Inc.     

On the mechanisms enabling myocardial edema contrast in bSSFP‐based imaging approaches

The biophysical mechanisms influencing balanced steady‐state free precession (bSSFP) based edema imaging in the setting of acute myocardial infarction are not well understood. To assess the various mechanisms that enable the detection of myocardial edema on bSSFP‐based imaging approaches (cine bSSFP and T₂‐prepared bSSFP), experiments were conducted in canine models subjected to ischemia‐reperfusion injury. Results showed that in addition to relaxation effects, the alteration in thermal equilibrium (M₀) (including magnetization transfer) has a significant contribution to the image contrast between edematous and healthy myocardium. The relative signal‐intensity ratios between edematous and healthy myocardium were: 1.51 ± 0.18 (cine bSSFP) and 1.58 ± 0.20 (T₂‐prepared bSSFP); the theoretically estimated relative relaxation and M₀ effects were: 1.17 ± 0.09 and 1.30 ± 0.19, respectively (cine bSSFP), and 1.49 ± 0.23 and 1.06 ± 0.07, respectively (T₂‐prepared bSSFP). There were no significant difference between cine bSSFP and T₂‐prep bSSFP relative signal‐intensity ratios. However, the relative relaxation effect in cine bSSFP was significantly lower than in T₂‐prep bSSFP (P < 0.05), and the M₀ effect in cine bSSFP was significantly higher than in T₂‐prep bSSFP (P < 0.05). Hence the acquisition strategies that wish to maximize myocardial edema contrast in cine bSSFP imaging should take both relaxation and M₀ effects into account. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Respiratory motion‐compensated radial dynamic contrast‐enhanced (DCE)‐MRI of chest and abdominal lesions

Dynamic contrast‐enhanced (DCE)‐MRI is becoming an increasingly important tool for evaluating tumor vascularity and assessing the effectiveness of emerging antiangiogenic and antivascular agents. In chest and abdominal regions, however, respiratory motion can seriously degrade the achievable image quality in DCE‐MRI studies. The purpose of this work is to develop a respiratory motion‐compensated DCE‐MRI technique that combines the self‐gating properties of radial imaging with the reconstruction flexibility afforded by the golden‐angle view‐order strategy. Following radial data acquisition, the signal at k‐space center is first used to determine the respiratory cycle, and consecutive views during the expiratory phase of each respiratory period (34–55 views, depending on the breathing rate) are grouped into individual segments. Residual intrasegment translation of lesion is subsequently compensated for by an autofocusing technique that optimizes image entropy, while intersegment translation (among different respiratory cycles) is corrected using 3D image correlation. The resulting motion‐compensated, undersampled dynamic image series is then processed to reduce image streaking and to enhance the signal‐to‐noise ratio (SNR) prior to perfusion analysis, using either the k‐space‐weighted image contrast (KWIC) radial filtering technique or principal component analysis (PCA). The proposed data acquisition scheme also allows for high frame‐rate arterial input function (AIF) sampling and free‐breathing baseline T₁ mapping. The performance of the proposed radial DCE‐MRI technique is evaluated in subjects with lung and liver lesions, and results demonstrate that excellent pixelwise perfusion maps can be obtained with the proposed methodology. Magn Reson Med 60:1135–1146, 2008. © 2008 Wiley‐Liss, Inc.     

High‐resolution magnetic resonance colonography and dynamic contrast‐enhanced magnetic resonance imaging in a murine model of colitis

Inflammatory bowel disease, including ulcerative colitis, is characterized by persistent or recurrent inflammation and can progress to colon cancer. Colitis is difficult to detect and monitor noninvasively. The goal of this work was to develop a preclinical imaging method for evaluating colitis. Herein, we report improved MRI methods for detecting and characterizing colitis noninvasively in mice, using high‐resolution in vivo MR images and dynamic contrast‐enhanced MRI studies, which were confirmed by histologic studies in a murine model of colitis. C57Bl6/J male mice were treated with 2.5% dextran sulfate sodium in their drinking water for 5 days to induce colitis. MR images were acquired using a 9.4‐T Bruker scanner from 5–25 days following dextran sulfate sodium treatment. In dynamic contrast‐enhanced MRI studies, Gd uptake (K^trans) and its distribution (v_e) were measured in muscle and normal and inflamed colons after administering Gd‐diethyltriaminepentaacetic acid (Gd‐DTPA). T₂‐weighted MR images distinguished normal colon from diffusely thickened colonic wall occurring in colitis (P <0.0005) and correlated with histologic features. Values of K^trans and v_e obtained from dynamic contrast‐enhanced MRI were also significantly different in inflamed colons compared to normal colon (P < 0.0005). The results demonstrate that both T₂‐weighted anatomic imaging and quantitative analysis of dynamic contrast‐enhanced MRI data can successfully distinguish colitis from normal colon in mice. Magn Reson Med 63:922–929, 2010. © 2010 Wiley‐Liss, Inc.     

Manganese‐enhanced MRI detection of neurodegeneration in neonatal hypoxic‐ischemic cerebral injury

In this study, we investigated the Mn‐enhanced MRI (MEMRI) for detecting neurodegenerative processes in neonatal hypoxic‐ischemic (H‐I) cerebral injury. Seven‐day‐old rats were induced with H‐I injury, and scanned for T₁‐weighted image (T₁WI) and T₂‐weighted image (T₂WI) with and without systemic MnCl₂ administration. Serial histological analysis was performed for Mn‐superoxide dismutase (Mn‐SOD) and glutamine synthetase (GS), which are Mn‐binding enzymes against the oxidative stress and glutamate excitotoxicity in neurodegeneration. In the acute phase (first 2 days), the ipsilateral lesion exhibited no Mn enhancement in T₁WIs, with histology showing no Mn‐SOD and GS production. In the mid‐phase (from day 3), Mn enhancement was found in the cortex, basal ganglia, and hippocampus, correlating with local Mn‐SOD and GS increase. In the late phase, the enhancement became more localized to the pericyst basal ganglia and cortex, and then gradually diminished. In T₂WIs, a signal decrease was observed from day 3 in the corresponding regions. Hypointense voids gradually formed in the late phase, correlating with the local iron accumulation. H‐I rats without Mn²⁺ administration exhibited similar but weak changes in T₁WIs and T₂WIs from days 14 and 7, respectively. These results indicate that Mn²⁺ may be a useful in vivo probe for monitoring Mn‐SOD and GS enzymatic activities. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.     

T(2) -weighted STIR imaging of myocardial edema associated with ischemia-reperfusion injury: the influence of proton density effect on image contrast

Purpose

To investigate the contribution of proton density (PD) in T₂‐STIR based edema imaging in the setting of acute myocardial infarction (AMI).

Materials and Methods

Canines (n = 5), subjected to full occlusion of the left anterior descending artery for 3 hours, underwent serial magnetic resonance imaging (MRI) studies 2 hours postreperfusion (day 0) and on day 2. During each study, T₁ and T₂ maps, STIR (TE = 7.1 msec and 64 msec) and late gadolinium enhancement (LGE) images were acquired. Using T₁ and T₂ maps, relaxation and PD contributions to myocardial edema contrast (EC) in STIR images at both TEs were calculated.

Results

Edematous territories showed significant increase in PD (20.3 ± 14.3%, P < 0.05) relative to healthy territories. The contributions of T₁ changes and T₂ or PD changes toward EC were in opposite directions. One‐tailed t‐test confirmed that the mean T₂ and PD‐based EC at both TEs were greater than zero. EC from STIR images at TE = 7.1 msec was dominated by PD than T₂ effects (94.3 ± 11.3% vs. 17.6 ± 2.5%, P < 0.05), while at TE = 64 msec, T₂ effects were significantly greater than PD effects (90.8 ± 20.3% vs. 12.5 ± 11.9%, P < 0.05). The contribution from PD in standard STIR acquisitions (TE = 64 msec) was significantly higher than 0 (P < 0.05).

Conclusion

In addition to T₂‐weighting, edema detection in the setting of AMI with T₂‐weighted STIR imaging has a substantial contribution from PD changes, likely stemming from increased free‐water content within the affected tissue. This suggests that imaging approaches that take advantage of both PD as well as T₂ effects may provide the optimal sensitivity for detecting myocardial edema. J. Magn. Reson. Imaging 2011;33:962–967. © 2011 Wiley‐Liss, Inc.     

3D flow‐independent peripheral vessel wall imaging using T2‐prepared phase‐sensitive inversion‐recovery steady‐state free precession

Purpose:

To develop a 3D flow‐independent peripheral vessel wall imaging method using T₂‐prepared phase‐sensitive inversion‐recovery (T₂PSIR) steady‐state free precession (SSFP).

Materials and Methods:

A 3D T₂‐prepared and nonselective inversion‐recovery SSFP sequence was designed to achieve flow‐independent blood suppression for vessel wall imaging based on T₁ and T₂ properties of the vessel wall and blood. To maximize image contrast and reduce its dependence on the inversion time (TI), phase‐sensitive reconstruction was used to restore the true signal difference between vessel wall and blood. The feasibility of this technique for peripheral artery wall imaging was tested in 13 healthy subjects. Image signal‐to‐noise ratio (SNR), wall/lumen contrast‐to‐noise ratio (CNR), and scan efficiency were compared between this technique and conventional 2D double inversion recovery – turbo spin echo (DIR‐TSE) in eight subjects.

Results:

3D T₂PSIR SSFP provided more efficient data acquisition (32 slices and 64 mm in 4 minutes, 7.5 seconds per slice) than 2D DIR‐TSE (2–3 minutes per slice). SNR of the vessel wall and CNR between vessel wall and lumen were significantly increased as compared to those of DIR‐TSE (P < 0.001). Vessel wall and lumen areas of the two techniques are strongly correlated (intraclass correlation coefficients: 0.975 and 0.937, respectively; P < 0.001 for both). The lumen area of T₂PSIR SSFP is slightly larger than that of DIR‐TSE (P = 0.008). The difference in vessel wall area between the two techniques is not statistically significant.

Conclusion:

T₂PSIR SSFP is a promising technique for peripheral vessel wall imaging. It provides excellent blood signal suppression and vessel wall/lumen contrast. It can cover a 3D volume efficiently and is flow‐ and TI‐independent. J. Magn. Reson. Imaging 2010;32:399–408. © 2010 Wiley‐Liss, Inc.     

Experimental protocol for activation‐induced manganese‐enhanced MRI (AIM‐MRI) based on quantitative determination of Mn content in rat brain by fast T1 mapping

In activation‐induced manganese‐enhanced MRI (AIM‐MRI) experiments, differential accumulation of Mn in activated and silent brain areas is generally assessed using T₁‐weighted images and quantified by the enhancement of signal intensity (SI), calculated with reference to SI before Mn administration or to SI of brain regions unaffected by the specific stimulus. However, SI enhancement can be unreliable when animals are removed from and reinserted into the magnet. We have developed an experimental protocol based on repeated intraperitoneal (i.p.) injections of Mn, quantitative determination of T₁, and coregistration of images to a rat brain atlas that allows absolute quantification of Mn concentration in selected brain areas. Results showed that interanimal variability of postcontrast T₁ values was very low (compared to the experimental error in T₁ determinations) allowing detection of differential regional Mn uptake in stimulated and unstimulated animals. In addition we have determined in vivo relaxivity of Mn in brain tissue and its frequency dependence. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Accelerated cardiac T2 mapping using breath‐hold multiecho fast spin‐echo pulse sequence with k‐t FOCUSS

Cardiac T₂ mapping is a promising method for quantitative assessment of myocardial edema and iron overload. We have developed a new multiecho fast spin echo (ME‐FSE) pulse sequence for breath‐hold T₂ mapping with acceptable spatial resolution. We propose to further accelerate this new ME‐FSE pulse sequence using k‐t focal underdetermined system solver adapted with a framework that uses both compressed sensing and parallel imaging (e.g., sensitivity encoding) to achieve higher spatial resolution. We imaged 12 control subjects in midventricular short‐axis planes and compared the accuracy of T₂ measurements obtained using ME‐FSE with generalized autocalibrating partially parallel acquisitions and ME‐FSE with k‐t focal underdetermined system solver. For image reconstruction, we used a bootstrapping two‐step approach, where in the first step fast Fourier transform was used as the sparsifying transform and in the final step principal component analysis was used as the sparsifying transform. When compared with T₂ measurements obtained using generalized autocalibrating partially parallel acquisitions, T₂ measurements obtained using k‐t focal underdetermined system solver were in excellent agreement (mean difference = 0.04 msec; upper/lower 95% limits of agreement were 2.26/?2.19 msec, respectively). The proposed accelerated ME‐FSE pulse sequence with k‐t focal underdetermined system solver is a promising investigational method for rapid T₂ measurement of the heart with relatively high spatial resolution (1.7 × 1.7 mm²). Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

lacZ as a genetic reporter for real‐time MRI

Molecular imaging based on MRI is currently hampered by the lack of genetic reporters for in vivo imaging. We determined that the commercially available substrate S‐Gal? can be used to detect genetically engineered β‐galactosidase expressing cells by MRI. The effect and specificity of the reaction between β‐galactosidase and S‐Gal? on MRI contrast were determined both in vitro and in vivo. β‐galactosidase activity in the presence of S‐Gal? resulted in enhanced T₂ and T*₂ MR‐contrast, which was amplified with increasing magnetic field strengths (4.7‐17.6 T) in phantom studies. Using both lacZ⁺ transgenic animals and lacZ⁺ tissue transplants, we were able to detect labeled cells in live animals in real time. Similar to phantom studies, detection of the labeled cells/tissues in vivo was enhanced at high magnetic fields. These results demonstrate that the genetic reporter, lacZ, can be used as an in vivo marker gene using high‐field‐strength MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin‐locked balanced steady‐state free‐precession (slSSFP) pulse sequence is described that combines a balanced gradient‐echo acquisition with an off‐resonance spin‐lock pulse for fast MRI. The transient and steady‐state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady‐state free‐precession (bSSFP) for a range of T₂/T₁ and flip angles, although the slSSFP steady‐state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal‐to‐noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Compensation of slice profile mismatch in combined spin‐ and gradient‐echo echo‐planar imaging pulse sequences

Combined acquisition of gradient‐echo and spin‐echo signals in MRI time series reveals additional information for perfusion‐weighted imaging and functional MRI because of differences in the sensitivity of gradient‐echo and spin‐echo measurements to the properties of the underlying vascular architecture. The acquisition of multiple echo trains within one time frame facilitates the simultaneous estimation of the transversal relaxation parameters R₂ and R. However, the simultaneous estimation of these parameters tends to be incorrect in the presence of slice profile mismatches between signal excitation and subsequent refocusing pulses. It is shown here that improvements in pulse design reduced R₂ and R estimation errors. Further improvements were achieved by augmented parameter estimation through the introduction of an additional parameter δ to correct for discordances in slice profiles to facilitate more quantitative measurements. Moreover, the analysis of time‐resolved acquisitions revealed that the temporal stability of R₂ estimates could be increased with improved pulse design, counteracting low contrast‐to‐noise ratios in spin‐echo‐based perfusion and functional MRI. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Non‐contrast‐enhanced four‐dimensional (4D) intracranial MR angiography: A feasibility study

T₁‐shortening contrast agents have been widely used in time‐resolved magnetic resonance angiography. To match imaging data acquisition with the short time period of the first pass of contrast agent, temporal resolution and/or spatial resolution have to be compromised in many cases. In this study, a novel non‐contrast‐enhanced technique was developed for time‐resolved magnetic resonance angiography. Alternating magnetization preparation was applied in two consecutive acquisitions of each measurement to eliminate the need for contrast media. Without the constraint of contrast media kinetics, temporal resolution is drastically improved from the order of a second as in conventional contrast‐enhanced approach to tens of milliseconds (50.9 msec) in this study, without compromising spatial resolution. Initial results from volunteer studies demonstrate the feasibility of this method to depict anatomic structure and dynamic filling of main vessels in the head. Magn Reson Med 63:835–841, 2010. © 2010 Wiley‐Liss, Inc.