Optimized and combinedT1 andB1 mapping technique for fast and accurateT1 quantification in contrast-enhanced abdominal MRI

Simultaneous whole-liver water T 1 $$ {\mathrm{T}}_1 $$ and T 2 $$ {\mathrm{T}}_2 $$ mapping with isotropic resolution during free-breathing

PURPOSE

To develop and validate a data acquisition scheme combined with a motion-resolved reconstruction and dictionary-matching-based parameter estimation to enable free-breathing isotropic resolution self-navigated whole-liver simultaneous water-specificT 1 $$ {\mathrm{T}}_1 $$ (wT 1 $$ {\mathrm{wT}}_1 $$ ) andT 2 $$ {\mathrm{T}}_2 $$ (wT 2 $$ {\mathrm{wT}}_2 $$ ) mapping for the characterization of diffuse and oncological liver diseases.

METHODS

The proposed data acquisition consists of a magnetization preparation pulse and a two-echo gradient echo readout with a radial stack-of-stars trajectory, repeated with different preparations to achieve differentT 1 $$ {\mathrm{T}}_1 $$ andT 2 $$ {\mathrm{T}}_2 $$ contrasts in a fixed acquisition time of 6 min. Regularized reconstruction was performed using self-navigation to account for motion during the free-breathing acquisition, followed by water-fat separation. Bloch simulations of the sequence were applied to optimize the sequence timing forB 1 $$ {B}_1 $$ insensitivity at 3 T, to correct for relaxation-induced blurring, and to mapT 1 $$ {\mathrm{T}}_1 $$ andT 2 $$ {\mathrm{T}}_2 $$ using a dictionary. The proposed method was validated on a water-fat phantom with varying relaxation properties and in 10 volunteers against imaging and spectroscopy reference values. The performance and robustness of the proposed method were evaluated in five patients with abdominal pathologies.

RESULTS

Simulations demonstrate goodB 1 $$ {B}_1 $$ insensitivity of the proposed method in measuringT 1 $$ {\mathrm{T}}_1 $$ andT 2 $$ {\mathrm{T}}_2 $$ values. The proposed method produces co-registeredwT 1 $$ {\mathrm{wT}}_1 $$ andwT 2 $$ {\mathrm{wT}}_2 $$ maps with a good agreement with reference methods (phantom:wT 1 = 1 . 02 wT 1,ref - 8 . 93 ms , R 2 = 0 . 991 $$ {\mathrm{wT}}_1=1.02\kern0.1em {\mathrm{wT}}_{1,\mathrm{ref}}-8.93\kern0.1em \mathrm{ms},{R}^2=0.991 $$ ;wT 2 = 1 . 03 wT 2,ref + 0 . 73 ms , R 2 = 0 . 995 $$ {\mathrm{wT}}_2=1.03\kern0.1em {\mathrm{wT}}_{2,\mathrm{ref}}+0.73\kern0.1em \mathrm{ms},{R}^2=0.995 $$ ). The proposedwT 1 $$ {\mathrm{wT}}_1 $$ andwT 2 $$ {\mathrm{wT}}_2 $$ mapping exhibits good repeatability and can be robustly performed in patients with pathologies.

CONCLUSIONS

The proposed method allows whole-liverwT 1 $$ {\mathrm{wT}}_1 $$ andwT 2 $$ {\mathrm{wT}}_2 $$ quantification with high accuracy at isotropic resolution in a fixed acquisition time during free-breathing.

T1 and T2-mapping in pancreatic MRI: Current evidence and future perspectives

Conventional T1- and T2-weighted magnetic resonance imaging (MRI) of the pancreas can vary significantly due to factors such as scanner differences and pulse sequence variations. This review explores T1 and T2 mapping techniques, modern MRI methods providing quantitative information about tissue relaxation times. Various T1 and T2 mapping pulse sequences are currently under investigation. Clinical and research applications of T1 and T2 mapping in the pancreas include their correlation with fibrosis, inflammation, and neoplasms. In chronic pancreatitis, T1 mapping and extracellular volume (ECV) quantification demonstrate potential as biomarkers, aiding in early diagnosis and classification. T1 mapping also shows promise in evaluating pancreatic exocrine function and detecting glucose metabolism disorders. T2* mapping is valuable in quantifying pancreatic iron, offering insights into conditions like thalassemia major. However, challenges persist, such as the lack of consensus on optimal sequences and normal values for healthy pancreas relaxometry. Large-scale studies are needed for validation, and improvements in mapping sequences are essential for widespread clinical integration. The future holds potential for mixed qualitative and quantitative models, extending the applications of relaxometry techniques to various pancreatic lesions and enhancing routine MRI protocols for pancreatic pathology diagnosis and prognosis.

Accelerated liver water T1 mapping using single-shot continuous inversion-recovery spiral imaging

PURPOSE

Liver T1 mapping techniques typically require long breath holds or long scan time in free-breathing, need correction for B 1 + inhomogeneities and process composite (water and fat) signals. The purpose of this work is to accelerate the multi-slice acquisition of liver water selective T1 (wT1) mapping in a single breath hold, improving the k-space sampling efficiency.

METHODS

The proposed continuous inversion-recovery (IR) Look-Locker methodology combines a single-shot gradient echo spiral readout, Dixon processing and a dictionary-based analysis for liver wT1 mapping at 3 T. The sequence parameters were adapted to obtain short scan times. The influence of fat, B 1 + inhomogeneities and TE on the estimation of T1 was first assessed using simulations. The proposed method was then validated in a phantom and in 10 volunteers, comparing it with MRS and the modified Look-Locker inversion-recovery (MOLLI) method. Finally, the clinical feasibility was investigated by comparing wT1 maps with clinical scans in nine patients.

RESULTS

The phantom results are in good agreement with MRS. The proposed method encodes the IR-curve for the liver wT1 estimation, is minimally sensitive to B 1 + inhomogeneities and acquires one slice in 1.2 s. The volunteer results confirmed the multi-slice capability of the proposed method, acquiring nine slices in a breath hold of 11 s. The present work shows robustness to B 1 + inhomogeneities (wT 1 , No B 1 + = 1.07 wT 1 , B 1 + - 45.63 , R 2 = 0.99 ) , good repeatability (wT 1 , 2 ° = 1 . 0 wT 1 , 1 ° - 2.14 , R 2 = 0.96 ) and is in better agreement with MRS (wT 1 = 0.92 wT 1 MRS + 103.28 , R 2 = 0.38 ) than is MOLLI (wT 1 MOLLI = 0.76 wT 1 MRS + 254.43 , R 2 = 0.44 ) . The wT1 maps in patients captured diverse lesions, thus showing their clinical feasibility.

CONCLUSION

A single-shot spiral acquisition can be combined with a continuous IR Look-Locker method to perform rapid repeatable multi-slice liver water T1 mapping at a rate of 1.2 s per slice without a B 1 + map. The proposed method is suitable for nine-slice liver clinical applications acquired in a single breath hold of 11 s.

A relaxometry method that emphasizes practicality and availability

PURPOSE

Although many methods have been proposed to quantitatively map the main MRI parameters (e.g., T₁ , T₂ , C × M₀ ), these methods often involve special sequences not readily available on clinical scanners and/or may require long scan times. In contrast, the proposed method can readily run on most scanners, offer flexible tradeoffs between scan time and image quality, and map MRI parameters jointly to ensure spatial alignment.

METHODS

The approach is based on the multi-shot spin-echo (SE) EPI sequence. The corresponding signal equation was derived and strategies for solving it were developed. As usual with multi-shot EPI, scan time can readily be traded-off against image quality by adjusting the echo train length. Validation was performed against reference relaxometry methods, in gel phantoms with varying concentrations of gadobutrol and gadoterate meglumine contrast agents. In vivo examples are further presented, from 3 neuroradiology patients.

RESULTS

Bland-Altman analysis was performed: for T₂ , as compared to 2D SE, bias was 0.29 ms and the 95% limits of agreement ranged from -1.15 to +1.73 ms. For T₁ , compared to inversion-recovery SE (and MOLLI), bias was -20.2 ms (and -14.5 ms) and the limits of agreement ranged from -62.4 to +22.0 ms (and -53.8 to +24.9 ms). The mean relative T₁ error between the proposed method and each of the 2 reference methods was similar to that of the reference methods among themselves.

CONCLUSION

In the constellation of existing relaxometry methods, the proposed method is meant to stand out in terms of its practicality and availability.

Assessment of Liver Function With MRI: Where Do We Stand?

Liver disease and hepatocellular carcinoma (HCC) have become a global health burden. For this reason, the determination of liver function plays a central role in the monitoring of patients with chronic liver disease or HCC. Furthermore, assessment of liver function is important, e.g., before surgery to prevent liver failure after hepatectomy or to monitor the course of treatment. Liver function and disease severity are usually assessed clinically based on clinical symptoms, biopsy, and blood parameters. These are rather static tests that reflect the current state of the liver without considering changes in liver function. With the development of liver-specific contrast agents for MRI, noninvasive dynamic determination of liver function based on signal intensity or using T1 relaxometry has become possible. The advantage of this imaging modality is that it provides additional information about the vascular structure, anatomy, and heterogeneous distribution of liver function. In this review, we summarized and discussed the results published in recent years on this technique. Indeed, recent data show that the T1 reduction rate seems to be the most appropriate value for determining liver function by MRI. Furthermore, attention has been paid to the development of automated tools for image analysis in order to uncover the steps necessary to obtain a complete process flow from image segmentation to image registration to image analysis. In conclusion, the published data show that liver function values obtained from contrast-enhanced MRI images correlate significantly with the global liver function parameters, making it possible to obtain both functional and anatomic information with a single modality.

Fast multi-parametric imaging in abdomen by B 1 + corrected dual-flip angle sequence with interleaved echo acquisition

PURPOSE

To achieve simultaneous T_{1, w} /proton density fat fraction (PDFF)/ R 2 ∗ mapping in abdomen within a single breadth-hold, and validate the accuracy using state-of-art measurement.

THEORY AND METHODS

An optimized multiple echo gradient echo (GRE) sequence with dual flip-angle acquisition was used to realize simultaneous water T₁ (T_{1, w} )/PDFF/ R 2 ∗ quantification. A new method, referred to as "solving the fat-water ambiguity based on their T₁ difference" (SORT), was proposed to address the fat-water separation problem. This method was based on the finding that compared to the true solution, the wrong (or aliased) solution to fat-water separation problem showed extra dependency on the applied flip angle due to the T₁ difference between fat and water. The B 1 + measurement sequence was applied to correct the B 1 + inhomogeneity for T_{1, w} relaxometry. The 2D parallel imaging was incorporated to enable the acquisition within a single breath-hold in abdomen.

RESULTS

The multi-parametric quantification results of the proposed method were consistent with the results of reference methods in phantom experiments (PDFF quantification: R²  = 0.993, mean error 0.73%; T_{1, w} quantification: R²  = 0.999, mean error 4.3%; R 2 ∗ quantification: R²  = 0.949, mean error 4.07 s^-1 ). For volunteer studies, robust fat-water separation was achieved without evident fat-water swaps. Based on the accurate fat-water separation, simultaneous T_{1, w} /PDFF/ R 2 ∗ quantification was realized for whole liver within a single breath-hold.

CONCLUSION

The proposed method accurately quantified T_{1, w} /PDFF/ R 2 ∗ for the whole liver within a single breath-hold. This technique serves as a quantitative tool for disease management in patients with hepatic steatosis.

Dual flip-angle IR-FLASH with spin history mapping for B1+ corrected T1 mapping: Application to T1 cardiovascular magnetic resonance multitasking

PURPOSE

To develop a single-scan method for B 1 + -corrected T1 mapping and apply it for free-breathing (FB) cardiac MR multitasking without electrocardiogram (ECG) triggering.

METHODS

One dual flip-angle (2FA) inversion recovery (IR)-FLASH scan provides two observations of T 1 ∗ (apparent T1 ) corresponding to two distinct combinations of the nominal FA α and B 1 + . Spatiotemporally coregistered T1 and B 1 + spin history maps are obtained by fitting the 2FA signal model. T1 estimate accuracy and repeatability for single flip-angle (1FA) and 2FA IR-FLASH sequence MR multitasking were evaluated at 3T. A T1 phantom was first imaged on the scanner table, then on two human subjects' thoraxes in both breath-hold (BH) and FB conditions. IR-turbo spin echo (IR-TSE) static phantom T1 measurements served as reference. In 10 healthy subjects, myocardial T1 was evaluated with ECG-free, FB multitasking sequences alongside ECG-triggered BH MOLLI.

RESULTS

For phantom-on-table T1 estimates, 2FA agreed better with IR-TSE (intraclass correlation coefficient [ICC] = 0.996, mean error ± SD = -1.6% ± 1.9%) than did 1FA (ICC = 0.922; mean error ± SD = -4.3% ± 12%). For phantom-on-thorax, 2FA was more repeatable and robust to respiration than 1FA (coefficient of variation [CoV] = 1.2% 2FA, = 11.3% 1FA). In vivo, in intrasession T1 repeatability, 2FA (septal CoV = 2.4%, six-segment CoV = 4.4%) outperformed 1FA (septal CoV = 3.1%, six-segment CoV = 5.5%). In six-segment T1 homogeneity, 2FA (CoV = 7.9%) also outperformed 1FA (CoV = 11.1%).

CONCLUSION

The 2FA IR-FLASH improves T1 estimate accuracy and repeatability over 1FA IR-FLASH, and enables single-scan B 1 + -corrected T1 mapping without BHs or ECG when used with MR multitasking.

Quantitative Magnetic Resonance Imaging in Perianal Crohn's Disease at 1.5 and 3.0 T: A Feasibility Study

Perianal Crohn's Disease (pCD) is a common manifestation of Crohn's Disease. Absence of reliable disease measures makes disease monitoring unreliable. Qualitative MRI has been increasingly used for diagnosing and monitoring pCD and has shown potential for assessing response to treatment. Quantitative MRI sequences, such as diffusion-weighted imaging (DWI), dynamic contrast enhancement (DCE) and magnetisation transfer (MT), along with T2 relaxometry, offer opportunities to improve diagnostic capability. Quantitative MRI sequences (DWI, DCE, MT and T2) were used in a cohort of 25 pCD patients before and 12 weeks after biological therapy at two different field strengths (1.5 and 3 T). Disease activity was measured with the Perianal Crohn's Disease Activity index (PDAI) and serum C-reactive protein (CRP). Diseased tissue areas on MRI were defined by a radiologist. A baseline model to predict outcome at 12 weeks was developed. No differences were seen in the quantitative MR measured in the diseased tissue regions from baseline to 12 weeks; however, PDAI and CRP decreased. Baseline PDAI, CRP, T2 relaxometry and surgical history were found to have a moderate ability to predict response after 12 weeks of biological treatment. Validation in larger cohorts with MRI and clinical measures are needed in order to further develop the model.

In vivo quantitative imaging biomarkers of bone quality and mineral density using multi-band-SWIFT magnetic resonance imaging

Bone is a composite biomaterial of mineral crystals, organic matrix, and water. Each contributes to bone quality and strength and may change independently, or together, with disease progression and treatment. Even so, there is a near ubiquitous reliance on ionizing x-ray-based approaches to measure bone mineral density (BMD) which is unable to fully characterize bone strength and may not adequately predict fracture risk. Characterization of treatment efficacy in bone diseases of altered remodeling is complicated by the lack of imaging modality able to safely monitor material-level and biochemical changes in vivo. To improve upon the current state of bone imaging, we tested the efficacy of Multi Band SWeep Imaging with Fourier Transformation (MB-SWIFT) magnetic resonance imaging (MRI) as a readout of bone derangement in an estrogen deficient ovariectomized (OVX) rat model during growth. MB-SWIFT MRI-derived BMD correlated significantly with BMD measured using micro-computed tomography (μCT). In this rodent model, growth appeared to overcome estrogen deficiency as bone mass continued to increase longitudinally over the duration of the study. Nonetheless, after 10 weeks of intervention, MB-SWIFT detected significant changes consistent with estrogen deficiency in cortical water, cortical matrix organization (T₁), and marrow fat. Findings point to MB-SWIFT's ability to quantify BMD in good agreement with μCT while providing additive quantitative outcomes about bone quality in a manner consistent with estrogen deficiency. These results indicate MB-SWIFT as a non-ionizing imaging strategy with value for bone imaging and may be a promising technique to progress to the clinic for monitoring and clinical management of patients with bone diseases such as osteoporosis.

Prostate magnetic resonance imaging technique

Multiparametric magnetic resonance (MR) imaging of the prostate is an excellent tool to detect clinically significant prostate cancer, and it has widely been incorporated into clinical practice due to its excellent tissue contrast and image resolution. The aims of this article are to describe the prostate MR imaging technique for detection of clinically significant prostate cancer according to PI-RADS v2.1, as well as alternative sequences and basic aspects of patient preparation and MR imaging artifact avoidance.

Multi-pathway multi-echo acquisition and neural contrast translation to generate a variety of quantitative and qualitative image contrasts

PURPOSE

Clinical exams typically involve acquiring many different image contrasts to help discriminate healthy from diseased states. Ideally, 3D quantitative maps of all of the main MR parameters would be obtained for improved tissue characterization. Using data from a 7-min whole-brain multi-pathway multi-echo (MPME) scan, we aimed to synthesize several 3D quantitative maps (T₁ and T₂ ) and qualitative contrasts (MPRAGE, FLAIR, T₁ -weighted, T₂ -weighted, and proton density [PD]-weighted). The ability of MPME acquisitions to capture large amounts of information in a relatively short amount of time suggests it may help reduce the duration of neuro MR exams.

METHODS

Eight healthy volunteers were imaged at 3.0T using a 3D isotropic (1.2 mm) MPME sequence. Spin-echo, MPRAGE, and FLAIR scans were performed for training and validation. MPME signals were interpreted through neural networks for predictions of different quantitative and qualitative contrasts. Predictions were compared to reference values at voxel and region-of-interest levels.

RESULTS

Mean absolute errors (MAEs) for T₁ and T₂ maps were 216 ms and 11 ms, respectively. In ROIs containing white matter (WM) and thalamus tissues, the mean T₁ /T₂ predicted values were 899/62 ms and 1139/58 ms, consistent with reference values of 850/66 ms and 1126/58 ms, respectively. For qualitative contrasts, signals were normalized to those of WM, and MAEs for MPRAGE, FLAIR, T₁ -weighted, T₂ -weighted, and PD-weighted contrasts were 0.14, 0.15, 0.13, 0.16, and 0.05, respectively.

CONCLUSIONS

Using an MPME sequence and neural-network contrast translation, whole-brain results were obtained with a variety of quantitative and qualitative contrast in ~6.8 min.

Evaluation of variable flip angle, MOLLI, SASHA, and IR-SNAPSHOT pulse sequences for T1 relaxometry and extracellular volume imaging of the pancreas and liver

PURPOSE

Compare fourT₁-mapping pulse sequences for T₁ relaxometry and extracellular volume (ECV) fraction of the pancreas and liver MATERIALS AND METHODS: In vitro phase of this prospective study was performed on a T₁ phantom, followed by imaging 22 patients. Variable flip angle (VFA), modified Look-Locker inversion recovery (MOLLI), prototype saturation recovery single-shot acquisition (SASHA), and prototype inversion recovery (IR-SNAPSHOT) pulse sequences were used to obtain T₁ and ECV maps on the same 1.5 T MR scanner using the same imaging protocol.

RESULTS

In vitro tests showed almost perfect precision of MOLLI (ρ_c = 0.9998), SASHA (ρ_c = 0.9985), and IR-SNAPSHOT (ρ_c = 0.9976), while VFA showed relatively less, however, substantial precision (ρ_c = 0.9862). Results of patient scans showed similar ECV fraction of the liver (p = 0.08), pancreas (p = 0.43), and T₁ of the liver (p = 0.08) with all pulse sequences. T₁ of the pancreas with MOLLI, SASHA, and IR-SNAPSHOT was statistically similar (p > 0.05).

CONCLUSION

MOLLI, SASHA, and IR-SNAPSHOT provided almost perfect in vitro precision and similar T₁ during in vivo scans. Similar ECV fractions of the liver and pancreas were obtained with all sequences. More refinement of pulse sequences to provide sufficient spatial coverage in one breath hold together with high precision would be desirable in abdominal imaging.

Radiofrequency transmit calibration: A multi-center evaluation of vendor-provided radiofrequency transmit mapping methods

PURPOSE

To determine the accuracy and test-retest repeatability of fast radiofrequency (RF) transmit measurement approaches used in Dynamic Contrast Enhanced Magnetic Resonance Imaging (DCE-MRI). Spatial variation in the transmitted RF field introduces bias and increased variance in quantitative DCE-MRI metrics including tracer kinetic parameter maps. If unaccounted for, these errors can dominate all other sources of bias and variance. The amount and pattern of variation depend on scanner-specific hardware and software.

METHODS

Human tissue mimicking torso and brain phantoms were constructed. RF transmit maps were measured and compared across eight different commercial scanners, from three major vendors, and three clinical sites. Vendor-recommended rapid methods for RF mapping were compared to a slower reference method. Imaging was repeated at all sites after 2 months. Ranges and magnitude of RF inhomogeneity were compared scanner-wise at two time points. Limits of Agreement of vendor-recommended methods and double-angle reference method were assessed.

RESULTS

At 3 T, B₁ ⁺ inhomogeneity spans across 35% in the head and 120% in the torso. Fast vendor provided methods are within 30% agreement with the reference double angle method for both the head and the torso phantom.

CONCLUSIONS

If unaccounted for, B₁ ⁺ inhomogeneity can severely impact tracer-kinetic parameter estimation. Depending on the scanner, fast vendor provided B₁ ⁺ mapping sequences allow unbiased and reproducible measurements of B₁ ⁺ inhomogeneity to correct for this source of bias.

Multipathway multi-echo (MPME) imaging: all main MR parameters mapped based on a single 3D scan

PURPOSE

Quantitative parameter maps, as opposed to qualitative grayscale images, may represent the future of diagnostic MRI. A new quantitative MRI method is introduced here that requires a single 3D acquisition, allowing good spatial coverage to be achieved in relatively short scan times.

METHODS

A multipathway multi-echo sequence was developed, and at least 3 pathways with 2 TEs were needed to generate T₁ , T₂ , T₂ ^* , B₁ ⁺ , and B₀ maps. The method required the central k-space region to be sampled twice, with the same sequence but with 2 very different nominal flip angle settings. Consequently, scan time was only slightly longer than that of a single scan. The multipathway multi-echo data were reconstructed into parameter maps, for phantom as well as brain acquisitions, in 5 healthy volunteers at 3 T. Spatial resolution, matrix size, and FOV were 1.2 × 1.0 × 1.2 mm³ , 160 × 192 × 160, and 19.2 × 19.2 × 19.2 cm³ (whole brain), acquired in 11.5 minutes with minimal acceleration. Validation was performed against T₁ , T₂ , and T₂ ^* maps calculated from gradient-echo and spin-echo data.

RESULTS

In Bland-Altman plots, bias and limits of agreement for T₁ and T₂ results in vivo and in phantom were -2.9/±125.5 ms (T₁ in vivo), -4.8/±20.8 ms (T₂ in vivo), -1.5/±18.1 ms (T₁ in phantom), and -5.3/±7.4 ms (T₂ in phantom), for regions of interest including given brain structures or phantom compartments. Due to relatively high noise levels, the current implementation of the approach may prove more useful for region of interest-based as opposed to pixel-based interpretation.

CONCLUSIONS

We proposed a novel approach to quantitatively map MR parameters based on a multipathway multi-echo acquisition.

Fast Interleaved Multislice T1 Mapping: Model-Based Reconstruction of Single-Shot Inversion-Recovery Radial FLASH

Purpose

To develop a high-speed multislice T1 mapping method based on a single-shot inversion-recovery (IR) radial FLASH acquisition and a regularized model-based reconstruction.

Methods

Multislice radial k-space data are continuously acquired after a single nonselective inversion pulse using a golden-angle sampling scheme in a spoke-interleaved manner with optimized flip angles. Parameter maps and coil sensitivities of each slice are estimated directly from highly undersampled radial k-space data using a model-based nonlinear inverse reconstruction in conjunction with joint sparsity constraints. The performance of the method has been validated using a numerical and experimental T1 phantom as well as demonstrated for studies of the human brain and liver at 3T.

Results

The proposed method allows for 7 simultaneous T1 maps of the brain at 0.5 × 0.5 × 4 mm³ resolution within a single IR experiment of 4 s duration. Phantom studies confirm similar accuracy and precision as obtained for a single-slice acquisition. For abdominal applications, the proposed method yields three simultaneous T1 maps at 1.25 × 1.25 × 6 mm³ resolution within a 4 s breath hold.

Conclusion

Rapid, robust, accurate, and precise multislice T1 mapping may be achieved by combining the advantages of a model-based nonlinear inverse reconstruction, radial sampling, parallel imaging, and compressed sensing.

Prostate DCE-MRI with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction using an approximated analytical approach

PURPOSE

To develop and evaluate a practical <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction method for prostate dynamic contrast-enhanced (DCE) MRI analysis.

THEORY

We proposed a simple analytical <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction method using a Taylor series approximation to the steady-state spoiled gradient echo signal equation. This approach only requires <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> maps and uncorrected pharmacokinetic (PK) parameters as input to estimate the corrected PK parameters.

METHODS

The proposed method was evaluated using a prostate digital reference object (DRO), and 82 in vivo prostate DCE-MRI cases. The approximated analytical correction was compared with the ground truth PK parameters in simulation, and compared with the reference numerical correction in in vivo experiments, using percentage error as the metric.

RESULTS

The prostate DRO results showed that our approximated analytical approach provided residual error less than 0.4% for both K^trans and v_e , compared to the ground truth. This noise-free residual error was smaller than the noise-induced error using the reference numerical correction, which had a minimum error of 2.1+4.3% with baseline signal-to-noise ratio of 234.5. For the 82 in vivo cases, K^trans and v_e percentage error compared to the reference numerical correction method had a mean of 0.1% (95% central range of [0.0%, 0.2%]) across the prostate volume.

CONCLUSION

The approximated analytical <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction method provides comparable results with less than 0.2% error within 95% central range, compared to reference numerical <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction. The proposed method is a practical solution for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:msubsup><mml:mi>B</mml:mi> <mml:mn>1</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:math> correction in prostate DCE-MRI because of its simple implementation.

Influence of k-space trajectory corrections on proton density mapping with ultrashort echo time imaging: Application for imaging of short T2 components in white matter

PURPOSE

To evaluate the impact of MR gradient system imperfections and limitations for the quantitative mapping of short T2* signals performed by ultrashort echo time (UTE) acquisition approach.

MATERIALS AND METHODS

The measurement of short T2* signals from a phantom and a healthy volunteer study (8 subjects of average age 28 ± 4 years) were performed on a 3T scanner. The characteristics of the gradient system were obtained using calibration method performed directly on the measured subject or phantom. This information was used to calculate the actual sampling trajectory with the help of a parametric eddy current model. The actual sample positions were used to reconstruct corrected images and compared with uncorrected data.

RESULTS

Comparison of both approaches, i.e., without and with correction of k-space sampling trajectories revealed substantial improvement when correction was applied. The phantom experiments demonstrate substantial in-plane signal intensity variations for uncorrected sampling trajectories. In the case of the volunteer study, this led to significant differences in relative proton density (RPD) estimation between the uncorrected and corrected data (P = 0.0117 by Wilcoxon matched-pairs test) and provides for about ~15% higher values for short T2* components of white matter (WM) in the case of uncorrected images.

CONCLUSION

The imperfection of the applied gradients could induce errors in k-space data sampling which further propagates into the fidelity of the UTE images and jeopardizes precision of quantification. However, the study proved that measurement of gradient errors together with correction of sample positions can contribute to increased accuracy and unbiased characterization of short T2* signals.

Multiparameter estimation using multi-echo spoiled gradient echo with variable flip angles and multicontrast compressed sensing

PURPOSE

To develop multiparameter mapping including T₁ , R2*, and proton density fat fraction with a single breath-hold to evaluate liver disease and liver function.

METHODS

A 6-echo spoiled gradient-echo sequence with dual flip angles was used to acquire a 12-set MRI volume data set. To shorten the scan time, undersampling and multicontrast compressed-sensing reconstruction were used. The scan time was 18 seconds. R2* and proton density fat fraction mapping were achieved by using the iterative least-squares method. T₁ mapping was estimated using driven equilibrium single-pulse observation of T₁ . Quantitative values were validated by performing phantom and volunteer studies.

RESULTS

Statistical analysis showed that the quantitative values measured using the proposed methods agreed with those measured using conventional methods. T₁ values of water proton measured by the proposed method in phantom and volunteer studies were in good agreement with those by MRS.

CONCLUSION

The results showed that accurate quantitative mapping of T₁ , R2*, and proton density fat fraction with a single breath-hold was achieved using our approach.

Technical Note: The effect of 2D excitation profile on T1 measurement accuracy using the variable flip angle method with an average flip angle assumption

PURPOSE

To study the accuracy and precision of T₁ estimates using the Variable Flip Angle (VFA) method in 2D and 3D acquisitions.

METHODS

Excitation profiles were simulated using numerical implementation of the Bloch equations for Hamming-windowed sinc excitation pulses with different time-bandwidth products (TBP) of 2, 6, and 10 and for T₁ values of 295 ms and 1045 ms. Experimental data were collected in 5° increments from 5° to 90° for the same T₁ and TBP values. T₁ was calculated for every combination of flip angle with and without a correction for B₁ and slice profile variation. Calculations were also made for flat slice profile such as obtained in 3D acquisition. Monte Carlo simulations were performed to obtain T₁ measurement uncertainty.

RESULTS

VFA T₁ measurements in 2D without correction can result in a 40-80% underestimation of true T₁ . Flip angle correction can reduce the underestimation, but results in accurate measurements of T₁ only within a narrow band of flip angle combinations. The narrow band of accuracy increases with TBP, but remains too narrow for any practical range of T₁ values or B₁ variation. Simulated noisy VFA T₁ measurements in 3D were accurate as long as the two angles chosen are on either side of the Ernst angle.

CONCLUSIONS

Accurate T1 estimates from VFA 2D acquisitions are possible, but only a narrow range of T1 values within a narrow range of flip angle combinations can be accurately calculated using a 2D slice. Unless a better flip angle correction method is used, these results demonstrate that accurate measurements of T1 in 2D cannot be obtained robustly enough for practical use and are more likely obtained by a thin slab 3D VFA acquisition than from multiple-slice 2D acquisitions. VFA T₁ measurements in 3D are accurate for wide ranges of flip angle combinations and T₁ values.

Improved hepatic arterial fraction estimation using cardiac output correction of arterial input functions for liver DCE MRI

Liver dynamic contrast enhanced (DCE) MRI pharmacokinetic modelling could be useful in the assessment of diffuse liver disease and focal liver lesions, but is compromised by errors in arterial input function (AIF) sampling. In this study, we apply cardiac output correction to arterial input functions (AIFs) for liver DCE MRI and investigate the effect on dual-input single compartment hepatic perfusion parameter estimation and reproducibility.

Thirteen healthy volunteers (28.7  ±  1.94 years, seven males) underwent liver DCE MRI and cardiac output measurement using aortic root phase contrast MRI (PCMRI), with reproducibility (n  =  9) measured at 7 d. Cardiac output AIF correction was undertaken by constraining the first pass AIF enhancement curve using the indicator-dilution principle. Hepatic perfusion parameters with and without cardiac output AIF correction were compared and 7 d reproducibility assessed.

Differences between cardiac output corrected and uncorrected liver DCE MRI portal venous (PV) perfusion (p  =  0.066), total liver blood flow (TLBF) (p  =  0.101), hepatic arterial (HA) fraction (p  =  0.895), mean transit time (MTT) (p  =  0.646), distribution volume (DV) (p  =  0.890) were not significantly different. Seven day corrected HA fraction reproducibility was improved (mean difference 0.3%, Bland–Altman 95% limits-of-agreement (BA95%LoA)  ±27.9%, coefficient of variation (CoV) 61.4% versus 9.3%, ±35.5%, 81.7% respectively without correction). Seven day uncorrected PV perfusion was also improved (mean difference 9.3 ml min⁻¹/100 g, BA95%LoA  ±506.1 ml min⁻¹/100 g, CoV 64.1% versus 0.9 ml min⁻¹/100 g, ±562.8 ml min⁻¹/100 g, 65.1% respectively with correction) as was uncorrected TLBF (mean difference 43.8 ml min⁻¹/100 g, BA95%LoA  ±586.7 ml min⁻¹/ 100 g, CoV 58.3% versus 13.3 ml min⁻¹/100 g, ±661.5 ml min⁻¹/100 g, 60.9% respectively with correction). Reproducibility of uncorrected MTT was similar (uncorrected mean difference 2.4 s, BA95%LoA  ±26.7 s, CoV 60.8% uncorrected versus 3.7 s, ±27.8 s, 62.0% respectively with correction), as was and DV (uncorrected mean difference 14.1%, BA95%LoA  ±48.2%, CoV 24.7% versus 10.3%, ±46.0%, 23.9% respectively with correction).

Cardiac output AIF correction does not significantly affect the estimation of hepatic perfusion parameters but demonstrates improvements in normal volunteer 7 d HA fraction reproducibility, but deterioration in PV perfusion and TLBF reproducibility. Improved HA fraction reproducibility maybe important as arterialisation of liver perfusion is increased in chronic liver disease and within malignant liver lesions.

Quantitative Liver Function Analysis: Volumetric T1 Mapping with Fast Multisection B1 Inhomogeneity Correction in Hepatocyte-specific Contrast-enhanced Liver MR Imaging

Purpose To determine whether B₁ inhomogeneity-corrected volumetric T1 maps of gadoxetic acid-enhanced liver magnetic resonance (MR) imaging are able to demonstrate global liver function and functional heterogeneity in patients with cirrhosis and to investigate their relationship with the development of hepatic insufficiency and decompensation. Materials and Methods This institutional review board-approved retrospective study with waiver of informed consent included 234 consecutive patients who underwent gadoxetic acid-enhanced liver MR imaging, including B₁ inhomogeneity-corrected volumetric T1 mapping. For all patients, T1 relaxation times of the liver and liver volumes were measured on T1 maps. Liver T1 and functional liver volume-to-weight ratio (liver volume divided by liver T1 and the patient's weight) were compared between Child-Pugh class A and class B cirrhosis. Associations between serum markers, MR parameters, hepatic insufficiency, and decompensation were investigated by using Cox proportional hazards analysis. Results Patients with Child-Pugh class B disease showed significantly longer liver T1 (548.2 msec ± 257.7 vs 372.2 msec ± 77.5, P < .0001) and lower kurtosis of liver T1 (29.1 ± 39.6 vs 43.9 ± 64.9, P = .016) than patients with Child-Pugh class A disease. Prolonged liver T1 (≥462 msec) (hazard ratio [HR], 5.9; 95% confidence interval [CI]: 1.1, 62.8) and an albumin level of less than 3.5 g/dL (HR, 20.7; 95% CI: 3.9, 221.9) were independently associated with the development of hepatic insufficiency. Functional liver volume-to-weight ratio was associated with the development of hepatic decompensation in patients with Child-Pugh class A disease (HR, 0.03; 95% CI: 0.004, 0.23). Conclusion B₁ inhomogeneity-corrected volumetric T1 mapping provided information on global liver function and demonstrated functional heterogeneity. In addition, prolonged liver T1 (≥462 msec) was associated with the development of hepatic insufficiency, and functional liver volume-to-weight ratio was negatively related with the development of decompensation in compensated cirrhosis. ^© RSNA, 2016 Online supplemental material is available for this article.

Emulsion Stability Modulates Gastric Secretion and Its Mixing with Emulsified Fat in Healthy Adults in a Randomized Magnetic Resonance Imaging Study

BACKGROUND

Oil-in-water emulsions have recently become of interest to nutritional sciences because of their ability to influence gastrointestinal digestive processes and ultimately benefit human health. MRI offers the potential to noninvasively characterize the interaction between emulsified lipids and gastric secretion within the stomach.

OBJECTIVES

We determined noninvasively how emulsion stability modulates volumes of fat and secretion, layering of fat, and the mixing of emulsified fat with secretion within the stomach. This required the development of MRI technology for quantifying fat and secretion concentrations inside the stomach.

METHODS

Twenty-one healthy adults [13 men, mean ± SD age: 22.5 ± 2.5 y, mean ± SD body mass index (in kg/m²): 22.7 ± 1.8] were analyzed in a single-blind, randomized, parallel design. MRI was used to acquire the distributions of fat and secretion in the stomach after ingestion of 2 emulsions: a stable emulsion (E1) or an unstable emulsion (E4) with 20% fat fraction and ∼0.3 mm droplet sizes. Layer, volume, and mixing variables were fitted to the data and compared between the 2 emulsions.

RESULTS

The intragastric mixing between fat and secretion was better with the E4 than the E1 [increase in content heterogeneity of 17.1% (95% CI: 12.3%, 21.9%)]. The E4 demonstrated a linear relation [slope 1.57 (95% CI: 0.86, 2.29)] between the degree of layering and mixing. In contrast, no such relation was detected for the E1. Accumulated secretion volume in the stomach was lower with the E4 [decrease in volume variable k_s of 2.3 (95% CI: -3.9, -0.7)] and correlated with the degree of layering (r = 0.62, P < 0.001).

CONCLUSIONS

In healthy adults, intragastric fat layering was influenced mainly by the degree of intragastric mixing, rather than the overall dominance of secretion. The E1 triggered a higher accumulation of gastric secretion, which in turn facilitated homogenization of intragastric content in comparison with its unstable counterpart. This trial was registered at clinicaltrials.gov as NCT02602158.

Assessment of Treatment Response With Diffusion-Weighted MRI and Dynamic Contrast-Enhanced MRI in Patients With Early-Stage Breast Cancer Treated With Single-Dose Preoperative Radiotherapy: Initial Results

Single-dose preoperative stereotactic body radiotherapy is a novel radiotherapy technique for the early-stage breast cancer, and the treatment response pattern of this technique needs to be investigated on a quantitative basis. In this work, dynamic contrast-enhanced magnetic resonance imaging and diffusion-weighted magnetic resonance imaging were used to study the treatment response pattern in a unique cohort of patients with early-stage breast cancer treated with preoperative radiation. Fifteen female qualified patients received single-dose preoperative radiotherapy with 1 of the 3 prescription doses: 15 Gy, 18 Gy, and 21 Gy. Magnetic resonance imaging scans including both diffusion-weighted magnetic resonance imaging and dynamic contrast-enhanced magnetic resonance imaging were acquired before radiotherapy for planning and after radiotherapy but before surgical resection. In diffusion-weighted magnetic resonance imaging, the regional averaged apparent diffusion coefficient was calculated. In dynamic contrast-enhanced magnetic resonance imaging, quantitative parameters K (trans) and v e were evaluated using the standard Tofts model based on the average contrast agent concentration within the region of interest, and the semiquantitative initial area under the concentration curve (iAUC6min) was also recorded. These parameters' relative changes after radiotherapy were calculated for gross tumor volume, clinical target volume, and planning target volume. The initial results showed that after radiotherapy, initial area under the concentration curve significantly increased in planning target volume (P < .006) and clinical target volume (P < .006), and v e significantly increased in planning target volume (P < .05) and clinical target volume (P < .05). Statistical studies suggested that linear correlations between treatment dose and the observed parameter changes exist in most examined tests, and among these tests, the change in gross tumor volume regional averaged apparent diffusion coefficient (P < .012) and between treatment dose and planning target volume K (trans) (P < .029) were found to be statistically significant. Although it is still preliminary, this pilot study may be useful to provide insights for future works.

Estimation of contrast agent bolus arrival delays for improved reproducibility of liver DCE MRI

Delays between contrast agent (CA) arrival at the site of vascular input function (VIF) sampling and the tissue of interest affect dynamic contrast enhanced (DCE) MRI pharmacokinetic modelling. We investigate effects of altering VIF CA bolus arrival delays on liver DCE MRI perfusion parameters, propose an alternative approach to estimating delays and evaluate reproducibility.

Thirteen healthy volunteers (28.7  ±  1.9 years, seven males) underwent liver DCE MRI using dual-input single compartment modelling, with reproducibility (n  =  9) measured at 7 days. Effects of VIF CA bolus arrival delays were assessed for arterial and portal venous input functions. Delays were pre-estimated using linear regression, with restricted free modelling around the pre-estimated delay. Perfusion parameters and 7 days reproducibility were compared using this method, freely modelled delays and no delays using one-way ANOVA. Reproducibility was assessed using Bland–Altman analysis of agreement.

Maximum percent change relative to parameters obtained using zero delays, were  −31% for portal venous (PV) perfusion, +43% for total liver blood flow (TLBF), +3247% for hepatic arterial (HA) fraction, +150% for mean transit time and  −10% for distribution volume. Differences were demonstrated between the 3 methods for PV perfusion (p  =  0.0085) and HA fraction (p  <  0.0001), but not other parameters. Improved mean differences and Bland–Altman 95% Limits-of-Agreement for reproducibility of PV perfusion (9.3 ml/min/100 g, ±506.1 ml/min/100 g) and TLBF (43.8 ml/min/100 g, ±586.7 ml/min/100 g) were demonstrated using pre-estimated delays with constrained free modelling.

CA bolus arrival delays cause profound differences in liver DCE MRI quantification. Pre-estimation of delays with constrained free modelling improved 7 days reproducibility of perfusion parameters in volunteers.

Role of Multiparametric MR Imaging in Malignancies of the Urogenital Tract

Multiparametric MR imaging (mpMRI) combine different sequences that, properly tailored, can provide qualitative and quantitative information about the tumor microenvironment beyond traditional tumor size measures and/or morphologic assessments. This article focuses on mpMRI in the evaluation of urogenital tract malignancies by first reviewing technical aspects and then discussing its potential clinical role. This includes insight into histologic subtyping and grading of renal cell carcinoma and assessment of tumor response to targeted therapies. The clinical utility of mpMRI in the staging and grading of ureteral and bladder tumors is presented. Finally, the evolving role of mpMRI in prostate cancer is discussed.

T1 mapping for diagnosis of mild chronic pancreatitis

PURPOSE

To determine if the T₁ relaxation time of the pancreas can detect parenchymal changes in mild chronic pancreatitis (CP).

MATERIALS AND METHODS

This Institutional Review Board (IRB)-approved, Health Insurance Portability and Accountability Act (HIPAA)-compliant retrospective study analyzed 98 patients with suspected mild CP. Patients were grouped as normal (n = 53) or mild CP (n = 45) based on history, presenting symptomatology, and concordant findings on both the secretin-enhanced magnetic resonance cholangiopancreatography (S-MRCP) and endoscopic retrograde cholangiopancreatography (ERCP). T₁ maps were obtained in all patients using the same 3D gradient echo technique on the same 3T scanner. T₁ relaxation times, fat signal fraction (FSF), and anterior-posterior (AP) diameter were correlated with the clinical diagnosis of CP.

RESULTS

There was a significant difference (P < 0.0001) in the T₁ relaxation times between the control (mean = 797 msec, 95% confidence interval [CI]: 730, 865) and mild CP group (mean = 1099 msec, 95% CI: 1032, 1166). A T₁ relaxation time threshold value of 900 msec was 80% sensitive (95% CI: 65, 90) and 69% specific (95% CI: 56, 82) for the diagnosis of mild CP (area under the curve [AUC]: 0.81). Multiple regression analysis showed that T₁ relaxation time was the only statistically significant variable correlating with the diagnosis of CP (P < 0.0001). T₁ relaxation times showed a weak positive correlation with the pancreatic FSF (ρ = 0.33, P = 0.01) in the control group, but not in the mild CP group.

CONCLUSION

The T₁ relaxation time of the pancreatic parenchyma was significantly increased in patients with mild CP. Therefore, T₁ mapping might be used as a practical quantitative imaging technique for the evaluation of suspected mild CP.

LEVEL OF EVIDENCE

3 J. Magn. Reson. Imaging 2017;45:1171-1176.

Optimization of Look-Locker Turbo-Field Echo-Planar Imaging and Evaluation of Its Accuracy in Head and Neck 3D T1 Mapping

Purpose:

We present a sequence for T₁ relaxation-time mapping that enables a rapid and accurate measuring. The sequence is based on the Look-Locker method by employing turbo-field echo-planar imaging (TFEPI) acquisitions and time to free relaxation after constant application of the radiofrequency (RF) pulses. We optimized the sequence, and then evaluated the accuracy of the method in imaging of head and neck.

Materials and Methods:

The method was implemented on a standard clinical scanner, and the accuracy of the T₁ value was evaluated against that with the two-dimensional (2D) inversion recovery method.

Results:

The percentage errors of the T₁ value, as validated by phantom imaging measurements, were 3.1% for slow-relaxing compartments (T₁ = 2736 msec) and 1.1% for fast-relaxing compartments (T₁ = 264.2 msec).

Conclusion:

We demonstrated a fast 3D sequence to obtain multiple slices, based on the Look-Locker method for T₁ measurement, which provided a rapid and accurate way of measuring the spin-lattice relaxation time. An acquisition time of approximately 5 min was achieved for T₁ mapping; in principle, this can provide head and neck coverage with 15 slices.

Dynamic contrast-enhanced MRI for oncology drug development

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) is a promising tool for evaluating tumor vascularity, as it can provide vasculature-derived, functional, and quantitative parameters. To implement DCE-MRI parameters as biomarkers for monitoring the effect of antiangiogenic or vascular-disrupting treatment, two crucial elements of surrogate endpoint, ie, validation and qualification, should be satisfied. Although early studies have shown the accuracy and reliability of DCE-MRI parameters for evaluating treatment-driven vascular alterations, there have been an increasing number of studies demonstrating the limitations of DCE-MRI parameters as surrogate endpoints. Therefore, in order to improve the application of DCE-MRI parameters in drug development, it is necessary to establish a standardized evaluation method and to determine the correct therapeutics-oriented meaning of individual DCE-MRI parameter. In this regard, this article describes the biophysical background and data acquisition/analysis techniques of DCE-MRI while focusing on the validation and qualification issues. Specifically, the causes of disagreement and confusion encountered in the preclinical and clinical trials using DCE-MRI are presented in detail. Finally, considering these limitations, we present potential strategies to optimize implementation of DCE-MRI. J. Magn. Reson. Imaging 2016;44:251-264.

MR Fingerprinting for Rapid Quantitative Abdominal Imaging

PURPOSE

To develop a magnetic resonance (MR) "fingerprinting" technique for quantitative abdominal imaging.

MATERIALS AND METHODS

This HIPAA-compliant study had institutional review board approval, and informed consent was obtained from all subjects. To achieve accurate quantification in the presence of marked B0 and B1 field inhomogeneities, the MR fingerprinting framework was extended by using a two-dimensional fast imaging with steady-state free precession, or FISP, acquisition and a Bloch-Siegert B1 mapping method. The accuracy of the proposed technique was validated by using agarose phantoms. Quantitative measurements were performed in eight asymptomatic subjects and in six patients with 20 focal liver lesions. A two-tailed Student t test was used to compare the T1 and T2 results in metastatic adenocarcinoma with those in surrounding liver parenchyma and healthy subjects.

RESULTS

Phantom experiments showed good agreement with standard methods in T1 and T2 after B1 correction. In vivo studies demonstrated that quantitative T1, T2, and B1 maps can be acquired within a breath hold of approximately 19 seconds. T1 and T2 measurements were compatible with those in the literature. Representative values included the following: liver, 745 msec ± 65 (standard deviation) and 31 msec ± 6; renal medulla, 1702 msec ± 205 and 60 msec ± 21; renal cortex, 1314 msec ± 77 and 47 msec ± 10; spleen, 1232 msec ± 92 and 60 msec ± 19; skeletal muscle, 1100 msec ± 59 and 44 msec ± 9; and fat, 253 msec ± 42 and 77 msec ± 16, respectively. T1 and T2 in metastatic adenocarcinoma were 1673 msec ± 331 and 43 msec ± 13, respectively, significantly different from surrounding liver parenchyma relaxation times of 840 msec ± 113 and 28 msec ± 3 (P < .0001 and P < .01) and those in hepatic parenchyma in healthy volunteers (745 msec ± 65 and 31 msec ± 6, P < .0001 and P = .021, respectively).

CONCLUSION

A rapid technique for quantitative abdominal imaging was developed that allows simultaneous quantification of multiple tissue properties within one 19-second breath hold, with measurements comparable to those in published literature.

Volume, distribution and acidity of gastric secretion on and off proton pump inhibitor treatment: a randomized double-blind controlled study in patients with gastro-esophageal reflux disease (GERD) and healthy subjects

Background

Postprandial accumulation of gastric secretions in the proximal stomach above the meal adjacent to the esophagogastric junction (EGJ), referred to as the ‘acid pocket’, has been proposed as a pathophysiological factor in gastro-esophageal reflux disease (GERD) and as a target for GERD treatment. This study assessed the effect of proton pump inhibitor (PPI) therapy on the volume, distribution and acidity of gastric secretions in GERD and healthy subjects (HS).

Methods

A randomized, double blind, cross-over study in 12 HS and 12 GERD patients pre-treated with 40 mg pantoprazole (PPI) or placebo b.i.d. was performed. Postprandial secretion volume (SV), formation of a secretion layer and contact between the layer and the EGJ were quantified by Magnetic Resonance Imaging (MRI). Multi-channel pH-monitoring assessed intragastric pH.

Results

A distinct layer of undiluted acid secretion was present on top of gastric contents in almost all participants on and off high-dose acid suppression. PPI reduced SV (193 ml to 100 ml, in HS, 227 ml to 94 ml in GERD; p < 0.01) and thickness of the acid layer (26 mm to 7 mm, 36 mm to 9 mm respectively, p < 0.01). No differences in secretion volume or layer thickness were observed between groups; however, off treatment, contact time between the secretion layer and EGJ was 2.6 times longer in GERD compared to HS (p = 0.012). This was not the case on PPI.

Conclusions

MRI can visualize and quantify the volume and distribution dynamics of gastric secretions that form a layer in the proximal stomach after ingestion of a liquid meal. The secretion volume and the secretion layer on top of gastric contents is similar in GERD patients and HS; however contact between the layer of undiluted secretion and the EGJ is prolonged in patients. High dose PPI reduced secretion volume by about 50 % and reduced contact time between secretion and EGJ towards normal levels.

Trial registration

NCT01212614.

Electronic supplementary material

The online version of this article (doi:10.1186/s12876-015-0343-x) contains supplementary material, which is available to authorized users.

Volume-assisted estimation of liver function based on Gd-EOB-DTPA-enhanced MR relaxometry

OBJECTIVES

To determine whether liver function as determined by indocyanine green (ICG) clearance can be estimated quantitatively from hepatic magnetic resonance (MR) relaxometry with gadoxetic acid (Gd-EOB-DTPA).

METHODS

One hundred and seven patients underwent an ICG clearance test and Gd-EOB-DTPA-enhanced MRI, including MR relaxometry at 3 Tesla. A transverse 3D VIBE sequence with an inline T1 calculation was acquired prior to and 20 minutes post-Gd-EOB-DTPA administration. The reduction rate of T1 relaxation time (rrT1) between pre- and post-contrast images and the liver volume-assisted index of T1 reduction rate (LVrrT1) were evaluated. The plasma disappearance rate of ICG (ICG-PDR) was correlated with the liver volume (LV), rrT1 and LVrrT1, providing an MRI-based estimated ICG-PDR value (ICG-PDRest).

RESULTS

Simple linear regression model showed a significant correlation of ICG-PDR with LV (r = 0.32; p = 0.001), T1post (r = 0.65; p < 0.001) and rrT1 (r = 0.86; p < 0.001). Assessment of LV and consecutive evaluation of multiple linear regression model revealed a stronger correlation of ICG-PDR with LVrrT1 (r = 0.92; p < 0.001), allowing for the calculation of ICG-PDRest.

CONCLUSIONS

Liver function as determined using ICG-PDR can be estimated quantitatively from Gd-EOB-DTPA-enhanced MR relaxometry. Volume-assisted MR relaxometry has a stronger correlation with liver function than does MR relaxometry.

KEY POINTS

• Measurement of T1 relaxation times in Gd-EOB-DTPA-enhanced MR imaging quantifies liver function. • Volume-assisted Gd-EOB-DTPA-enhanced MR relaxometry has stronger correlation with ICG-PDR than does Gd-EOB-DTPA-enhanced MR relaxometry. • Gd-EOB-DTPA-enhanced MR relaxometry may provide robust parameters for detecting and characterizing liver disease. • Gd-EOB-DTPA-enhanced MR relaxometry may be useful for monitoring liver disease progression. • Gd-EOB-DTPA-enhanced MR relaxometry has the potential to become a novel liver function index.

B1 and T1 mapping of the breast with a reference tissue method

PURPOSE

To develop a method for mapping the B1 field using a reference signal from a tissue with known T1.

METHODS

Flip angle correction factors were calculated in a region with a known "gold standard" T1; by comparing T1 values from a variable flip angle (VFA) sequence to the "gold standard" and correcting the value of the Ernst angle. The resulting partial B1 map was interpolated for all other regions. In the breast, fat is an ideal reference tissue because its T1 is spatially homogeneous and interpatient variability is low. This method was tested with scans of phantoms and patients (n = 4) on a 3T magnet. The performance of the method was evaluated by comparing the results of VFA T1 mapping with and without B1 correction to inversion recovery (IR) T1 maps.

RESULTS

Phantom data determined that a linear inverse distance weighted interpolation accurately recovered the full B1 map. Use of interpolated maps to correct the VFA data in vivo, reduced the average difference in the T1 of parenchyma between VFA and IR results from 58% to 8%.

CONCLUSION

This proof-of-principle study showed that it is possible to recover a full and accurate map of the B1 field in the breast by using a reference tissue (fat) with an accurately measured T1.

Improved T1, contrast concentration, and pharmacokinetic parameter quantification in the presence of fat with two-point Dixon for dynamic contrast-enhanced magnetic resonance imaging

PURPOSE

To evaluate the impact of fat and fat-suppression on the quantification of T1, gadolinium concentration, and pharmacokinetic parameters in DCE-MRI.

METHODS

T1 values were measured in fat-free phantoms using variable flip angle with no fat suppression, quick or interleaved fat saturation (QFS), or two-point Dixon and were compared with reference values measured with inversion recovery-prepared turbo spin echo. Relaxivity of gadolinium-benzyloxypropionictetraacetate (Gd-BOPTA) was measured in emulsions of Gd-BOPTA solution and fat using Dixon in-phase and water-only images. Liver T1 and pharmacokinetic parameters of 15 patients were calculated from Dixon in-phase and water-only images and were correlated with liver fat signal fraction.

RESULTS

T1 values measured using Dixon water-only and non-fat-suppressed images matched the reference values; while T1 values measured using QFS showed large deviations. Relaxivities and Gd measured in the Dixon water-only images were less affected by the fat than those measured in the in-phase images. The correlation between liver fat fraction and the differences in measured pharmacokinetic parameters using Dixon in-phase and water-only images were significant (P < 0.05) for T1, K(trans), and incremental area under the curve, but not Ve (P = 0.1).

CONCLUSION

Dixon water-only images provided more reliable estimation of T1, Gd, and pharmacokinetic parameters when fat was present.

Rapid volumetric T1 mapping of the abdomen using three-dimensional through-time spiral GRAPPA

PURPOSE

To develop an ultrafast T1 mapping method for high-resolution, volumetric T1 measurements in the abdomen.

METHODS

The Look-Locker method was combined with a stack-of-spirals acquisition accelerated using three-dimensional (3D) through-time spiral GRAPPA reconstruction for fast data acquisition. A segmented k-space acquisition scheme was proposed and the time delay between segments for the recovery of longitudinal magnetization was optimized using Bloch equation simulations. The accuracy of this method was validated in a phantom experiment and in vivo T1 measurements were performed with 35 asymptomatic subjects on both 1.5 Tesla (T) and 3T MRI systems.

RESULTS

Phantom experiments yielded close agreement between the proposed method and gold standard measurements for a large range of T1 values (200 to 1600 ms). The in vivo results further demonstrate that high-resolution T1 maps (2 × 2 × 4 mm(3)) for 32 slices can be achieved in a single clinically feasible breath-hold of approximately 20 s. The T1 values for multiple organs and tissues in the abdomen are in agreement with the published literature.

CONCLUSION

A high-resolution 3D abdominal T1 mapping technique was developed, which allows fast and accurate T1 mapping of multiple abdominal organs and tissues in a single breath-hold.

Variable flip angle T1 mapping in the human brain with reduced T2 sensitivity using fast radiofrequency-spoiled gradient echo imaging

PURPOSE

Variable flip angle (VFA) T1 quantification using three-dimensional (3D) radiofrequency (RF) spoiled gradient echo imaging offers the acquisition of whole-brain T1 maps in clinically acceptable times. However, conventional VFA T1 relaxometry is biased by incomplete spoiling (i.e., residual T2 dependency). A new postprocessing approach is proposed to overcome this T2-related bias.

METHODS

T1 is quantified from the signal ratio of two spoiled gradient echo (SPGR) images acquired at different flip angles using an analytical solution for the RF-spoiled steady-state signal in combination with a global T2 guess. T1 accuracy is evaluated from simulations and in vivo 3D SPGR imaging of the human brain at 3 Tesla.

RESULTS

The simulations demonstrated that the sensitivity of VFA T1 mapping to T2 can considerably be reduced using a global T2 guess. The method proved to deliver reliable and accurate T1 values in vivo for white and gray matter in good agreement with inversion recovery reference measurements.

CONCLUSION

Based on a global T2 estimate, the accuracy of VFA T1 relaxometry in the human brain can substantially be improved compared with conventional approaches which rely on the generally wrong assumption of ideal spoiling.

Incorporating relaxivities to more accurately reconstruct MR images

PURPOSE

To develop a mathematical model that incorporates the magnetic resonance relaxivities into the image reconstruction process in a single step.

MATERIALS AND METHODS

In magnetic resonance imaging, the complex-valued measurements of the acquired signal at each point in frequency space are expressed as a Fourier transformation of the proton spin density weighted by Fourier encoding anomalies: T2(⁎), T1, and a phase determined by magnetic field inhomogeneity (∆B) according to the MR signal equation. Such anomalies alter the expected symmetry and the signal strength of the k-space observations, resulting in images distorted by image warping, blurring, and loss in image intensity. Although T1 on tissue relaxation time provides valuable quantitative information on tissue characteristics, the T1 recovery term is typically neglected by assuming a long repetition time. In this study, the linear framework presented in the work of Rowe et al., 2007, and of Nencka et al., 2009 is extended to develop a Fourier reconstruction operation in terms of a real-valued isomorphism that incorporates the effects of T2(⁎), ∆B, and T1. This framework provides a way to precisely quantify the statistical properties of the corrected image-space data by offering a linear relationship between the observed frequency space measurements and reconstructed corrected image-space measurements. The model is illustrated both on theoretical data generated by considering T2(⁎), T1, and/or ∆B effects, and on experimentally acquired fMRI data by focusing on the incorporation of T1. A comparison is also made between the activation statistics computed from the reconstructed data with and without the incorporation of T1 effects.

RESULT

Accounting for T1 effects in image reconstruction is shown to recover image contrast that exists prior to T1 equilibrium. The incorporation of T1 is also shown to induce negligible correlation in reconstructed images and preserve functional activations.

CONCLUSION

With the use of the proposed method, the effects of T2(⁎) and ∆B can be corrected, and T1 can be incorporated into the time series image-space data during image reconstruction in a single step. Incorporation of T1 provides improved tissue segmentation over the course of time series and therefore can improve the precision of motion correction and image registration.

Analyzing the blood-brain barrier: the benefits of medical imaging in research and clinical practice

A dysfunctional BBB is a common feature in a variety of brain disorders, a fact stressing the need for diagnostic tools designed to assess brain vessels' permeability in space and time. Biological research has benefited over the years various means to analyze BBB integrity. The use of biomarkers for improper BBB functionality is abundant. Systemic administration of BBB impermeable tracers can both visualize brain regions characterized by BBB impairment, as well as lead to its quantification. Additionally, locating molecular, physiological content in regions from which it is restricted under normal BBB functionality undoubtedly indicates brain pathology-related BBB disruption. However, in-depth research into the BBB's phenotype demands higher analytical complexity than functional vs. pathological BBB; criteria which biomarker based BBB permeability analyses do not meet. The involvement of accurate and engineering sciences in recent brain research, has led to improvements in the field, in the form of more accurate, sensitive imaging-based methods. Improvements in the spatiotemporal resolution of many imaging modalities and in image processing techniques, make up for the inadequacies of biomarker based analyses. In pre-clinical research, imaging approaches involving invasive procedures, enable microscopic evaluation of BBB integrity, and benefit high levels of sensitivity and accuracy. However, invasive techniques may alter normal physiological function, thus generating a modality-based impact on vessel's permeability, which needs to be corrected for. Non-invasive approaches do not affect proper functionality of the inspected system, but lack in spatiotemporal resolution. Nevertheless, the benefit of medical imaging, even in pre-clinical phases, outweighs its disadvantages. The innovations in pre-clinical imaging and the development of novel processing techniques, have led to their implementation in clinical use as well. Specialized analyses of vessels' permeability add valuable information to standard anatomical inspections which do not take the latter into consideration.

Blood-brain barrier imaging in human neuropathologies

The blood-brain barrier (BBB) is essential for normal function of the brain, and its role in many brain pathologies has been the focus of numerous studies during the last decades. Dysfunction of the BBB is not only being shown in numerous brain diseases, but animal studies have indicated that it plays a direct key role in the genesis of neurovascular dysfunction and associated neurodegeneration. As such evidence accumulates, the need for robust and clinically applicable methods for minimally invasive assessment of BBB integrity is becoming urgent. This review provides an introduction to BBB imaging methods in the clinical scenario. First, imaging modalities are reviewed, with a focus on dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). We then proceed to review image analysis methods, including quantitative and semi-quantitative methods. The advantages and limitations of each approach are discussed, and future directions and questions are highlighted.

Early detection of antiangiogenic treatment responses in a mouse xenograft tumor model using quantitative perfusion MRI

Angiogenesis plays a major role in tumor growth and metastasis, with tumor perfusion regarded as a marker for angiogenesis. To evaluate antiangiogenic treatment response in vivo, we investigated arterial spin labeling (ASL) magnetic resonance imaging (MRI) to measure tumor perfusion quantitatively. Chronic and 24-h acute treatment responses to bevacizumab were assessed by ASL and dynamic-contrast-enhanced (DCE) MRI in the A498 xenograft mouse model. After the MRI, tumor vasculature was assessed by CD34 staining. After 39 days of chronic treatment, tumor perfusion decreased to 44.8 ± 16.1 mL/100 g/min (P < 0.05), compared to 92.6 ± 42.9 mL/100 g/min in the control group. In the acute treatment study, tumor perfusion in the treated group decreased from 107.2 ± 32.7 to 73.7 ± 27.8 mL/100 g/min (P < 0.01; two-way analysis of variance), as well as compared with control group post dosing. A significant reduction in vessel density and vessel size was observed after the chronic treatment, while only vessel size was reduced 24 h after acute treatment. The tumor perfusion correlated with vessel size (r = 0.66; P < 0.005) after chronic, but not after acute treatment. The results from DCE-MRI also detected a significant change between treated and control groups in both chronic and acute treatment studies, but not between 0 and 24 h in the acute treatment group. These results indicate that tumor perfusion measured by MRI can detect early vascular responses to antiangiogenic treatment. With its noninvasive and quantitative nature, ASL MRI would be valuable for longitudinal assessment of tumor perfusion and in translation from animal models to human.