Understanding and optimizing the amplitude modulated control for multiple-slice continuous arterial spin labeling

Investigating the origin of pH-sensitive magnetization transfer ratio asymmetry MRI contrast during the acute stroke: Correction of T1 change reveals the dominant amide proton transfer MRI signal

PURPOSE

Amide proton transfer (APT) MRI is promising to serve as a surrogate metabolic imaging biomarker of acute stroke. Although the magnetization transfer ratio asymmetry (MTRasym ) has been used commonly, the origin of pH-weighted MRI effect remains an area of investigation, including contributions from APT, semisolid MT contrast asymmetry, and nuclear Overhauser enhancement effects. Our study aimed to determine the origin of pH-weighted MTRasym contrast following acute stroke.

METHODS

Multiparametric MRI, including T1 , T2 , diffusion and Z-spectrum, were performed in rats after middle cerebral artery occlusion. We analyzed the conventional Z-spectrum I Δ ω I 0 and the apparent exchange spectrum R ex Δ ω , being the difference between the relaxation-scaled inverse Z-spectrum and the intrinsic spinlock relaxation rate R 1 · cos 2 θ · I 0 I Δ ω - R 1 ρ Δ ω . The ischemia-induced change was calculated as the spectral difference between the diffusion lesion and the contralateral normal area.

RESULTS

The conventional Z-spectrum signal change at -3.5 ppm dominates that at +3.5 ppm (-1.16 ± 0.39% vs. 0.76 ± 0.26%, P < .01) following acute stroke. In comparison, the magnitude of ΔRex change at 3.5 ppm becomes significantly larger than that at -3.5 ppm (-2.80 ± 0.40% vs. -0.94 ± 0.80%, P < .001), with their SNR being 7.0 and 1.2, respectively. We extended the magnetization transfer and relaxation normalized APT concept to the apparent exchange-dependent relaxation image, documenting an enhanced pH contrast between the ischemic lesion and the intact tissue, over that of MTRasym .

CONCLUSION

Our study shows that after the relaxation-effect correction, the APT effect is the dominant contributing factor to pH-weighted MTRasym following acute stroke.

Development of fast multi-slice apparent T1 mapping for improved arterial spin labeling MRI measurement of cerebral blood flow

PURPOSE

To develop fast multi-slice apparent T₁ (T_1app ) mapping for accurate cerebral blood flow (CBF) quantification with arterial spin labeling (ASL) MRI.

METHODS

Fast multi-slice T_1app was measured using a modified inversion recovery echo planar imaging (EPI) sequence with simultaneous application of ASL tagging radiofrequency (RF) and gradient pulses. The fast multi-slice T_1app measurement was compared with the single-slice T_1app imaging approach, repeated per slice. CBF was assessed in healthy adult Wistar rats (N = 5) and rats with acute stroke 24 hours after a transient middle cerebral artery occlusion (N = 5).

RESULTS

The fast multi-slice T_1app measurement was in good agreement with that of a single-slice T_1app imaging approach (Lin's concordance correlation coefficient = 0.92). CBF calculated using T_1app reasonably accounted for the finite labeling RF duration, whereas the routine T₁ -normalized ASL MRI underestimated the CBF, particularly at short labeling durations. In acute stroke rats, the labeling time and the CBF difference (ΔCBF) between the contralateral normal area and the ischemic lesion were significantly correlated when using T₁ -normalized perfusion calculation (R = 0.844, P = .035). In comparison, T_1app -normalized ΔCBF had little labeling time dependence based on the linear regression equation of ΔCBF = -0.0247*τ + 1.579 mL/g/min (R = -0.352, P = .494).

CONCLUSIONS

Our study found fast multi-slice T_1app imaging improves the accuracy and reproducibility of CBF measurement.

Demonstration of magnetization transfer and relaxation normalized pH-specific pulse-amide proton transfer imaging in an animal model of acute stroke

PURPOSE

pH-weighted amide proton transfer (APT) MRI is promising to serve as a new surrogate metabolic imaging biomarker for refined ischemic tissue demarcation. APT MRI with pulse-RF irradiation (pulse-APT) is an alternative to the routine continuous wave (CW-) APT MRI that overcomes the RF duty cycle limit. Our study aimed to generalize the recently developed pH-specific magnetization transfer and relaxation-normalized APT (MRAPT) analysis to pulse-APT MRI in acute stroke imaging.

METHODS

Multiparametric MRI, including CW- and pulse-APT MRI scans, were performed following middle cerebral artery occlusion in rats. We calculated pH-sensitive MTR_asym and pH-specific MRAPT contrast between the ipsilateral diffusion lesion and contralateral normal area.

RESULTS

An inversion pulse of 10 to 15 ms maximizes the pH-sensitive MRI contrast for pulse-APT MRI. The contrast-to-noise ratio of pH-specific MRAPT effect between the contralateral normal area and ischemic lesion from both methods are comparable (3.25 ± 0.65 vs. 3.59 ± 0.40, P > .05). pH determined from both methods were in good agreement, with their difference within 0.1.

CONCLUSIONS

Pulse-APT MRI provides highly pH-specific mapping for acute stroke imaging.

Fast correction of B0 field inhomogeneity for pH-specific magnetization transfer and relaxation normalized amide proton transfer imaging of acute ischemic stroke without Z-spectrum

PURPOSE

The magnetization transfer and relaxation normalized amide proton transfer (MRAPT) analysis is promising to provide a highly pH-specific mapping of tissue acidosis, complementing commonly used CEST asymmetry analysis. We aimed to develop a fast B₀ inhomogeneity correction algorithm for acute stroke magnetization transfer and relaxation normalized amide proton transfer imaging without Z-spectral interpolation.

METHODS

The proposed fast field inhomogeneity correction describes B₀ artifacts with linear regression. We compared the new algorithm with the routine interpolation correction approach in CEST imaging of a dual-pH phantom. The fast B₀ correction was further evaluated in amide proton transfer imaging of normal and acute stroke rats.

RESULTS

Our phantom data showed that the proposed fast B₀ inhomogeneity correction significantly improved pH MRI contrast, recovering over 80% of the pH MRI contrast-to-noise-ratio difference between the raw magnetization transfer ratio asymmetry and that using the routine interpolation-based B₀ correction approach. In normal rat brains, the proposed fast B₀ correction improved pH-specific MRI uniformity across the intact tissue, with the ratio of magnetization transfer and relaxation normalized amide proton transfer ratio being 10% of that without B₀ inhomogeneity correction. In acute stroke rats, fast B₀ inhomogeneity-corrected pH MRI reveals substantially improved pH lesion conspicuity, particularly in regions with nonnegligible B₀ inhomogeneity. The pH MRI contrast-to-noise ratio between the ipsilateral diffusion lesion and contralateral normal tissue improved significantly with fast B₀ correction (from 1.88 ± 0.48 to 2.20 ± 0.44, P < .01).

CONCLUSIONS

Our study established an expedient B₀ inhomogeneity correction algorithm for fast pH imaging of acute ischemia.

Posterior reversible encephalopathy syndrome in stroke-prone spontaneously hypertensive rats on high-salt diet

Stroke-prone spontaneously hypertensive rats (SHRSP) on high-salt diet are characterized by extremely high arterial pressures, and have been endorsed as a model for hypertensive small vessel disease and vascular cognitive impairment. However, rapidly developing malignant hypertension is a well-known cause of posterior reversible encephalopathy syndrome (PRES) in humans, associated with acute neurological deficits, seizures, vasogenic cerebral edema and microhemorrhages. In this study, we aimed to examine the overlap between human PRES and SHRSP on high-salt diet. In SHRSP, arterial blood pressure progressively increased after the onset of high-salt diet and seizure-like signs emerged within three to five weeks. MRI revealed progressive T2-hyperintense lesions suggestive of vasogenic edema predominantly in the cortical watershed and white matter regions. Histopathology confirmed severe blood-brain barrier disruption, white matter vacuolization and microbleeds that were more severe posteriorly. Hematological data suggested a thrombotic microangiopathy as a potential underlying mechanism. Unilateral common carotid artery occlusion protected the ipsilateral hemisphere from neuropathological abnormalities. Notably, all MRI and histopathological abnormalities were acutely reversible upon switching to regular diet and starting antihypertensive treatment. Altogether our data suggest that SHRSP on high-salt diet recapitulates the neurological, histopathological and imaging features of human PRES rather than chronic progressive small vessel disease.

Mapping tissue pH in an experimental model of acute stroke - Determination of graded regional tissue pH changes with non-invasive quantitative amide proton transfer MRI

pH-weighted amide proton transfer (APT) MRI is sensitive to tissue pH change during acute ischemia, complementing conventional perfusion and diffusion stroke imaging. However, the currently used pH-weighted magnetization transfer (MT) ratio asymmetry (MTR_asym) analysis is of limited pH specificity. To overcome this, MT and relaxation normalized APT (MRAPT) analysis has been developed that to homogenize the background signal, thus providing highly pH conspicuous measurement. Our study aimed to calibrate MRAPT MRI toward absolute tissue pH mapping and determine regional pH changes during acute stroke. Using middle cerebral artery occlusion (MCAO) rats, we performed lactate MR spectroscopy and multi-parametric MRI. MRAPT MRI was calibrated against a region of interest (ROI)-based pH spectroscopy measurement (R² = 0.70, P < 0.001), showing noticeably higher correlation coefficient than the simplistic MTR_asym index. Capitalizing on this, we mapped brain tissue pH and semi-automatically segmented pH lesion, in addition to routine perfusion and diffusion lesions. Tissue pH from regions of the contralateral normal, perfusion/diffusion lesion mismatch and diffusion lesion was found to be 7.03 ± 0.04, 6.84 ± 0.10, 6.52 ± 0.19, respectively. Most importantly, we delineated the heterogeneous perfusion/diffusion lesion mismatch into perfusion/pH and pH/diffusion lesion mismatches, with their pH being 7.01 ± 0.04 and 6.71 ± 0.12, respectively (P < 0.05). To summarize, our study calibrated pH-sensitive MRAPT MRI toward absolute tissue pH mapping, semi-automatically segmented and determined graded tissue pH changes in ischemic tissue and demonstrated its feasibility for refined demarcation of heterogeneous metabolic disruption following acute stroke.

JOURNAL CLUB: Evaluation of Diffusion Kurtosis Imaging of Stroke Lesion With Hemodynamic and Metabolic MRI in a Rodent Model of Acute Stroke

OBJECTIVE

Diffusion kurtosis imaging (DKI) has emerged as a new acute stroke imaging approach, augmenting routine DWI. Although it has been shown that a diffusion lesion without kurtosis abnormality is more likely to recover after reperfusion, whereas a kurtosis lesion shows poor response, little is known about the underlying pathophysiologic profile of the kurtosis lesion versus the kurtosis lesion-diffusion lesion mismatch.

MATERIALS AND METHODS

We performed multiparametric MRI, including arterial spin labeling, pH-sensitive amide proton transfer, and DKI, in a rodent model of acute stroke caused by embolic middle cerebral artery occlusion. Diffusion and kurtosis lesions were semiautomatically segmented, and multiparametric MRI indexes were compared among the kurtosis lesion, diffusion lesion, kurtosis lesion-diffusion lesion mismatch, and the contralateral normal tissue area.

RESULTS

We confirmed a significant difference between diffusion lesion and kurtosis lesion volumes (mean [± SD] volume, 151 ± 65 vs 125 ± 47 mm³; p < 0.05). Although ischemic lesions have significantly reduced cerebral blood flow compared with contralateral normal tissue, we did not find a significant difference in cerebral blood flow between the kurtosis lesion and the kurtosis lesion-diffusion lesion mismatch (mean cerebral blood flow, 0.53 ± 0.10 vs 0.47 ± 0.14 mL/g of tissue per minute; p > 0.05). Of importance, the pH of the kurtosis lesion was significantly lower than that of the lesion mismatch (mean pH, 6.81 ± 0.08 vs 6.89 ± 0.09; p < 0.01).

CONCLUSION

The present study confirms that DKI provides an expedient approach for refining the heterogeneous DWI lesion that is associated with graded metabolic derangement, which is promising for improving the infarction core definition and ultimately helping to guide stroke treatment.

Antiangiogenic Effect of Bevacizumab: Application of Arterial Spin-Labeling Perfusion MR Imaging in a Rat Glioblastoma Model

BACKGROUND AND PURPOSE

The usefulness of arterial spin-labeling for the evaluation of the effect of the antiangiogenic therapy has not been elucidated. Our aim was to evaluate the antiangiogenic effect of bevacizumab in a rat glioblastoma model based on arterial spin-labeling perfusion MR imaging.

MATERIALS AND METHODS

DSC and arterial spin-labeling perfusion MR imaging were performed by using a 9.4T MR imaging scanner in nude rats with glioblastoma. Rats were randomly assigned to the following 3 groups: control, 3-day treatment, and 10-day treatment after bevacizumab injection. One-way analysis of variance with a post hoc test was used to compare perfusion parameters (eg, normalized CBV and normalized CBF from DSC MR imaging and normalized CBF based on arterial spin-labeling) with microvessel area on histology. The Pearson correlations between perfusion parameters and microvessel area were also determined.

RESULTS

All of the normalized CBV from DSC, normalized CBF from DSC, normalized CBF from arterial spin-labeling, and microvessel area values showed significant decrease after treatment (P < .001, P < .001, P = .005, and P < .001, respectively). In addition, normalized CBV and normalized CBF from DSC and normalized CBF from arterial spin-labeling strongly correlated with microvessel area (correlation coefficient, r = 0.911, 0.869, and 0.860, respectively; P < .001 for all).

CONCLUSIONS

Normalized CBF based on arterial spin-labeling and normalized CBV and normalized CBF based on DSC have the potential for evaluating the effect of antiangiogenic therapy on glioblastomas treated with bevacizumab, with a strong correlation with microvessel area.

pH-sensitive MRI demarcates graded tissue acidification during acute stroke - pH specificity enhancement with magnetization transfer and relaxation-normalized amide proton transfer (APT) MRI

pH-sensitive amide proton transfer (APT) MRI provides a surrogate metabolic biomarker that complements the widely-used perfusion and diffusion imaging. However, the endogenous APT MRI is often calculated using the asymmetry analysis (MTRasym), which is susceptible to an inhomogeneous shift due to concomitant semisolid magnetization transfer (MT) and nuclear overhauser (NOE) effects. Although the intact brain tissue has little pH variation, white and gray matter appears distinct in the MTRasym image. Herein we showed that the heterogeneous MTRasym shift not related to pH highly correlates with MT ratio (MTR) and longitudinal relaxation rate (R1w), which can be reasonably corrected using the multiple regression analysis. Because there are relatively small MT and R1w changes during acute stroke, we postulate that magnetization transfer and relaxation-normalized APT (MRAPT) analysis increases MRI specificity to acidosis over the routine MTRasym image, hence facilitates ischemic lesion segmentation. We found significant differences in perfusion, pH and diffusion lesion volumes (P<0.001, ANOVA). Furthermore, MRAPT MRI depicted graded ischemic acidosis, with the most severe acidosis in the diffusion lesion (-1.05±0.29%/s), moderate acidification within the pH/diffusion mismatch (i.e., metabolic penumbra, -0.67±0.27%/s) and little pH change in the perfusion/pH mismatch (i.e., benign oligemia, -0.04±0.14%/s), providing refined stratification of ischemic tissue injury.

Imaging acute ischemic tissue acidosis with pH-sensitive endogenous amide proton transfer (APT) MRI--correction of tissue relaxation and concomitant RF irradiation effects toward mapping quantitative cerebral tissue pH

Amide proton transfer (APT) MRI is sensitive to ischemic tissue acidosis and has been increasingly used as a research tool to investigate disrupted tissue metabolism during acute stroke. However, magnetization transfer asymmetry (MTR(asym)) analysis is often used for calculating APT contrast, which only provides pH-weighted images. In addition to pH-dependent APT contrast, in vivo MTR(asym) is subject to a baseline shift (ΔMTR'(asym)) attributable to the slightly asymmetric magnetization transfer (MT) effect. Additionally, APT contrast approximately scales with T(1) relaxation time. Tissue relaxation time may also affect the experimentally obtainable APT contrast via saturation efficiency and RF spillover effects. In this study, we acquired perfusion, diffusion, relaxation and pH-weighted APT MRI data, and spectroscopy (MRS) in an animal model of acute ischemic stroke. We modeled in vivo MTR(asym) as a superposition of pH-dependent APT contrast and a baseline shift ΔMTR'(asym) (i.e., MTR(asym)=APTR(pH)+ΔMTR'(asym)), and quantified tissue pH. We found pH of the contralateral normal tissue to be 7.03±0.05 and the ipsilateral ischemic tissue pH was 6.44±0.24, which correlated with tissue perfusion and diffusion rates. In summary, our study established an endogenous and quantitative pH imaging technique for improved characterization of ischemic tissue acidification and metabolism disruption.

Association between pH-weighted endogenous amide proton chemical exchange saturation transfer MRI and tissue lactic acidosis during acute ischemic stroke

The ischemic tissue becomes acidic after initiation of anaerobic respiration, which may result in impaired tissue metabolism and, ultimately, in severe tissue damage. Although changes in the major cerebral metabolites can be studied using magnetic resonance (MR) spectroscopy (MRS)-based techniques, their spatiotemporal resolution is often not sufficient for routine examination of fast-evolving and heterogeneous acute stroke lesions. Recently, pH-weighted MR imaging (MRI) has been proposed as a means to assess tissue acidosis by probing the pH-dependent chemical exchange of amide protons from endogenous proteins and peptides. In this study, we characterized acute ischemic tissue damage using localized proton MRS and multiparametric imaging techniques that included perfusion, diffusion, pH, and relaxation MRI. Our study showed that pH-weighted MRI can detect ischemic lesions and strongly correlates with tissue lactate content measured by (1)H MRS, indicating lactic acidosis. Our results also confirmed the correlation between apparent diffusion coefficient and lactate; however, no significant relationship was found for perfusion, T(1), and T(2). In summary, our study showed that optimized endogenous pH-weighted MRI, by sensitizing to local tissue pH, remains a promising tool for providing a surrogate imaging marker of lactic acidosis and altered tissue metabolism, and augments conventional techniques for stroke diagnosis.

Improved pseudo-continuous arterial spin labeling for mapping brain perfusion

PURPOSE

To investigate arterial spin labeling (ASL) methods for improved brain perfusion mapping. Previously, pseudo-continuous ASL (pCASL) was developed to overcome limitations inherent with conventional continuous ASL (CASL), but the control scan (null pulse) in the original method for pCASL perturbs the equilibrium magnetization, diminishing the ASL signal. Here, a new modification of pCASL, termed mpCASL is reported, in which the perturbation caused by the null pulse is reduced and perfusion mapping improved.

MATERIALS AND METHODS

improvements with mpCASL are demonstrated using numerical simulations and experiments. ASL signal intensity as well as contrast and reproducibility of in vivo brain perfusion images were measured in four volunteers who had MRI scans at 4 Tesla and the data compared across the labeling methods.

RESULTS

Perfusion maps with mpCASL showed, on average, higher ASL signal intensity and higher image contrast than those from CASL or pCASL. Furthermore, mpCASL yielded better reproducibility in repeat scans than the other methods.

CONCLUSION

The experimental results are consistent with the hypothesis that the new null pulse of mpCASL leads to improved brain perfusion images.

Continuous flow-driven inversion for arterial spin labeling using pulsed radio frequency and gradient fields

Continuous labeling by flow-driven adiabatic inversion is advantageous for arterial spin labeling (ASL) perfusion studies, but details of the implementation, including inefficiency, magnetization transfer, and limited support for continuous-mode operation on clinical scanners, have restricted the benefits of this approach. Here a new approach to continuous labeling that employs rapidly repeated gradient and radio frequency (RF) pulses to achieve continuous labeling with high efficiency is characterized. The theoretical underpinnings, numerical simulations, and in vivo implementation of this pulsed continuous ASL (PCASL) method are described. In vivo PCASL labeling efficiency of 96% relative to continuous labeling with comparable labeling parameters far exceeded the 33% duty cycle of the PCASL RF pulses. Imaging at 3T with body coil transmission was readily achieved. This technique should help to realize the benefits of continuous labeling in clinical imagers.

Regional variation of cerebral blood flow and arterial transit time in the normal and hypoperfused rat brain measured using continuous arterial spin labeling MRI

Continuous arterial spin labeling (CASL) is a noninvasive magnetic resonance (MR) method for measuring cerebral perfusion. In its most widely used form, CASL incorporates a postlabeling delay to minimize the sensitivity of the technique to transit time effects, which otherwise corrupt cerebral blood flow (CBF) quantification. For this delay to work effectively, it must be longer than the longest transit time present in the system. In this work, CASL measurements were made in four coronal slices in the rat brain using a range of postlabeling delays. By doing this, direct estimation of both CBF and arterial transit time (delta(a)) was possible. These measurements were performed in the normal brain and during hypoperfusion induced by occlusion of the common carotid arteries. It was found that, in the normal rat brain, significant regional variation exists for both CBF and delta(a). Mean values of CBF and delta(a) in the selected gray matter regions of interest were 233 mL/100 g min and 266 ms, respectively, with the latter ranging from 100 to 500 ms. Therefore, use of a 500-ms postlabeling delay is suitable for any location in the normal rat brain. After common carotid artery occlusion, CBF decreased and delta(a) increased by regionally dependent amounts. In the sensory cortex, delta(a) increased to a mean value of 740 ms, significantly greater than 500 ms. These results highlight the importance of either (a) determining delta(a) as part of the CASL measurement or (b) knowing the approximate range of values delta(a) is likely to take for a given application, so that the parameters of the CASL sequence can be chosen appropriately.