Improving the robustness of pseudo-continuous arterial spin labeling to off-resonance and pulsatile flow velocity

PURPOSE

To improve pseudo-continuous arterial spin labeling (PCASL) robustness to off-resonance and pulsatile blood flow velocity.

METHODS

Bloch equations were solved to evaluate the effect of labeling parameters in a pulsatile flow model for a range of off-resonance. Experimental confirmation was achieved in volunteers using linear phase increase between labeling pulses to approximate off-resonance errors. We first assessed the location of the labeling plane in four volunteers. Next, we explored a range of parameters-including balanced and unbalanced gradients-in five more volunteers at an optimal labeling plane location.

RESULTS

Simulations demonstrated that 1) high velocities are vulnerable to off-resonance, 2) unbalanced PCASL outperforms balanced PCASL, 3) increased B1 and low average gradient improve the labeling efficiency for high-velocity flow, and 4) a low ratio of selective to average gradient improves off-resonance robustness. A good agreement between theory and experiment was observed.

CONCLUSION

The robustness of PCASL can be increased by selecting an unbalanced scheme with a low average gradient (0.5 mT/m), a low ratio (7×) of selective to average gradients, and the highest feasible B1 (1.8 μT). Placing the labeling plane above the carotid bifurcation and below the V3 segment, usually between the second and third vertebrae, yielded robust results. Magn Reson Med 78:1342-1351, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Longitudinal diffusion changes following postoperative delirium in older people without dementia

OBJECTIVE

To investigate the effect of postoperative delirium on longitudinal brain microstructural changes, as measured by diffusion tensor imaging.

METHODS

We studied a subset of the larger Successful Aging after Elective Surgery (SAGES) study cohort of older adults (≥70 years) without dementia undergoing elective surgery: 113 participants who had diffusion tensor imaging before and 1 year after surgery. Postoperative delirium severity and occurrence were assessed during the hospital stay using the Confusion Assessment Method and a validated chart review method. We investigated the association of delirium severity and occurrence with longitudinal diffusion changes across 1 year, adjusting for age, sex, vascular comorbidity, and baseline cognitive performance. We also assessed the association between changes in diffusion and cognitive performance across the 1-year follow-up period, adjusting for age, sex, education, and baseline cognitive performance.

RESULTS

Postoperative delirium occurred in 25 participants (22%). Delirium severity and occurrence were associated with longitudinal diffusion changes in the periventricular, frontal, and temporal white matter. Diffusion changes were also associated with changes in cognitive performance across 1 year, although the cognitive changes did not show significant association with delirium severity or occurrence.

CONCLUSIONS

Our study raises the possibility that delirium has an effect on the development of brain microstructural abnormalities, which may reflect brain changes underlying cognitive trajectories. Future studies are warranted to clarify whether delirium is the driving factor of the observed changes or rather a correlate of a vulnerable brain that is at high risk for neurodegenerative processes.

Methodologic considerations in the design and analysis of nested case-control studies: association between cytokines and postoperative delirium

Background

The nested case-control study (NCC) design within a prospective cohort study is used when outcome data are available for all subjects, but the exposure of interest has not been collected, and is difficult or prohibitively expensive to obtain for all subjects. A NCC analysis with good matching procedures yields estimates that are as efficient and unbiased as estimates from the full cohort study. We present methodological considerations in a matched NCC design and analysis, which include the choice of match algorithms, analysis methods to evaluate the association of exposures of interest with outcomes, and consideration of overmatching.

Methods

Matched, NCC design within a longitudinal observational prospective cohort study in the setting of two academic hospitals. Study participants are patients aged over 70 years who underwent scheduled major non-cardiac surgery. The primary outcome was postoperative delirium from in-hospital interviews and medical record review. The main exposure was IL-6 concentration (pg/ml) from blood sampled at three time points before delirium occurred. We used nonparametric signed ranked test to test for the median of the paired differences. We used conditional logistic regression to model the risk of IL-6 on delirium incidence. Simulation was used to generate a sample of cohort data on which unconditional multivariable logistic regression was used, and the results were compared to those of the conditional logistic regression. Partial R-square was used to assess the level of overmatching.

Results

We found that the optimal match algorithm yielded more matched pairs than the greedy algorithm. The choice of analytic strategy—whether to consider measured cytokine levels as the predictor or outcome-- yielded inferences that have different clinical interpretations but similar levels of statistical significance. Estimation results from NCC design using conditional logistic regression, and from simulated cohort design using unconditional logistic regression, were similar. We found minimal evidence for overmatching.

Conclusions

Using a matched NCC approach introduces methodological challenges into the study design and data analysis. Nonetheless, with careful selection of the match algorithm, match factors, and analysis methods, this design is cost effective and, for our study, yields estimates that are similar to those from a prospective cohort study design.

Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D ) filtering

A pulsed inhomogeneous magnetization transfer (ihMT)-prepared fast imaging sequence was implemented at 11.75 T for preclinical studies on mouse central nervous system. A strategy based on filtering the ihMT signal originating from short dipolar relaxation time (T_1D ) components is proposed. It involves increasing the repetition time of consecutive radiofrequency (RF) pulses of the dual saturation and allows improved signal specificity for long T_1D myelinated structures. Furthermore, frequency offset, power and timing saturation parameters were adjusted to optimize the ihMT sensitivity. The optimization of the ihMT sensitivity, whilst preserving the strong specificity for the long T_1D component of myelinated tissues, allowed measurements of ihMT ratios on the order of 4-5% in white matter (WM), 2.5% in gray matter (GM) and 1-1.3% in muscle. This led to high relative ihMT contrasts between myelinated tissues and others (~3-4 between WM and muscle, and ≥2 between GM and muscle). Conversely, higher ihMT ratios (~6-7% in WM) could be obtained using minimal T_1D filtering achieved with short saturation pulse repetition time or cosine-modulated pulses for the dual-frequency saturation. This study represents a first stage in the process of validating ihMT as a myelin biomarker by providing optimized ihMT preclinical sequences, directly transposable and applicable to other preclinical magnetic fields and scanners. Finally, ihMT ratios measured in various central nervous system areas are provided for future reference.

High C-Reactive Protein Predicts Delirium Incidence, Duration, and Feature Severity After Major Noncardiac Surgery

OBJECTIVES

To examine associations between the inflammatory marker C-reactive protein (CRP) measured preoperatively and on postoperative day 2 (POD2) and delirium incidence, duration, and feature severity.

DESIGN

Prospective cohort study.

SETTING

Two academic medical centers.

PARTICIPANTS

Adults aged 70 and older undergoing major noncardiac surgery (N = 560).

MEASUREMENTS

Plasma CRP was measured using enzyme-linked immunosorbent assay. Delirium was assessed from Confusion Assessment Method (CAM) interviews and chart review. Delirium duration was measured according to number of hospital days with delirium. Delirium feature severity was defined as the sum of CAM-Severity (CAM-S) scores on all postoperative hospital days. Generalized linear models were used to examine independent associations between CRP (preoperatively and POD2 separately) and delirium incidence, duration, and feature severity; prolonged hospital length of stay (LOS, >5 days); and discharge disposition.

RESULTS

Postoperative delirium occurred in 24% of participants, 12% had 2 or more delirium days, and the mean ± standard deviation sum CAM-S was 9.3 ± 11.4. After adjusting for age, sex, surgery type, anesthesia route, medical comorbidities, and postoperative infectious complications, participants with preoperative CRP of 3 mg/L or greater had a risk of delirium that was 1.5 times as great (95% confidence interval (CI) = 1.1-2.1) as that of those with CRP less than 3 mg/L, 0.4 more delirium days (P < .001), more-severe delirium (3.6 CAM-S points higher, P < .001), and a risk of prolonged LOS that was 1.4 times as great (95% CI = 1.1-1.8). Using POD2 CRP, participants in the highest quartile (≥235.73 mg/L) were 1.5 times as likely to develop delirium (95% CI = 1.0-2.4) as those in the lowest quartile (≤127.53 mg/L), had 0.2 more delirium days (P < .05), and had more severe delirium (4.5 CAM-S points higher, P < .001).

CONCLUSION

High preoperative and POD2 CRP were independently associated with delirium incidence, duration, and feature severity. CRP may be useful to identify individuals who are at risk of developing delirium.

Cerebral blood flow MRI in the nondemented elderly is not predictive of post-operative delirium but is correlated with cognitive performance

Three-dimensional Arterial Spin Labeling (ASL) MRI was performed before surgery in a cohort of 146 prospectively enrolled subjects ≥ 70 years old scheduled to undergo elective surgery. We investigated the prospective association between ASL-derived measures of cerebral blood flow (CBF) before surgery with postoperative delirium incidence and severity using whole-brain and globally normalized voxel-wise analysis. We also investigated the cross-sectional association of CBF with patients' baseline performance on specific neuropsychological tests, and with a composite general cognitive performance measure (GCP). Out of 146 subjects, 32 (22%) developed delirium. We found no significant association between global and voxel-wise CBF with delirium incidence or severity. We found the most significant positive associations between CBF of the posterior cingulate and precuneus and the Hopkins Verbal Learning Test - Revised total score, Visual Search and Attention Test (VSAT) score and the GCP composite. VSAT score was also strongly associated with right parietal lobe CBF. ASL can be employed in a large, well-characterized older cohort to examine associations between CBF and age-related cognitive performance. Although ASL CBF measures in regions previously associated with preclinical Alzheimer's Disease were correlated with cognition, they were not found to be indicators of baseline pathology that may increase risk for delirium.

Using Anatomic Magnetic Resonance Image Information to Enhance Visualization and Interpretation of Functional Images: A Comparison of Methods Applied to Clinical Arterial Spin Labeling Images

Functional imaging provides hemodynamic and metabolic information and is increasingly being incorporated into clinical diagnostic and research studies. Typically functional images have reduced signal-to-noise ratio and spatial resolution compared to other non-functional cross sectional images obtained as part of a routine clinical protocol. We hypothesized that enhancing visualization and interpretation of functional images with anatomic information could provide preferable quality and superior diagnostic value. In this work, we implemented five methods (frequency addition, frequency multiplication, wavelet transform, nonsubsampled contourlet transform and intensity-hue-saturation) and a newly proposed ShArpening by Local Similarity with Anatomic images (SALSA) method to enhance the visualization of functional images, while preserving the original functional contrast and quantitative signal intensity characteristics over larger spatial scales. Arterial spin labeling blood flow MR images of the brain were visualization enhanced using anatomic images with multiple contrasts. The algorithms were validated on a numerical phantom and their performance on images of brain tumor patients were assessed by quantitative metrics and neuroradiologist subjective ratings. The frequency multiplication method had the lowest residual error for preserving the original functional image contrast at larger spatial scales (55%-98% of the other methods with simulated data and 64%-86% with experimental data). It was also significantly more highly graded by the radiologists (p<0.005 for clear brain anatomy around the tumor). Compared to other methods, the SALSA provided 11%-133% higher similarity with ground truth images in the simulation and showed just slightly lower neuroradiologist grading score. Most of these monochrome methods do not require any prior knowledge about the functional and anatomic image characteristics, except the acquired resolution. Hence, automatic implementation on clinical images should be readily feasible.

Higher C-Reactive Protein Levels Predict Postoperative Delirium in Older Patients Undergoing Major Elective Surgery: A Longitudinal Nested Case-Control Study

BACKGROUND

Delirium is a common, morbid, and costly postoperative complication. We aimed to identify blood-based postoperative delirium markers in a nested case-control study of older surgical patients using a proteomics approach followed by enzyme-linked immunosorbent assay (ELISA) validation.

METHODS

The Successful Aging after Elective Surgery study enrolled dementia-free adults ≥70 years old undergoing major scheduled noncardiac surgery (N = 566; 24% delirium). Plasma was collected at four time points: preoperative, postanesthesia care unit, postoperative day 2, and 1 month postoperative. Matched pairs were selected for the independent discovery (39 pairs) and replication cohorts (36 pairs), which were subsequently combined into the pooled cohort (75 pairs). Isobaric tags for relative and absolute quantitation-based relative quantitation mass spectrometry proteomics were performed to identify the strongest delirium-related protein, which was selected for ELISA validation. Using the ELISA results, statistical analyses using nonparametric signed rank tests were performed in all cohorts examining the association between the identified protein and delirium.

RESULTS

C-reactive protein emerged from the proteomics analysis as the strongest delirium-related protein. Validation by ELISA confirmed that compared with controls, cases had significantly higher C-reactive protein levels in the discovery, replication, and pooled cohorts at the preoperative (median paired difference [MPD] 1.97 mg/L [p < .05], 0.29 mg/L, 1.56 mg/L [p < .01]), postanesthesia care unit (MPD 2.83 mg/L, 2.22 mg/L [p < .05], 2.53 mg/L [p < .01]) and postoperative day 2 (MPD 71.97 mg/L [p < .01], 35.18 mg/L [p < .05], 63.76 mg/L [p < .01]) time points, but not 1 month postoperative (MPD 2.72 mg/L, -0.66 mg/L, 1.10 mg/L).

CONCLUSIONS

Elevated preoperative and postoperative plasma levels of C-reactive protein were associated with delirium, suggesting that a preinflammatory state and heightened inflammatory response to surgery are potential pathophysiologic mechanisms of delirium.

Association Between Hospital Readmission and Acute and Sustained Delays in Functional Recovery During 18 Months After Elective Surgery: The Successful Aging after Elective Surgery Study

OBJECTIVES

To examine the effect of hospital readmission on functional recovery after elective surgery in older adults.

DESIGN

Prospective cohort of individuals aged 70 and older undergoing elective surgery, enrolled from June 2010 to August 2013.

SETTING

Two academic medical centers.

PARTICIPANTS

Community-dwelling older adults (N = 566; mean age ± standard deviation 77 ± 5) undergoing major elective surgery and expected to be admitted for at least 3 days.

MEASUREMENTS

Readmission was assessed in multiple interviews with participants and family members over 18 months and validated against medical record review. Physical function was assessed according to ability to perform instrumental activities of daily living (IADLs) and activities of daily living (ADL), Medical Outcomes Study 12-item Short-Form Survey Physical Component Summary score, and a standardized functional composite.

RESULTS

Two hundred fifty-five (45%) participants experienced 503 readmissions. Readmissions were associated with delays in functional recovery in all measures of physical function. Having two or more readmissions over 18 months was associated with persistent and significantly greater risk of IADL dependence (relative risk (RR) = 1.8, 95% confidence interval (CI) = 1.5-2.3) and ADL dependence (RR = 3.3, 95% CI = 1.7-6.4). Degree of functional impairment increased progressively with number of readmissions. Readmissions within 2 months resulted in delayed functional recovery to baseline by 18 months, and readmissions between 12 and 18 months after surgery resulted in loss of functional recovery previously achieved.

CONCLUSION

Readmission after elective surgery may contribute to delays in functional recovery and persistent functional deficits in older adults.

Phase I study of low-dose metronomic temozolomide for recurrent malignant gliomas

Background

The treatment goal for recurrent malignant gliomas centers on disease stabilization while minimizing therapy-related side effects. Metronomic dosing of cytotoxic chemotherapy has emerged as a promising option to achieve this objective.

Methods

This phase I study was performed using metronomic temozolomide (mTMZ) at 25 or 50 mg/m²/day continuously in 42-day cycles. Correlative studies were incorporated using arterial spin labeling MRI to assess tumor blood flow, analysis of matrix metalloproteinase-2 (MMP-2) and MMP-9 activities in the cerebrospinal fluid (CSF) as surrogates for tumor angiogenesis and invasion, as well as determination of CSF soluble interleukin-2 receptor alpha (sIL-2Rα) levels as a marker of immune modulation.

Results

Nine subjects were enrolled and toxicity consisted of primarily grade 1 or 2 hematological and gastrointestinal side effects; only one patient had a grade 3 elevated liver enzyme level that was reversible. Tumor blood flow was variable across subjects and time, with two experiencing a transient increase before a decrease to below baseline level while one exhibited a gradual drop in blood flow over time. MMP-2 activity correlated with overall survival but not with progression free survival, while MMP-9 activity did not correlate with either outcome parameters. Baseline CSF sIL-2Rα level was inversely correlated with time from initial diagnosis to first progression, suggesting that subjects with higher sIL-2Rα may have more aggressive disease. But they lived longer when treated with mTMZ, probably due to drug-related changes in T-cell constituency.

Conclusions

mTMZ possesses efficacy against recurrent malignant gliomas by altering blood flow, slowing invasion and modulating antitumor immune function.

In vivo measurement of a new source of contrast, the dipolar relaxation time, T1D , using a modified inhomogeneous magnetization transfer (ihMT) sequence

PURPOSE

This paper describes a technique that can be used in vivo to measure the dipolar relaxation time, T_1D , of macromolecular protons contributing to magnetization transfer (MT) in tissues and to produce quantitative T_1D maps.

THEORY AND METHODS

The technique builds upon the inhomogeneous MT (ihMT) technique that is particularly sensitive to tissue components with long T_1D . A standard ihMT experiment was altered to introduce a variable time for switching between positive and negative offset frequencies for RF saturation. A model for the dependence of ihMT was developed and used to fit data acquired in vivo.

RESULTS

Application of the method to images from brains of healthy volunteers produced values of T_1D  = (5.9 ± 1.2) ms in gray matter and T_1D  = (6.2 ± 0.4) ms in white matter regions and provided maps of the T_1D parameter.

CONCLUSION

The model and experiments described provide access to a new relaxation characteristic of tissue with potentially unique diagnostic information. Magn Reson Med 78:1362-1372, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Preoperative Cognitive Performance Dominates Risk for Delirium Among Older Adults

BACKGROUND

Cognitive impairment is a well-recognized risk factor for delirium. Our goal was to determine whether the level of cognitive performance across the nondemented cognitive ability spectrum is correlated with delirium risk and to gauge the importance of cognition relative to other known risk factors for delirium.

METHODS

The Successful Aging after Elective Surgery study enrolled 566 adults aged ≥70 years scheduled for major surgery. Patients were assessed preoperatively and daily during hospitalization for the occurrence of delirium using the Confusion Assessment Method. Cognitive function was assessed preoperatively with an 11-test neuropsychological battery combined into a composite score for general cognitive performance (GCP). We examined the risk for delirium attributable to GCP, as well as demographic factors, vocabulary ability, and informant-rated cognitive decline, and compared the strength of association with risk factors identified in a previously published delirium prediction rule for delirium.

RESULTS

Delirium occurred in 135 (24%) patients. Lower GCP score was strongly and linearly predictive of delirium risk (relative risk = 2.0 per each half standard deviation difference in GCP score, 95% confidence interval, 1.5-2.5). This effect was not attenuated by statistical adjustment for demographics, vocabulary ability, and informant-rated cognitive decline. The effect was stronger than, and largely independent from, both standard delirium risk factors and comorbidity.

CONCLUSION

Risk of delirium is linearly and strongly related to presurgical cognitive performance level even at levels above the population median, which would be considered unimpaired.

Effects of arterial transit delay on cerebral blood flow quantification using arterial spin labeling in an elderly cohort

PURPOSE

To investigate whether measurement of arterial transit time (ATT) can improve the accuracy of arterial spin labeling (ASL) cerebral blood flow (CBF) quantification in an elderly cohort due to the potentially prolonged ATT in the cohort.

MATERIALS AND METHODS

We employed a 1-minute, low-resolution (12 mm in-plane), sequential multidelay ATT measurement (both with and without vessel suppression) approach to characterize and correct ATT errors in CBF imaging of an elderly, clinical cohort. In all, 140 nondemented subjects greater than 70 years old were imaged at 3T with a single delay, volumetric continuous ASL sequence and also with the fast ATT measurement method. Nine healthy young subjects (28 ± 6 years old) were also imaged.

RESULTS

ATTs measured without vessel suppression (superior frontal: 1.51 ± 0.27 sec) in the elderly were significantly shorter than those with suppression (P < 0.0001). Correction of CBF for ATT significantly increased average CBF in multiple brain regions where ATT was longer than the postlabeling delay (P < 0.01) and decreased intersubject variability of CBF in frontal, parietal, and occipital regions (P < 10^-8 ). Measured ATT with vessel suppression was significantly longer in the elderly subjects (eg, superior frontal: 1.76 ± 0.25 sec) compared to the younger adults (superior frontal: 1.59 ± 0.19 sec) in basal ganglia and frontal cortical regions (P < 0.05).

CONCLUSION

The ATT measurement is beneficial for imaging of elderly clinical populations. If ATT mapping is not feasible or available, postlabeling delays of 2-2.3 seconds should be used for elderly populations based on longest measured regional ATTs.

LEVEL OF EVIDENCE

1 J. Magn. Reson. Imaging 2017;45:472-481.

Magnetization transfer from inhomogeneously broadened lines (ihMT): Improved imaging strategy for spinal cord applications

PURPOSE

Inhomogeneous magnetization transfer (ihMT) shows great promise for specific imaging of myelinated tissues. Whereas the ihMT technique has been previously applied in brain applications, the current report presents a strategy for cervical spinal cord (SC) imaging free of cerebrospinal fluid (CSF) pulsatility artifacts.

METHODS

A pulsed ihMT preparation was combined with a single-shot HASTE readout. Electrocardiogram (ECG) synchronization was used to acquire all images during the quiescent phase of SC motion. However ihMT signal quantification errors may occur when a variable recovery delay is introduced in the sequence as a consequence of variable cardiac cycle. A semiautomatic retrospective correction algorithm, based on repetition time (TR) -matching, is proposed to correct for signal variations of long T₁ -components (e.g., CSF).

RESULTS

The proposed strategy combining ECG synchronization and retrospective data pairing led to clean SC images free of CSF artifacts. Lower variability of the ihMT metrics were obtained with the correction algorithm, and allowed for shorter TR to be used, hence improving signal-to-noise ratio efficiency.

CONCLUSION

The proposed methodology enabled faster acquisitions, while offering robust ihMT quantification and exquisite SC image quality. This opens great perspectives for widening the in vivo characterization of SC physiopathology using MRI, such as studying white matter tracts microstructure or impairment in degenerative pathologies. Magn Reson Med 77:581-591, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Quantifying fluctuations of resting state networks using arterial spin labeling perfusion MRI

Blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) has been widely used to investigate spontaneous low-frequency signal fluctuations across brain resting state networks. However, BOLD only provides relative measures of signal fluctuations. Arterial Spin Labeling (ASL) MRI holds great potential for quantitative measurements of resting state network fluctuations. This study systematically quantified signal fluctuations of the large-scale resting state networks using ASL data from 20 healthy volunteers by separating them from global signal fluctuations and fluctuations caused by residual noise. Global ASL signal fluctuation was 7.59% ± 1.47% relative to the ASL baseline perfusion. Fluctuations of seven detected resting state networks vary from 2.96% ± 0.93% to 6.71% ± 2.35%. Fluctuations of networks and residual noise were 6.05% ± 1.18% and 6.78% ± 1.16% using 4-mm resolution ASL data applied with Gaussian smoothing kernel of 6mm. However, network fluctuations were reduced by 7.77% ± 1.56% while residual noise fluctuation was markedly reduced by 39.75% ± 2.90% when smoothing kernel of 12 mm was applied to the ASL data. Therefore, global and network fluctuations are the dominant structured noise sources in ASL data. Quantitative measurements of resting state networks may enable improved noise reduction and provide insights into the function of healthy and diseased brain.

Neural substrates of vulnerability to postsurgical delirium as revealed by presurgical diffusion MRI

Despite the significant impact of postoperative delirium on surgical outcomes and the long-term prognosis of older patients, its neural basis has not yet been clarified. In this study we investigated the impact of premorbid brain microstructural integrity, as measured by diffusion tensor imaging before surgery, on postoperative delirium incidence and severity, as well as the relationship among presurgical cognitive performance, diffusion tensor imaging abnormalities and postoperative delirium. Presurgical diffusion tensor imaging scans of 136 older (≥70 years), dementia-free subjects from the prospective Successful Aging after Elective Surgery study were analysed blind to the clinical data and delirium status. Primary outcomes were postoperative delirium incidence and severity during the hospital stay, as assessed by the Confusion Assessment Method. We measured cognition before surgery using general cognitive performance, a composite score based on a battery of neuropsychological tests. We investigated the association between presurgical diffusion tensor imaging parameters of brain microstructural integrity (i.e. fractional anisotropy, axial, mean and radial diffusivity) with postoperative delirium incidence and severity. Analyses were adjusted for the following potential confounders: age, gender, vascular comorbidity status, and general cognitive performance. Postoperative delirium occurred in 29 of 136 subjects (21%) during hospitalization. Presurgical diffusion tensor imaging abnormalities of the cerebellum, cingulum, corpus callosum, internal capsule, thalamus, basal forebrain, occipital, parietal and temporal lobes, including the hippocampus, were associated with delirium incidence and severity, after controlling for age, gender and vascular comorbidities. After further controlling for general cognitive performance, diffusion tensor imaging abnormalities of the cerebellum, hippocampus, thalamus and basal forebrain still remained associated with delirium incidence and severity. This study raises the intriguing possibility that structural dysconnectivity involving interhemispheric and fronto-thalamo-cerebellar networks, as well as microstructural changes of structures involved in limbic and memory functions predispose to delirium under the stress of surgery. While the diffusion tensor imaging abnormalities observed in the corpus callosum, cingulum, and temporal lobe likely constitute the neural substrate for the association between premorbid cognition, as measured by general cognitive performance, and postoperative delirium, the microstructural changes observed in the cerebellum, hippocampus, thalamus and basal forebrain seem to constitute a separate phenomenon that predisposes to postsurgical delirium independent of presurgical cognitive status.

Volumetric Arterial Spin-labeled Perfusion Imaging of the Kidneys with a Three-dimensional Fast Spin Echo Acquisition

RATIONALE AND OBJECTIVES

Renal perfusion measurements using noninvasive arterial spin-labeled (ASL) magnetic resonance imaging techniques are gaining interest. Currently, focus has been on perfusion in the context of renal transplant. Our objectives were to explore the use of ASL in patients with renal cancer, and to evaluate three-dimensional (3D) fast spin echo (FSE) acquisition, a robust volumetric imaging method for abdominal applications. We evaluate 3D ASL perfusion magnetic resonance imaging in the kidneys compared to two-dimensional (2D) ASL in patients and healthy subjects.

MATERIALS AND METHODS

Isotropic resolution (2.6 × 2.6 × 2.8 mm(3)) 3D ASL using segmented FSE was compared to 2D single-shot FSE. ASL used pseudo-continuous labeling, suppression of background signal, and synchronized breathing. Quantitative perfusion values and signal-to-noise ratio (SNR) were compared between 3D and 2D ASL in four healthy volunteers and semiquantitative assessments were made by four radiologists in four patients with known renal masses (primary renal cell carcinoma).

RESULTS

Renal cortex perfusion in healthy subjects was 284 ± 21 mL/100 g/min, with test-retest repeatability of 8.8%. No significant differences were found between the quantitative perfusion value and SNR in volunteers between 3D ASL and 2D ASL, or in 3D ASL with synchronized or free breathing. In patients, semiquantitative assessment by radiologists showed no significant difference in image quality between 2D ASL and 3D ASL. In one case, 2D ASL missed a high perfusion focus in a mass that was seen by 3D ASL.

CONCLUSIONS

3D ASL renal perfusion imaging provides isotropic-resolution images, with comparable quantitative perfusion values and image SNR in similar imaging time to single-slice 2D ASL.

Minimizing the effects of magnetization transfer asymmetry on inhomogeneous magnetization transfer (ihMT) at ultra-high magnetic field (11.75 T)

OBJECTIVES

The recently reported inhomogeneous magnetization transfer technique (ihMT) has been proposed for specific imaging of inhomogeneously broadened lines, and has shown great promise for characterizing myelinated tissues. The ihMT contrast is obtained by subtracting magnetization transfer images obtained with simultaneous saturation at positive and negative frequency offsets (dual frequency saturation experiment, MT (+/-)) from those obtained with single frequency saturation (MT (+)) at the same total power. Hence, ihMT may be biased by MT-asymmetry, especially at ultra-high magnetic field. Use of the average of single positive and negative frequency offset saturation MT images, i.e., (MT (+)+MT (-)) has been proposed to correct the ihMT signal from MT-asymmetry signal.

MATERIALS AND METHODS

The efficiency of this correction method was experimentally assessed in this study, performed at 11.75 T on mice. Quantitative corrected ihMT and MT-asymmetry ratios (ihMTR and MTRasym) were measured in mouse brain structures for several MT-asymmetry magnitudes and different saturation parameter sets.

RESULTS

Our results indicated a "safe" range of magnitudes (/MTRasym/<4 %) for which MT-asymmetry signal did not bias the corrected ihMT signal. Moreover, experimental evidence of the different natures of both MT-asymmetry and inhomogeneous MT contrasts were provided. In particular, non-zero ihMT ratios were obtained at zero MTRasym values.

CONCLUSION

MTRasym is not a confounding factor for ihMT quantification, even at ultra-high field, as long as MTRasym is restricted to ±4 %.

The Successful Aging after Elective Surgery (SAGES) Study: Cohort Description and Data Quality Procedures

BACKGROUND/OBJECTIVES

Delirium is the most common complication of major elective surgery in older patients. The Successful Aging after Elective Surgery (SAGES) study was designed to examine novel risk factors and long-term outcomes associated with delirium. This report describes the cohort, quality assurance procedures, and results.

DESIGN

Long-term prospective cohort study.

SETTING

Three academic medical centers.

PARTICIPANTS

A total of 566 patients age 70 and older without recognized dementia scheduled for elective major surgery.

MEASUREMENTS

Participants were assessed preoperatively, daily during hospitalization, and at variable monthly intervals for up to 36 months post-discharge. Delirium was assessed in hospital by trained study staff. Study outcomes included cognitive and physical function. Novel risk factors for delirium were assessed including genetic and plasma biomarkers, neuroimaging markers, and cognitive reserve markers. Interrater reliability (kappa and weighted kappa) was assessed for key variables in 119 of the patient interviews.

RESULTS

Participants were an average of 77 years old and 58% were female. The majority of patients (81%) were undergoing orthopedic surgery and 24% developed delirium post-operatively. Over 95% of eligible patients were followed for 18 months. There was >99% capture of key study outcomes (cognitive and functional status) at every study interview and interrater reliability was high (weighted kappas for delirium = 0.92 and for overall cognitive and functional outcomes = 0.94 -1.0). Completion rates for plasma biomarkers (4 timepoints) were 95%-99% and for neuroimaging (one year follow-up) was 86%.

CONCLUSION

The SAGES study will contribute to the understanding of novel risk factors, pathophysiology and long-term outcomes of delirium. This manuscript describes the cohort and data quality procedures, and will serve as a reference source for future studies based on SAGES.

Interpretation of magnetization transfer from inhomogeneously broadened lines (ihMT) in tissues as a dipolar order effect within motion restricted molecules

Comparison of off-resonance saturation with single and dual frequency irradiation indicates a contribution of inhomogeneously broadened lines to magnetization transfer in tissues. This inhomogeneous magnetization transfer (ihMT) phenomenon can be exploited to produce images that highlight tissues containing myelin, in vivo. Here, a model for ihMT is described that includes dipolar order effects from magnetization associated with motion-restricted macromolecules. In this model, equal irradiation at positive and negative frequency offsets eliminates dipolar order and achieves greater saturation than irradiation at a single offset frequency using the same power. Fitting of mouse and human volunteer brain data at different irradiation powers and offset frequencies was performed to assess the relevance of the model and approximate tissue parameters. A key parameter in determining ihMT signal was found to be the relaxation time T1D associated with the dipolar order reservoir and the fraction f of the semi-solid, bound magnetization that possessed a nonzero T1D. Indeed, better fits of myelinated tissue were achieved when assuming f≠1. From such fits, estimated T1Ds of mice in the white matter, (34±14) ms, were much longer than in muscle, T1D=(1±1) ms and the average f from white matter volunteer data was 2.2 times greater than that in grey matter. The combination of f and longer T1Ds was primarily responsible for the much higher ihMT in myelinated tissues, and provided explanation for the species variation. This dipolar order ihMT model should help guide future research, pulse sequence optimization, and clinical applications.

Selective spectroscopic imaging of hyperpolarized pyruvate and its metabolites using a single-echo variable phase advance method in balanced SSFP

PURPOSE

In balanced steady state free precession (bSSFP), the signal intensity has a well-known dependence on the off-resonance frequency, or, equivalently, the phase advance between successive radiofrequency (RF) pulses. The signal profile can be used to resolve the contributions from the spectrally separated metabolites. This work describes a method based on use of a variable RF phase advance to acquire spatial and spectral data in a time-efficient manner for hyperpolarized 13C MRI.

THEORY AND METHODS

The technique relies on the frequency response from a bSSFP acquisition to acquire relatively rapid, high-resolution images that may be reconstructed to separate contributions from different metabolites. The ability to produce images from spectrally separated metabolites was demonstrated in vitro, as well as in vivo following administration of hyperpolarized 1-13C pyruvate in mice with xenograft tumors.

RESULTS

In vivo images of pyruvate, alanine, pyruvate hydrate, and lactate were reconstructed from four images acquired in 2 s with an in-plane resolution of 1.25 × 1.25 mm(2) and 5 mm slice thickness.

CONCLUSION

The phase advance method allowed acquisition of spectroscopically selective images with high spatial and temporal resolution. This method provides an alternative approach to hyperpolarized 13C spectroscopic MRI that can be combined with other techniques such as multiecho or fluctuating equilibrium bSSFP. Magn Reson Med 76:1102-1115, 2016. © 2015 Wiley Periodicals, Inc.

Cytokines and Postoperative Delirium in Older Patients Undergoing Major Elective Surgery

BACKGROUND

A proinflammatory state has been associated with several age-associated conditions; however, the inflammatory mechanisms of delirium remain poorly characterized.

METHODS

Using the Successful Aging after Elective Surgery Study of adults age ≥70 undergoing major noncardiac surgery, 12 cytokines were measured at four timepoints: preoperative, postanesthesia care unit, postoperative day 2 (POD2) and 30 days later (POD1M). We conducted a nested, longitudinal matched (on age, sex, surgery type, baseline cognition, vascular comorbidity, and Apolipoprotein E genotype) case-control study: delirium cases and no-delirium controls were selected from the overall cohort (N = 566; 24% delirium). Analyses were independently conducted in discovery, replication, and pooled cohorts (39, 36, 75 matched pairs, respectively). Nonparametric signed-rank tests evaluating differences in cytokine levels between matched pairs were used to identify delirium-associated cytokines.

RESULTS

In the discovery and replication cohorts, matching variables were similar in cases and controls. Compared to controls, cases had (*p < .05, **p < .01) significantly higher interleukin-6 on POD2 in the discovery, replication, and pooled cohorts (median difference [pg/mL] 50.44**, 20.17*, 39.35**, respectively). In the pooled cohort, cases were higher than controls for interleukin-2 (0.99*, 0.77*, 1.07**, 0.73* at preoperative, postanesthesia care unit, POD2, POD1M, respectively), vascular endothelial growth factor (4.10* at POD2), and tumor necrosis factor-alpha (3.10* at POD1M), while cases had lower interleukin-12 at POD1M (-4.24*).

CONCLUSIONS

In this large, well-characterized cohort assessed at multiple timepoints, we observed an inflammatory signature of delirium involving elevated interleukin-6 at POD2, which may be an important disease marker for delirium. We also observed preliminary evidence for involvement of other cytokines.

Brain atrophy and white-matter hyperintensities are not significantly associated with incidence and severity of postoperative delirium in older persons without dementia

Postoperative delirium is a common complication in older people and is associated with increased mortality, morbidity, institutionalization, and caregiver burden. Although delirium is an acute confusional state characterized by global impairments in attention and cognition, it has been implicated in permanent cognitive impairment and dementia. The pathogenesis of delirium and the mechanisms leading to these disabling consequences remain unclear. The present study is the first to address the potential predisposing role of brain morphologic changes toward postoperative delirium in a large prospective cohort of patients undergoing elective surgery using state-of-the-art magnetic resonance imaging (MRI) techniques conducted before admission. We investigated the association of MRI-derived quantitative measures of white-matter damage, global brain, and hippocampal volume with the incidence and severity of delirium. Presurgical white-matter hyperintensities (WMHs), whole brain, and hippocampal volume were measured in 146 consecutively enrolled subjects, ≥70 years old, without dementia who were undergoing elective surgery. These 3 presurgical MRI indices were tested as predictors of incidence and severity of subsequent delirium. Out of 146 subjects, 32 (22%) developed delirium. We found no statistically significant differences in WMH, whole brain, or hippocampal volume between subjects with and without delirium. Both unadjusted and adjusted (age, gender, vascular comorbidity, and general cognitive performance) regression analyses demonstrated no statistically significant association between any of the MRI measures with respect to delirium incidence or severity. In persons without dementia, preexisting cerebral WMHs, general and hippocampal atrophy may not predispose to postoperative delirium or worsen its severity.