APTw CEST MRI suffers from long preparation times and consequently long acquisition times (~5 min). Recently, a consensus on the preparation module for clinical APTw CEST at 3 T was found in the ...community, and we present a fast whole-brain APTw CEST MRI sequence following this consensus preparation of pulsed RF irradiation of 2 s duration at 90% RF duty-cycle and a B
of 2 μT. After optimization of the snapshot CEST approach for APTw imaging regarding flip angle, voxel size and frequency offset sampling, we extend it by undersampled GRE acquisition and compressed sensing reconstruction. This allows 2 mm isotropic whole-brain APTw imaging for clinical research at 3 T below 2 min. With this sequence, a fast snapshot APTw imaging method is now available for larger clinical studies of brain tumors.
Purpose
In this work, we investigated the ability of neural networks to rapidly and robustly predict Lorentzian parameters of multi‐pool CEST MRI spectra at 7 T with corresponding uncertainty maps to ...make them quickly and easily available for routine clinical use.
Methods
We developed a deepCEST 7 T approach that generates CEST contrasts from just 1 scan with robustness against B1 inhomogeneities. The input data for a neural feed‐forward network consisted of 7 T in vivo uncorrected Z‐spectra of a single B1 level, and a B1 map. The 7 T raw data were acquired using a 3D snapshot gradient echo multiple interleaved mode saturation CEST sequence. These inputs were mapped voxel‐wise to target data consisting of Lorentzian amplitudes generated conventionally by 5‐pool Lorentzian fitting of normalized, denoised, B0‐ and B1‐corrected Z‐spectra. The deepCEST network was trained with Gaussian negative log‐likelihood loss, providing an uncertainty quantification in addition to the Lorentzian amplitudes.
Results
The deepCEST 7 T network provides fast and accurate prediction of all Lorentzian parameters also when only a single B1 level is used. The prediction was highly accurate with respect to the Lorentzian fit amplitudes, and both healthy tissues and hyperintensities in tumor areas are predicted with a low uncertainty. In corrupted cases, high uncertainty indicated wrong predictions reliably.
Conclusion
The proposed deepCEST 7 T approach reduces scan time by 50% to now 6:42 min, but still delivers both B0‐ and B1‐corrected homogeneous CEST contrasts along with an uncertainty map, which can increase diagnostic confidence. Multiple accurate 7 T CEST contrasts are delivered within seconds.
To evaluate the benefits and challenges of dynamic parallel transmit (pTx) pulses for fat saturation (FS) and water-excitation (WE), in the context of CEST MRI.
"Universal" k
-points (for FS) and ...spiral non-selective (for WE) trajectories were optimized offline for flip angle (FA) homogeneity. Routines to optimize the pulse shape online, based on the subject's fields maps, were implemented (target FA of 110°/0° for FS, 0°/5° for WE at fat/water frequencies). The pulses were inserted in a CEST sequence with a pTx readout. The different fat suppression schemes and their effects on CEST contrasts were compared in 12 volunteers at 7T.
With a 25%-shorter pulse duration, pTx FS largely improved the FA homogeneity (root-mean-square-error (RMSE) = 12.3° vs. 53.4° with circularly-polarized mode, at the fat frequency). However, the spectral selectivity was degraded mainly in the cerebellum and close to the sinuses (RMSE = 5.8° vs. 0.2° at the water frequency). Similarly, pTx WE showed a trade-off between FA homogeneity and spectral selectivity compared to pTx non-selective pulses (RMSE = 0.9° and 1.1° at the fat and water frequencies, vs. 4.6° and 0.5°). In the brain, CEST metrics were reduced by up to 31.9% at -3.3 ppm with pTx FS, suggesting a mitigated lipid-induced bias.
This clinically compatible implementation of dynamic pTx pulses improved the fat suppression homogeneity at 7T taking into account the subject-specific B
heterogeneities online. This study highlights the lipid-induced biases on the CEST z-spectrum. The results are promising for body applications where B
heterogeneities and fat are more substantial.
The objective of the current study was to optimize the postprocessing pipeline of 7 T chemical exchange saturation transfer (CEST) imaging for reproducibility and to prove this optimization for the ...detection of age differences and differences between patients with Parkinson's disease versus normal subjects. The following 7 T CEST MRI experiments were analyzed: repeated measurements of a healthy subject, subjects of two age cohorts (14 older, seven younger subjects), and measurements of 12 patients with Parkinson's disease. A slab‐selective,
B1+‐homogeneous parallel transmit protocol was used. The postprocessing, consisting of motion correction, smoothing,
B0‐correction, normalization, denoising,
B1+‐correction and Lorentzian fitting, was optimized regarding the intrasubject and intersubject coefficient of variation (CoV) of the amplitudes of the amide pool and the aliphatic relayed nuclear Overhauser effect (rNOE) pool within the brain. Seven “tricks” for postprocessing accomplished an improvement of the mean voxel CoV of the amide pool and the aliphatic rNOE pool amplitudes of less than 5% and 3%, respectively. These postprocessing steps are: motion correction with interpolation of the motion of low‐signal offsets (1) using the amide pool frequency offset image as reference (2), normalization of the Z‐spectrum using the outermost saturated measurements (3),
B0 correction of the Z‐spectrum with moderate spline smoothing (4), denoising using principal component analysis preserving the 11 highest intensity components (5),
B1+ correction using a linear fit (6) and Lorentzian fitting using the five‐pool fit model (7). With the optimized postprocessing pipeline, a significant age effect in the amide pool can be detected. Additionally, for the first time, an aliphatic rNOE contrast between subjects with Parkinson's disease and age‐matched healthy controls in the substantia nigra is detected. We propose an optimized postprocessing pipeline for CEST multipool evaluation. It is shown that by the use of these seven “tricks”, the reproducibility and, thus, the statistical power of a CEST measurement, can be greatly improved and subtle changes can be detected.
The presented postprocessing pipeline provides increased homogeneity and reproducibility. The mean voxel CoV is decreased to less than 5% for amide and to less than 3% for aliphatic rNOE contrast. Healthy aging‐related changes and altered aliphatic rNOE pools in the substantia nigra of patients in the early stages of Parkinson's disease are detected.
Isolated evaluation of multiparametric in vivo chemical exchange saturation transfer (CEST) MRI often requires complex computational processing for both correction of B0 and B1 inhomogeneity and ...contrast generation. For that, sufficiently densely sampled Z‐spectra need to be acquired. The list of acquired frequency offsets largely determines the total CEST acquisition time, while potentially representing redundant information. In this work, a linear projection‐based multiparametric CEST evaluation method is introduced that offers fast B0 and B1 inhomogeneity correction, contrast generation and feature selection for CEST data, enabling reduction of the overall measurement time. To that end, CEST data acquired at 7 T in six healthy subjects and in one brain tumor patient were conventionally evaluated by interpolation‐based inhomogeneity correction and Lorentzian curve fitting. Linear regression was used to obtain coefficient vectors that directly map uncorrected data to corrected Lorentzian target parameters. L1‐regularization was applied to find subsets of the originally acquired CEST measurements that still allow for such a linear projection mapping. The linear projection method allows fast and interpretable mapping from acquired raw data to contrast parameters of interest, generalizing from healthy subject training data to unseen healthy test data and to the tumor patient dataset. The L1‐regularization method shows that a fraction of the acquired CEST measurements is sufficient to preserve tissue contrasts, offering up to a 2.8‐fold reduction of scan time. Similar observations as for the 7‐T data can be made for data from a clinical 3‐T scanner. Being a fast and interpretable computation step, the proposed method is complementary to neural networks that have recently been employed for similar purposes. The scan time acceleration offered by the L1‐regularization (“CEST‐LASSO”) constitutes a step towards better applicability of multiparametric CEST protocols in a clinical context.
A data‐driven evaluation approach for multiparametric in vivo CEST MRI is proposed that allows mapping of uncorrected Z‐spectra to target contrasts (APT, NOE, MT, amine) by a simple, fast, and interpretable linear projection. Applying L1‐regularization–based feature selection (CEST‐LASSO) shows that a fraction of the originally acquired CEST measurements is sufficient to preserve tissue contrasts, offering up to a 2.8‐fold reduction of scan time. This represents a step towards better applicability of multiparametric CEST protocols in a clinical context.
The aim was to analyse the prevalence of computed tomographic (CT) morphological predictors and their influence on early chronic phase aortic diameter expansion in patients with uncomplicated acute ...Stanford type B aortic dissection (ATBAD).
This retrospective analysis reviewed the CT imaging of 140 patients admitted with uncomplicated ATBAD to two tertiary centres between March 2003 and April 2016. The prevalence of the following CT-morphological predictors was determined at baseline: primary entry tear (PET) diameter ≥ 10 mm, its location at the concavity of the aortic arch; maximum descending aortic diameter ≥ 40 mm; false lumen (FL) diameter ≥ 22 mm; partial FL thrombosis and a fusiform index (FI) of ≥0.64. Thoracic aortic diameter expansion (ADE) was evaluated in 65 patients treated by best medical therapy (BMT) (median CT follow up 11.6 months). Study end points were predictor prevalence and ADE.
A mean ± SD of 2.45 ± 1.35 predictors were registered among all 140 patients; 75.0% of patients showed at least two predictors. In 7.9% of patients, no predictor was found. The prevalence of PET at the arch concavity was 18.6%, PET diameter ≥10 mm in 60.0%, maximal descending aortic diameter ≥40 mm in 51.4%, FL diameter ≥22 mm in 47.9%, partial FL thrombosis in 47.9%, and FI ≥ 0.64 in 20.7%. An ADE ≥5 mm was observed in 38 of 65 patients. Median observed ADE was 5.1 mm (median follow up (FU) 11.6 months, range −3.2–27.4 mm). Regression analysis for multiple predictors showed a basic ADE of 2.5 mm plus 1.9 mm per predictor at the median FU of 11.6 months (2.5 mm ± 1.9; 95% confidence interval CI –0.2–5.2 mm ± 0.7–3.0 mm; p = .003).
In the majority of patients, at least one of the investigated morphological predictors of disease progression in uncomplicated ATBAD was detected. An ADE ≥5 mm affected 38 of 65 BMT patients. CT based predictors help to define TBAD patients at risk of progression.
Objectives We compared aortic root reconstructions using conduits with biological valves and mechanical valves. Methods Of 597 patients (1995–2008), 307 (mean age 71 years 23–89 years) had biological ...valves and 290 (mean age 51 years 21–82 years) had mechanical valves. The subgroup of 242 patients aged 50 to 70 years included 133 with biological and 109 with mechanical valves. Results Overall hospital mortality was 3.9% with biological valves (n = 15; elective: 3.7% n = 10) versus 2.8% with mechanical valves (n = 8; elective: 1.4% n = 3). In patients 50 to 70 years, age greater than 65 years (relative risk: 3.3 P = .0001), clot (relative risk: 2.5 P = .05), coronary artery disease (relative risk:3.5 P < .0001), and degenerative etiology (relative risk: 0.4 P = .006) were independent risk factors for long-term survival (after postoperative day 30); there was no difference in long-term survival between biological and mechanical valves (relative risk: 0.9 P = .74). The linearized rate for valve/ascending aorta reoperation was 0.86%/pt-y (2 in 2310 pt-y) after mechanical valves and 2.5%/pt-y (4 in 1586 pt-y) after Bentall procedures with the biological valve. Conclusions The choice of valve for aortic root reconstruction seems to have no influence on long-term outcome. Emergency operation and the presence of clot/atheroma have a significant impact on short-term outcome. Reoperation for either ascending aorta and/or aortic valve is low.
Purpose
Colonic ischaemia (CI) represents a serious complication after aortic surgery. This study aimed to analyse risk factors and outcome of patients suffering from postoperative CI.
Methods
Data ...of 1404 patients who underwent aortic surgery were retrospectively analysed regarding CI occurrence. Co-morbidities, procedural parameters, colon blood supply, procedure-related morbidity and mortality as well as survival during follow-up (FU) were compared with patients without CI using matched-pair analysis (1:3).
Results
Thirty-five patients (2.4%) with CI were identified. Cardiovascular, pulmonary and renal comorbidity were more common in CI patients. Operation time was longer (283 ± 22 vs. 188 ± 7 min,
p
< 0.0001) and blood loss was higher (2174 ± 396 vs. 1319 ± 108 ml,
p
= 0.0049) in the CI group. Patients with ruptured abdominal aortic aneurysm (AAA) showed a higher rate of CI compared to patients with intact AAA (5.4 vs. 1.9%,
p
= 0.0177). CI was predominantly diagnosed by endoscopy (26/35), generally within the first 4 postoperative days (20/35). Twenty-eight patients underwent surgery, all finalised with stoma creation. Postoperative bilateral occlusion and/or relevant stenosis of hypogastric arteries were more frequent in CI patients (57.8 vs. 20.8%,
p
= 0.0273). In-hospital mortality was increased in the CI group (26.7 vs. 2.9%,
p
< 0.0001). Survival was significantly reduced in CI patients (median: 28.2 months vs. 104.1 months,
p
< 0.0001).
Conclusion
CI after aortic surgery is associated with considerable perioperative sequelae and reduced survival. Especially in patients at risk, such as those with rAAA, complicated intraoperative course, severe cardiovascular morbidity and/or perioperative deterioration of the hypogastric perfusion, vigilant postoperative multimodal monitoring is required in order to initiate diagnosis and treatment.
Iatrogenic paraplegia has been accompanying cardiovascular surgery since its beginning. As a result, surgeons have been developing many theories about the exact mechanisms of this devastating ...complication. Thus, the impact of single arteries that contribute to the spinal perfusion is one of the most discussed subjects in modern surgery. The subsequent decision of reattachment or the permanent disconnection of these intercostal arteries divides the surgical community. On the one hand, the anatomical or vascular approach pleads for the immediate reimplantation to reconstruct the anatomical situation. On the other hand, the decision of the permanent disconnection aims at avoiding stealing phenomenon away from the spinal vascular network. This spinal collateral network can be described as consisting of three components-the intraspinal and two paraspinal compartments-that feed the nutrient arteries of the spinal cord. The exact functional impact of the different compartments of the collateral network remains poorly understood. In this review, the function of the intraspinal compartment in the context of collateral network principle as an immediate emergency backup system is described. The exact structure and architectural principles of the intraspinal compartment are described. The critical parameters with regard to the risk of postoperative spinal cord ischaemia are the number of anterior radiculomedullary arteries (ARMAs) and the distance between them in relation to the longitudinal extent of aortic disease. The paraspinal network as a sleeping reserve is proposed as the long-term backup system. This sleeping reserve has to be activated by arteriogenic stimuli. These are presented briefly, and prior findings regarding arteriogenesis are discussed in the light of the collateral network concept. Finally, the role of preoperative visualization of the ARMAs in order to evaluate the risk of postoperative paraplegia is emphasized.
In this study, we assessed the dynamic segmental anatomy of the entire ascending aorta (AA), enabling the determination of a favorable proximal landing zone and appropriate aortic sizing for the most ...proximal thoracic endovascular aortic repair (TEVAR). Methods: Patients with a non-operated AA (diameter < 40 mm) underwent electrocardiogram-gated computed tomography angiography (ECG-CTA) of the entire AA in the systolic and diastolic phases. For each plane of each segment, the maximum and minimum diameters in the systole and diastole phases were recorded. The Wilcoxon signed-rank test was used to compare aortic size values. Results: A total of 100 patients were enrolled (53% male; median age 82.1 years; age range 76.8−85.1). Analysis of the dynamic plane dimensions of the AA during the cardiac cycle showed significantly higher systolic values than diastolic values (p < 0.001). Analysis of the proximal AA segment showed greater distal plane values than proximal plane values (p < 0.001), showing a reversed funnel form. At the mid-ascending segment, the dynamic values did not notably differ between the distal plane and the proximal segmental plane, demonstrating a cylindrical form. At the distal segment of the AA, the proximal plane values were larger than the distal segmental plane values (p < 0.001), thus generating a funnel form. Conclusions: The entire AA showed greater systolic than diastolic aortic dimensions throughout the cardiac cycle. The mid-ascending and distal-ascending segments showed favorable forms for TEVAR using a regular cylindrical endograft design. The most proximal segment of the AA showed a pronounced conical form; therefore, a specific endograft design should be considered.
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