Sidebands in CEST MR—How to recognize and avoid them Schüre, Jan‐Rüdiger; Weinmüller, Simon; Kamm, Lukas ...
Magnetic resonance in medicine,
June 2024, 2024-Jun, 2024-06-00, 20240601, Letnik:
91, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Purpose
Clinical scanners require pulsed CEST sequences to maintain amplifier and specific absorption rate limits. During off‐resonant RF irradiation and interpulse delay, the magnetization can ...accumulate specific relative phases within the pulse train. In this work, we show that these phases are important to consider, as they can lead to unexpected artifacts when no interpulse gradient spoiling is performed during the saturation train.
Methods
We investigated sideband artifacts using a CEST‐3D snapshot gradient‐echo sequence at 3 T. Initially, Bloch‐McConnell simulations were carried out with Pulseq‐CEST, while measurements were performed in vitro and in vivo.
Results
Sidebands can be hidden in Z‐spectra, and their structure becomes clearly visible only at high sampling. Sidebands are further influenced by B0 inhomogeneities and the RF phase cycling within the pulse train. In vivo, sidebands are mostly visible in liquid compartments such as CSF. Multi‐pulse sidebands can be suppressed by interpulse gradient spoiling.
Conclusion
We provide new insights into sidebands occurring in pulsed CEST experiments and show that, similar as in imaging sequences, gradient and RF spoiling play an important role. Gradient spoiling avoids misinterpretations of sidebands as CEST effects especially in liquid environments including pathological tissue or for CEST resonances close to water. It is recommended to simulate pulsed CEST sequences in advance to avoid artifacts.
A method is presented for correcting the effects of stimulated and indirect echoes on quantitative T2 mapping data acquired with multiple spin echo techniques, such as turbo spin echo. In contrast to ...similar correction techniques proposed in the literature, the method does not require a priori knowledge of the radio frequency (RF) pulse profiles. In a first step, for the T2 mapping protocol under investigation, signal decay curves S(TE) are simulated for a range of different RF pulse profiles. The actual signal decay S(TE) is then measured on a phantom with known T2, so the approximate RF pulse profiles can be derived via comparison with the simulated decay curves. In a second step, with the RF pulses obtained from step one, signal decay curves S(TE) are simulated for different T2 values and fitted mono-exponentially, thus allowing to deduce the relationship between true T2 and the apparent T2 (T2app) values. Results show that this relationship is approximately linear, allowing for a direct correction of T2app maps. If the amplitude of the transmitted RF field (B1) does not exceed the nominal value by more than 10%, it is shown that a B1-independent correction of T2app maps yields sufficiently accurate results for T2. A B1-dependent version is also presented. The method is tested in vitro on a phantom with different T2 values and in vivo on healthy subjects.
•Fast quantitative T2 mapping is often based on multiple spin echo acquisition.•The signal decay is distorted by stimulated and indirect echoes requiring correction.•The correction proposed here does not require knowledge of RF pulse shapes.•The relationship between distorted and corrected T2 maps was found to be linear.•Both a B1-independent and a B1-dependent correction method is presented.
Amide proton transfer‐chemical exchange saturation transfer (APT‐CEST) imaging provides important information for the diagnosis and monitoring of tumors. For such analysis, complete coverage of the ...brain is advantageous, especially when registration is performed with other magnetic resonance (MR) modalities, such as MR spectroscopy (MRS). However, the acquisition of Z‐spectra across several slices via multislice imaging may be time‐consuming. Therefore, in this paper, we present a new approach for fast multislice imaging, allowing us to acquire 16 slices per frequency offset within 8 s. The proposed fast CEST‐EPI sequence employs a presaturation module, which drives the magnetization into the steady‐state equilibrium for the first frequency offset. A second module, consisting of a single CEST pulse (for maintaining the steady‐state) followed by an EPI acquisition, passes through a loop to acquire multiple slices and adjacent frequency offsets. Thus, the whole Z‐spectrum can be recorded much faster than the conventional saturation scheme, which employs a presaturation for each single frequency offset. The validation of the CEST sequence parameters was performed by using the conventional saturation scheme. Subsequently, the proposed and a modified version of the conventional CEST sequence were compared in vitro on a phantom with different T1 times and in vivo on a brain tumor patient. No significant differences between both sequences could be found in vitro. The in vivo data yielded almost identical MTRasym contrasts for the white and gray matter as well as for tumor tissue. Our results show that the proposed fast CEST‐EPI sequence allows for rapid data acquisition and provides similar CEST contrasts as the modified conventional scheme while reducing the scanning time by approximately 50%.
In this study, we present a CEST‐EPI sequence for rapid multislice data acquisition, which records multiple slices and adjacent frequency offsets after reaching steady‐state magnetization once. The time saving compared with an already modified conventional multislice acquisition scheme, which rebuilds the steady state repeatedly during a frequency offset change, amounts to almost 50%.
Purpose
Amide proton transfer‐weighted (APTw) MRI at 3T provides a unique contrast for brain tumor imaging. However, APTw imaging suffers from hyperintensities in liquid compartments such as cystic ...or necrotic structures and provides a distorted APTw signal intensity. Recently, it has been shown that heuristically motivated fluid suppression can remove such artifacts and significantly improve the readability of APTw imaging.
Theory and Methods
In this work, we show that the fluid suppression can actually be understood by the known concept of spillover dilution, which itself can be derived from the Bloch‐McConnell equations in comparison to the heuristic approach. Therefore, we derive a novel post‐processing formula that efficiently removes fluid artifact, and explains previous approaches. We demonstrate the utility of this APTw assessment in silico, in vitro, and in vivo in brain tumor patients acquired at MR scanners from different vendors.
Results
Our results show a reduction of the CEST signals from fluid environments while keeping the APTw‐CEST signal intensity almost unchanged for semi‐solid tissue structures such as the contralateral normal appearing white matter. This further allows us to use the same color bar settings as for conventional APTw imaging.
Conclusion
Fluid suppression has considerable value in improving the readability of APTw maps in the neuro‐oncological field. In this work, we derive a novel post‐processing formula from the underlying Bloch‐McConnell equations that efficiently removes fluid artifact, and explains previous approaches which justify the derivation of this metric from a theoretical point of view, to reassure the scientific and medical field about its use.
The pH value is a potential physiological marker for clinical diagnosis as it is altered in pathologies such as tumors. While intracellular pH can be measured noninvasively via phosphorus ...spectroscopy (31P MRSI), Amide Proton Transfer‐Chemical Exchange Saturation Transfer (APT‐CEST) MRI has been suggested as an alternative method for pH quantification.
To assess the suitability of APT‐CEST contrast for pH quantification, two approaches (magnetization transfer ratio asymmetry MTRasym and Lorentzian difference analysis LDA) for analyzing the Z‐spectrum have been correlated with pH values obtained by 31P MRSI. Fourteen patients with glioblastoma and 12 healthy controls were included. In contrast to MTRasym, the LDA is modeling the direct water saturation and the semi‐solid magnetization transfer, allowing a separate evaluation of the aliphatic nuclear Overhauser effect and the APT‐CEST.
The results of our study show that the pH values obtained by 31P MRSI correspond well with both methods describing the APT‐CEST contrast. Two‐sample t‐test showed significant differences in MTRasym, LDA and pH obtained by 31P MRSI for regions of interest in glioblastoma, contralateral control areas and normal appearing white matter (P < 0.001). A slightly improved correlation between the amide signal and pH was found after performing LDA (r = 0.78) compared with MTRasym (r = 0.70).
While both methods can be used to monitor pH changes, the LDA approach appears to be better suited.
In this in vivo study we compare pH obtained by 31P‐MRSI with the pH sensitivity of Amide Proton Transfer‐Chemical Exchange Saturation Transfer (APT‐CEST) contrast, analyzed by either the asymmetric magnetization transfer rate (MTRasym) or by Lorentzian Difference Analysis (LDA). While both approaches show a good correlation with 31P spectroscopic data, LDA appears to be more suitable for the detection of pH changes.
Chemical exchange saturation transfer (CEST) is a magnetic resonance (MR) imaging method providing molecular image contrasts based on indirect detection of low concentrated solutes. Previous CEST ...studies focused predominantly on the imaging of single CEST exchange regimes (e.g., slow, intermediate or fast exchanging groups). In this work, we aim to establish a so-called comprehensive CEST protocol for 7 T, covering the different exchange regimes by three saturation B1 amplitude regimes: low, intermediate and high. We used the results of previous publications and our own simulations in pulseq-CEST to produce a 7 T CEST protocol that has sensitivity to these three B1 regimes. With postprocessing optimization (simultaneous mapping of water shift and B1, B0-fitting, multiple interleaved mode saturation B1 correction, neural network employment (deepCEST) and analytical input feature reduction), we are able to shorten our initially 40 min protocol to 15 min and generate six CEST contrast maps simultaneously. With this protocol, we measured four healthy subjects and one patient with a brain tumor. We established a comprehensive CEST protocol for clinical 7 T MRI, covering three different B1 amplitude regimes. We were able to reduce the acquisition time significantly by more than 50%, while still maintaining decent image quality and contrast in healthy subjects and one patient with a tumor. Our protocol paves the way to perform comprehensive CEST studies in clinical scan times for hypothesis generation regarding molecular properties of certain pathologies, for example, ischemic stroke or high-grade brain tumours.
•Increased T2′ values in SVD, suggesting reduced oxygen extraction fraction (OEF).•Vascular dysfunction and microstructural impairment limit OEF capacity.•Association between prolonged T2′ and more ...alkaline intracellular pH.•Adaptation of intracellular energy metabolism compensates for reduced OEF.
We aimed to investigate whether combined phosphorous (31P) magnetic resonance spectroscopic imaging (MRSI) and quantitative T2′ mapping are able to detect alterations of the cerebral oxygen extraction fraction (OEF) and intracellular pH (pHi) as markers the of cellular energy metabolism in cerebral small vessel disease (SVD).
32 patients with SVD and 17 age-matched healthy control subjects were examined with 3-dimensional 31P MRSI and oxygenation-sensitive quantitative T2′ mapping (1/T2′ = 1/T2* - 1/T2) at 3 Tesla (T). PHi was measured within the white matter hyperintensities (WMH) in SVD patients. Quantitative T2′ values were averaged across the entire white matter (WM). Furthermore, T2′ values were extracted from normal-appearing WM (NAWM) and the WMH and compared between patients and controls.
Quantitative T2′ values were significantly increased across the entire WM and in the NAWM in patients compared to control subjects (149.51 ± 16.94 vs. 138.19 ± 12.66 ms and 147.45 ± 18.14 vs. 137.99 ± 12.19 ms, p < 0.05). WM T2′ values correlated significantly with the WMH load (ρ=0.441, p = 0.006). Increased T2′ was significantly associated with more alkaline pHi (ρ=0.299, p < 0.05). Both T2′ and pHi were significantly positively correlated with vascular pulsatility in the distal carotid arteries (ρ=0.596, p = 0.001 and ρ=0.452, p = 0.016).
This exploratory study found evidence of impaired cerebral OEF in SVD, which is associated with intracellular alkalosis as an adaptive mechanism. The employed techniques provide new insights into the pathophysiology of SVD with regard to disease-related consequences on the cellular metabolic state.
Previous diffusion tensor imaging (DTI) studies indicate that impaired microstructural integrity of the normal-appearing white matter (NAWM) is related to cognitive impairment in cerebral small ...vessel disease (SVD). This study aimed to investigate whether quantitative T2 relaxometry is a suitable imaging biomarker for the assessment of tissue changes related to cognitive abnormalities in patients with SVD. 39 patients and 18 age-matched healthy control subjects underwent 3 T magnetic resonance imaging (MRI) with T2-weighted multiple spin echo sequences for T2 relaxometry and DTI sequences, as well as comprehensive cognitive assessment. Averaged quantitative T2, fractional anisotropy (FA) and mean diffusivity (MD) were determined in the NAWM and related to cognitive parameters controlling for age, normalized brain volume, white matter hyperintensity volume and other conventional SVD markers. In SVD patients, quantitative T2 values were significantly increased compared to controls (p = 0.002) and significantly negatively correlated with the global cognitive performance (r= –0.410, p = 0.014) and executive function (r= –0.399, p = 0.016). DTI parameters did not correlate with cognitive function. T2 relaxometry of the NAWM seems to be sensitive to microstructural tissue damage associated with cognitive impairment in SVD and might be a promising imaging biomarker for evaluation of disease progression and possible effects of therapeutic interventions.
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