Magnetic Resonance Materials in Physics, Biology and Medicine

Chúng tôi nhằm điều tra ảnh hưởng của việc hiệu chỉnh $$ T_{2}^{*} $$ đến ước lượng các tham số động học từ dữ liệu chụp cộng hưởng từ động tương phản (DCE) T 1-được trọng số khi một hàm đầu vào động mạch (AIF) từ mô tham chiếu được sử dụng. Dữ liệu DCE-MRI được thu thập từ bảy con chuột có khối u vú 4T1 bằng cách sử dụng chuỗi echo gradient đôi ở 7 T. AIF được ước lượng từ một vùng quan tâm trong cơ. Mô hình Tofts mở rộng được sử dụng để ước lượng các tham số dược động học trong phần khối u đang tăng cường, có và không có hiệu chỉnh $$ T_{2}^{*} $$ của tổn thương và AIF. Các tham số được ước lượng với việc hiệu chỉnh $$ T_{2}^{*} $$ cả của AIF và đường cong cường độ thời gian của tổn thương được cho là tiêu chuẩn tham chiếu. Đối với toàn bộ quần thể, có sự khác biệt có ý nghĩa thống kê (p < 0.05) trong hằng số chuyển giao (K trans) giữa phương pháp đã hiệu chỉnh $$ T_{2}^{*} $$ và không hiệu chỉnh, nhưng không có sự khác biệt trong tỷ lệ thể tích liên kẽ (v e). Về mặt cá nhân, không có sự khác biệt có ý nghĩa nào được tìm thấy trong K trans và v e của bốn và sáu khối u, tương ứng, giữa các phương pháp đã hiệu chỉnh $$ T_{2}^{*} $$ và không hiệu chỉnh. Ngược lại, K trans bị đánh giá quá thấp một cách có ý nghĩa nếu việc hiệu chỉnh $$ T_{2}^{*} $$ không được sử dụng, trong các khối u khác mà K trans trung vị lớn hơn 0.4 min−1. Ảnh hưởng của $$ T_{2}^{*} $$ đến các khối u có K trans cao có thể không thể xem nhẹ trong phân tích mô hình động học, ngay cả khi AIF được ước lượng từ mô tham chiếu nơi nồng độ của tác nhân tương phản tương đối thấp.

To determine whether cumulative brain damage produced adjacent to a minor stroke that is followed by a mild transient ischemia is detectable with MRI and histology, and whether acute or chronic recovery between insults influences this damage. A minor photothrombotic (PT) stroke was followed acutely (1–2 days) or chronically (7 days) by a mild transient middle cerebral artery occlusion (tMCAO). MRI was performed after each insult, followed by final histology. The initial PT produced small hyperintense T2 and DW infarct lesions and peri-lesion regions of scattered necrosis and modestly increased T2. Following tMCAO, in a slice and a region adjacent to the PT, a region of T2 augmentation was observed when recovery between insults was acute but not chronic. Within the PT slice, a modest region of exacerbated T2 change proximate to the PT was also observed in the chronic group. Corresponding histological changes within regions of augmented T2 included increased vacuolation and cell death. Within regions adjacent to an experimental minor stroke, a recurrence of a mild transient cerebral ischemia augmented T2 above increases produced by tMCAO alone, reflecting increased damage in this region. Exacerbation appeared broader with acute versus chronic recovery between insults.

The PASADENA method has achieved hyperpolarization of 16–20% (exceeding 40,000-fold signal enhancement at 4.7 T), in liquid samples of biological molecules relevant to in vivo MRI and MRS. However, there exists no commercial apparatus to perform this experiment conveniently and reproducibly on the routine basis necessary for translation of PASADENA to questions of biomedical importance. The present paper describes equipment designed for rapid production of six to eight liquid samples per hour with high reproducibility of hyperpolarization. Drawing on an earlier, but unpublished, prototype, we provide diagrams of a delivery circuit, a laminar-flow reaction chamber within a low field NMR contained in a compact, movable housing. Assembly instructions are provided from which a computer driven, semi-automated PASADENA polarizer can be constructed. Together with an available parahydrogen generator, the polarizer, which can be operated by a single investigator, completes one cycle of hyperpolarization each 52 s. Evidence of efficacy is presented. In contrast to competing, commercially available devices for dynamic nuclear polarization which characteristically require 90 min per cycle, PASADENA provides a low-cost alternative for high throughput. This equipment is suited to investigators who have an established small animal NMR and wish to explore the potential of heteronuclear (13 C and 15 N) MRI, MRS, which harnesses the enormous sensitivity gain offered by hyperpolarization.

To develop and evaluate a model for describing the S1 (S+) and S2 (S−) phase in the presence of off-resonance frequency fluctuations, and to evaluate the performance of a novel interleaved navigator echo scheme. Using the extended phase graph model, a linear phase term was added to the evolution of transverse states. An approximation for the total S2 phase was derived with one fit parameter τl, which serves as an effective lifetime of the S2 signal. The model was evaluated using synthetic and in vivo phase evolution data. In addition, a novel interleaved phase correction scheme for the nb-SSFP sequence was applied to BOLD-fMRI data, and the number of activated voxels before and after phase correction was determined. The phases of S1 and S2 signals are significantly different from each other. The proposed nb-SSFP phase model provided a good description of the measured phase evolution data, and the approximate model for the S2 phase provided both at good fit to the data, as well as an effective lifetime of the S2 signal. In some subjects the phase contribution from older pathways was underestimated. In the BOLD-fMRI data, a twofold increase of the number of activated voxels for the S2 signal was observed, compared to no correction and a conventional navigator echo method. The different phase evolution of S1 and S2 signals can be qualitatively described by the proposed model, and detrimental phase history effects are significant at 7 Tesla when not appropriately corrected.

The quantification of choline-containing compounds (Cho) in breast tumors by proton MR spectroscopy (1H-MRS) has been of great interest because such compounds have been linked to malignancy. In this study, an internal reference method for the absolute quantification of Cho metabolite in malignant breast tumors was presented using a clinical 1.5 T scanner. We performed in vitro measurements to examine the accuracy of absolute quantification using four phantoms of known choline chloride concentrations. There was a high correlation between the calculated concentrations by MRS and the known concentrations (r 2 > 0.98). We applied the technique to in vivo breast study conducted on 45 patients with biopsy-confirmed breast cancer. After T 1 and T 2 relaxation times were corrected, the Cho levels in this work had a range of 0.76 – 21.20 mmol/kg from 34 MR spectra of 32 patients with malignant breast lesions. This result was rather consistent with the previously published value (i.e., 1.38 – 10 mmol/kg, Bolan et al. in Magn Reson Med 50:1134–1143, 2003). Therefore, we conclude that the internal method using the fully relaxed water as a reference could be used for quantifying Cho metabolite accurately in breast cancer patients using a clinical 1.5 T scanner.

We aimed to modify our previously published method for arterial input function measurements for evaluation of cerebral perfusion (dynamic susceptibility contrast MRI) such that it can be applied in humans in a clinical setting. Similarly to our previous work, a conventional measurement sequence for dynamic susceptibility contrast MRI is extended with an additional measurement slice at the neck. Measurement parameters at this slice were optimized for the blood signal (short echo time, background suppression, magnitude and phase images). Phase-based evaluation of the signal in the carotid arteries is used to obtain quantitative arterial input functions. In all pilot measurements, quantitative arterial input functions were obtained. The resulting absolute perfusion parameters agree well with literature values (gray and white matter mean values of 46 and 24 mL/100 g/min, respectively, for cerebral blood flow and 3.0% and 1.6%, respectively, for cerebral blood volume). The proposed method has the potential to quantify arterial input functions in the carotid arteries from a direct measurement without any additional normalization.

To review the major hardware components of low-field point-of-care MRI systems which affect the overall sensitivity. Designs for the following components are reviewed and analyzed: magnet, RF coils, transmit/receive switches, preamplifiers, data acquisition system, and methods for grounding and mitigating electromagnetic interference. High homogeneity magnets can be produced in a variety of different designs including C- and H-shaped as well as Halbach arrays. Using Litz wire for RF coil designs enables unloaded Q values of ~ 400 to be reached, with body loss representing about 35% of the total system resistance. There are a number of different schemes to tackle issues arising from the low coil bandwidth with respect to the imaging bandwidth. Finally, the effects of good RF shielding, proper electrical grounding, and effective electromagnetic interference reduction can lead to substantial increases in image signal-to-noise ratio. There are many different magnet and RF coil designs in the literature, and to enable meaningful comparisons and optimizations to be performed it would be very helpful to determine a standardized set of sensitivity measures, irrespective of design.