We used fMRI to assess the human brain areas activated for execution, observation and 1st person motor imagery of a visually guided tracing task with the index finger. Voxel-level conjunction ...analysis revealed several cortical areas activated in common across all three motor conditions, namely, the upper limb representation of the primary motor and somatosensory cortices, the dorsal and ventral premotor, the superior and inferior parietal cortices as well as the posterior part of the superior and middle temporal gyrus including the temporo-parietal junction (TPj) and the extrastriate body area (EBA). Functional connectivity analyses corroborated the notion that a common sensory-motor fronto-parieto-temporal cortical network is engaged for execution, observation, and imagination of the very same action. Taken together these findings are consistent with the more parsimonious account of motor cognition provided by the mental simulation theory rather than the recently revised mirror neuron view Action imagination and observation were each associated with several additional functional connections, which may serve the distinction between overt action and its covert counterparts, and the attribution of action to the correct agent. For example, the central position of the right middle and inferior frontal gyrus in functional connectivity during motor imagery may reflect the suppression of movements during mere imagination of action, and may contribute to the distinction between ‘imagined’ and ‘real’ action. Also, the central role of the right EBA in observation, assessed by functional connectivity analysis, may be related to the attribution of action to the ‘external agent’ as opposed to the ‘self’.
•Action execution, observation and imagination share a largely overlapping sensory-motor system.•Brain imaging data are consistent with the more parsimonious mental simulation account of motor cognition.•EBA circuits may contribute to the distinction between ‘executed’ and ‘observed’ actions.•MFG circuits and the preSMA may contribute to the differentiation between ‘real’ and ‘imagined’ actions.•IFG-IPL circuits may contribute to the distinction between overt and covert actions.
Blood Oxygen Level Dependent (BOLD) is a commonly-used MR imaging technique in studying brain function. The BOLD signal can be strongly affected by specific sequence parameters, especially in small ...field strengths. Previous small-scale studies have investigated the effect of TE on BOLD contrast. This study evaluates the dependence of fMRI results on echo time (TE) during concurrent activation of the visual and motor cortex at 1.5 T in a larger sample of 21 healthy volunteers. The experiment was repeated using two different TE values (50 and 70 ms) in counterbalanced order. Furthermore, T2* measurements of the gray matter were performed. Results indicated that both peak beta value and number of voxels were significantly higher using TE = 70 than TE = 50 ms in primary motor, primary somatosensory and supplementary motor cortices (p < 0.007). In addition, the amplitude of activation in visual cortices and the dorsal premotor area was also higher using TE = 70 ms (p < 0.001). Gray matter T2* of the corresponding areas did not vary significantly. In conclusion, the optimal TE value (among the two studied) for visual and motor activity is 70 ms affecting both the amplitude and extent of regional hemodynamic activation.
The generation of laser based relativistic electron sources involves impressive basic science as well as innovative applications. This study reports first novel qualitative results on polymer-gel ...radiation dosimetry of ultrafast laser-based relativistic electron beams. The fabricated polymer-gels are irradiated by the ultra-high dose rate (FLASH) laser-generated electron beams and then are analysed using magnetic resonance imaging. The reading of the irradiated dosimeters is performed using a clinical 1.5 T Magnetic Resonance Imaging system. Three-dimensional colour parametric T2 maps are then constructed from the original PD-T2 weighted images obtained from the clinical MRI scanner. For comparison, the gels are also irradiated with standard electron beams of various energies utilizing a radiotherapy clinical linear accelerator system. For the calibration measurements of the gel dosimetry method, special calibrated dosimetric films are also implemented. The preliminary results demonstrate the potential of polymer gel dosimetry for 3D-dose-distribution of FLASH type irradiation of laser generated electron beams. Furthermore, they illustrate potential issues related to the polymer gel based dosimetry in challenging irradiation arrangements, such as the oxygen sensitivity and necessity for oxygen impermeable container material.
Objective
To propose a new method of simulating the BOLD contrast using a dynamic, easy to construct and operate, low-cost physical phantom.
Materials and methods
A structure of thin pipelines ...passing through a gel volume was used to simulate blood vessels in human tissue. Quantitative T2*, R2* measurements were used to study the signal change of the phantom. BOLD fMRI experiments and analysis were performed to evaluate its potential use as an fMRI simulator.
Results
Experimental T2*, R2* measurements showed similar behavior with published references. BOLD contrast was successfully achieved with the proposed method. In addition, there were several proposed parameters, like the angle of the phantom relative to B0, which can easily adjust the signal change and the activation area. Coefficients of variation showed good reproducibility within a month period. Statistical
t
maps were produced with in-house software for the BOLD measurements.
Discussion
T2*maps and BOLD images confirm the potential use of this phantom as an fMRI simulator and also as a tool for studying sensitivity and specificity of BOLD sequences/algorithms.
Online magnetic resonance (MR)‐guided radiotherapy is expected to benefit brain stereotactic radiosurgery (SRS) due to superior soft tissue contrast and capability of daily adaptive planning. The ...purpose of this study was to investigate daily adaptive plan quality with setup variations and to perform an end‐to‐end test for brain SRS with multiple metastases treated with a 1.5‐Tesla MR‐Linac (MRL). The RTsafe PseudoPatient Prime brain phantom was used with a delineation insert that includes two predefined structures mimicking gadolinium contrast‐enhanced brain lesions. Daily adaptive plans were generated using six preset and six random setup variations. Two adaptive plans per daily MR image were generated using the adapt‐to‐position (ATP) and adapt‐to‐shape (ATS) workflows. An adaptive patient plan was generated on a diagnostic MR image with simulated translational and rotational daily setup variation and was compared with the reference plan. All adaptive plans were compared with the reference plan using the target coverage, Paddick conformity index, gradient index (GI), Brain V12 or V20, optimization time and total monitor units. Target doses were measured as an end‐to‐end test with two ionization chambers inserted into the phantom. With preset translational variations, V12 from the ATS plan was 17% lower than that of the ATP plan. With a larger daily setup variation, GI and V12 of the ATS plan were 10% and 16% lower than those of the ATP plan, respectively. Compared to the ATP plans, the plan quality index of the ATS plans was more consistent with the reference plan, and within 5% in both phantom and patient plans. The differences between the measured and planned target doses were within 1% for both treatment workflows. Treating brain SRS using an MRL is feasible and could achieve satisfactory dosimetric goals. Setup uncertainties could be accounted for using online plan adaptation. The ATS workflow achieved better dosimetric results than the ATP workflow at the cost of longer optimization time.
•Polyacrylamide gels are better in simulating ADC values compared to sucrose solutions used in this study.•The coefficient of variation of polyacrylamide gels has lower values from sucrose ...solutions.•Weighted Linear is faster than the Non Linear and equally precise for ADC measurements.•The fitting method is a significant parameter in ADC measurements.•An acceptance condition for precise ADC measurements is that SSNR should be higher than 5.
The aim of this study is to introduce a novel DWI-MRI phantom and to compare Apparent Diffusion Coefficient (ADC) measurements, utilizing EPI-DWI and HASTE-DWI sequences and two different fitting algorithms.
23 test tubes with different sucrose concentrations and polyacrylamide gels were used as a phantom for ADC measurements. The phantom was scanned on a clinical MRI system (1.5 T) over a two-month period utilizing an EPI-DWI and a HASTE-DWI sequence. ADC maps were calculated using a Weighted Linear (WL) and a Non Linear (NL) fitting algorithm. Measurements were performed with two sequences and two fitting algorithms. Geometric Distortions (GD), Ghosting Ratios (GR) and Signal to Structured Noise Ratios (SSNRs) were estimated using both sequences from the resultant ADC parametric maps.
Polyacrylamide gels reveal lower coefficient of variation (CV%) as compared to sucrose solutions. ADC measurements performed with WL and NL algorithms reveal identical results with both sequences. WL and NL algorithms require approx. 3 s and 7 min respectively, for a single slice. EPI-DWI reveals a mean percent ADC value difference of (+4.5%) as compared to HASTE-DWI, regardless the type of fitting algorithm.
Polyacrylamide gels can serve as a better means for simulating ADC values, compared with sucrose solutions used in this study. WL can be proposed as the method for ADC measurements in daily clinical practice. WL is significantly faster than NL fitting method and equally precise. SSNR measured directly on ADC maps is an excellent means for testing the precision of ADC measurements.
The Gamma Knife Icon allows the treatment of brain tumors mask‐based single‐fraction or fractionated treatment schemes. In clinic, uniform axial expansion of 1 mm around the gross tumor volume (GTV) ...and a 1.5 mm expansion in the superior and inferior directions are used to generate the planning target volume (PTV). The purpose of the study was to validate this margin scheme with two clinical scenarios: (a) the patient’s head remaining right below the high‐definition motion management (HDMM) threshold, and (b) frequent treatment interruptions followed by plan adaptation induced by large pitch head motion. A remote‐controlled head assembly was used to control the motion of a PseudoPatient® Prime head phantom; for dosimetric evaluations, an ionization chamber, EBT3 films, and polymer gels were used. These measurements were compared with those from the Gamma Knife plan. For the absolute dose measurements using an ionization chamber, the percentage differences for both targets were less than 3.0% for all scenarios, which was within the expected tolerance. For the film measurements, the two‐dimensional (2D) gamma index with a 2%/2 mm criterion showed the passing rates of ≥87% in all scenarios except the scenario 1. The results of Gel measurements showed that GTV (D100) was covered by the prescription dose and PTV (D95) was well above the planned dose by up to 5.6% and the largest geometric PTV offset was 0.8 mm for all scenarios. In conclusion, the current margin scheme with HDMM setting is adequate for a typical patient’s intrafractional motion.
To optimize clinical T1w and T2w sequences at multiple 1.5T MRI systems of different vendors, using an in house developed phantom simulating neonatal brain relaxometric characteristics.
A neonatal ...brain phantom, fabricated from paramagnetically doped agarose gel solutions, underwent quantitative MR relaxometry utilizing an Inversion Recovery Spin Echo (IRTSE) method to estimate T1 relaxation times and a Car-Purcell-Meiboom-Gill (CPMG) method to estimate T2 relaxation times at multiple 1.5T MRI systems. Multi-TI turbo spin-echo and multi-TE spin-echo sequences were utilized for direct T1 and T2 relaxation time measurements.
Signal data S(TR, TE) were reproduced mathematically according to the standard SE signal acquisition equation: S(TR, TE) = PD*(1-exp(-TR/T1)*(exp(-TE/T2). Consequently, TR and TE values at which the maximum CNR between neonatal gray and white matter was recorded were used for the final optimization of T1w and T2w clinical sequences respectively.
Based on the above values, TR and TE parameters were modified resulting in an optimized T1wSE sequence TR/TE = 1200 ms/10 ms and an optimized T2wTSE sequence TR/TE = 9500 ms/280 ms as estimated on a clinical MAGNETOM Sonata 1.5T system.
Clinical MRI inter-equipment variability for T1 and T2 measurements remained <5% during a total sampling period of 2 months.
TR and TE parameter variation in optimized T1w and T2w sequences remained less than 7% amongst the selected MRI systems included in this study.
CNR optimization of T1w and T2w sequences for neonatal brain imaging at multiple 1.5T clinical MRI systems was accomplished with the aid of an in house developed tissue-mimicking phantom.
To develop an anthropomorphic neonatal 3D printed head phantom which could be used as a quality assurance means for the overall optimization of the neonatal brain clinical T1-weighted (T1w) and ...T2-weighted (T2w) MRI pulse sequences.
Tissue-mimicking materials simulating the MR relaxation times of gray and white matter of the neonatal brain were fabricated and enclosed in five glass tube vials.
A 3D printed neonatal head anthropomorphic phantom, with radiologically bone-equivalent material was fabricated using anonymized CT scans of a specific neonatal subject. The aforementioned tube vials were encapsidated inside the neonatal head phantom cavity. The anthropomorphic phantom with the encapsidated neonatal brain tissue mimicking glass vials underwent quantitative MR relaxometry utilizing an Inversion Recovery Turbo Spin Echo (IRTSE) sequence to estimate T1 relaxation times and a Car-Purcell-Meiboom-Gill (CPMG) sequence to estimate T2 relaxation times on a clinical 1.5T MRI system.
Based on the relaxometric T1 and T2 measurements, the optimal TR and TE values at which the maximum Contrast to Noise Ratio (CNR) between neonatal gray and white matter was recorded were used for the final optimization of the clinical T1w and T2w clinical sequences respectively.
T1 and T2 relaxation times of the tissue mimicking materials were in concordance with the relaxation times of a real neonatal brain subject.
Clinical MRI system’s reproducibility for T1 and T2 measurements remained <5% during a sampling period of 2 months.
The anthropomorphic neonatal 3D printed head phantom served as an excellent means for the general optimization of the geometrical, MR signaling and RF safety characteristics, as well as the optimal acquisition times of the clinical neonatal T1w and T2w MRI pulse sequences.
Optimal values for the TR and TE MRI parameters were assessed in reference to the maximum CNR between neonatal brain gray and white matter. These were: (TR/TE = 1200 ms/10 ms) for the T1wSE and (TR/TE = 9500 ms/280 ms) for the (T2wTSE) sequences respectively.
Overall clinical T1w and T2w sequence optimization for neonatal brain imaging at a 1.5T clinical MRI system was accomplished with the aid of an anthropomorphic neonatal 3D printed head phantom encapsidated with neonatal brain tissue mimicking materials.
Introduction Increasing interest in developing reliable and reproducible models to study angiogenesis has emerged due to recent advances in the treatment of eye disease with pathologic angiogenesis. ...Purpose The purpose of this study was to evaluate Corneal Neovascularization in rabbits using High Resolution Magnetic Resonance Imaging (HR-MRI). Materials and methods Animal experiments were performed with institutional Animal Care Committee approval. Corneal neovascularization was induced with a silk suture of the corneal stroma in 3 rabbits. A High-Resolution (HR) 84 × 84 × 600 μ m3 MRI protocol was developed on a 1.5-T clinical system and applied in the left eyes, utilizing a small field of view surface coil. The area of corneal neovascularization was evaluated before and after the intravenous injection of different concentrations (0.2–0.02 mmol/kgr) of gadolinium-DTPA. Signal-to-Noise Ratios (SNR) and Contrast-to-Noise Ratios (CNR) amongst specific eye anatomical areas were calculated for each sequence. Results The neovascularized area was clearly visualized using MRI system. High concentration of gadolinium provides better delineation and visualization of the neovascularized area as compared with lower concentrations of the contrast agent. Even though the highest concentration of the contrast agent provides the best overall image quality, the neovascularized area presents high SNR even with the lowest concentration of the contrast agent. Conclusion MRI offers unique advantages over existing ocular imaging techniques, including the ability to image multiple layers without depth limitation and to provide multiple clinically relevant data in a single setting.
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