Meral Reyhan

Fourier analysis of STimulated echoes (FAST) for the quantitative analysis of left ventricular twist.

To validate a novel method for the rapid and facile quantification of left ventricular (LV) twist from tagged magnetic resonance images and demonstrate the potential clinical utility in a series of 20 healthy volunteers.

Characterization of the effect of MRI on Gafchromic film dosimetry.

Magnetic resonance (MR) imaging of Gafchromic film causes perturbation to absolute dosimetry measurements; the purpose of this work was to characterize the perturbation and develop a correction method for it. Three sets of Gafchromic EBT2 film were compared: radiation (control), radiation followed by MR imaging (RAD+B), and MR imaging followed by radiation (B+RAD). The T1‐weighted and T2‐weighted MR imaging was performed using a 1.5T scanner with the films wedged between two chicken legs. Doses from 0 to 800 cGy were delivered with a 6MV linac. The time interval between radiation and MR imaging was less than 10 min. Film calibration was generated from the red channel. Microscopic imaging was performed on two pieces of film. The effect of specific absorption rate (SAR) was determined by exposing another three sets of films to low, medium, and high levels of SAR through a series of pulse sequences. No discernible preferential alignment was detected on the microscopic images of the irradiated film exposed to MRI. No imaging artifacts were introduced by Gafchromic film on any MR images. On average, 4% dose difference was observed between B+RAD or RAD+B and the control, using the same calibration curve. The pixel values between the B+RAD or RAD+B and the control films were found to follow a linear relationship pixel(Control)=1.02×pixel(B+RAD or RAD+B). By applying this correction, the average dose error was reduced to approximately 2%. The SAR experiment revealed a dose overestimation with increasing SAR even when the correction was applied. It was concluded that MR imaging introduces perturbation on Gafchromic film dose measurements by 4% on average, compared to calibrating the film without the presence of MRI. This perturbation can be corrected by applying a linear correction to the pixel values. Additionally, Gafchromic film did not introduce any imaging artifacts in any of the MR images acquired. PACS number: 87.50.cmMagnetic resonance (MR) imaging of Gafchromic film causes perturbation to absolute dosimetry measurements; the purpose of this work was to characterize the perturbation and develop a correction method for it. Three sets of Gafchromic EBT2 film were compared: radiation (control), radiation followed by MR imaging (RAD+B), and MR imaging followed by radiation (B+RAD). The T1-weighted and T2-weighted MR imaging was performed using a 1.5T scanner with the films wedged between two chicken legs. Doses from 0 to 800 cGy were delivered with a 6MV linac. The time interval between radiation and MR imaging was less than 10 min. Film calibration was generated from the red channel. Microscopic imaging was performed on two pieces of film. The effect of specific absorption rate (SAR) was determined by exposing another three sets of films to low, medium, and high levels of SAR through a series of pulse sequences. No discernible preferential alignment was detected on the microscopic images of the irradiated film exposed to MRI. No imaging artifacts were introduced by Gafchromic film on any MR images. On average, 4% dose difference was observed between B+RAD or RAD+B and the control, using the same calibration curve. The pixel values between the B+RAD or RAD+B and the control films were found to follow a linear relationship pixel(Control)=1.02×pixel(B+RAD or RAD+B). By applying this correction, the average dose error was reduced to approximately 2%. The SAR experiment revealed a dose overestimation with increasing SAR even when the correction was applied. It was concluded that MR imaging introduces perturbation on Gafchromic film dose measurements by 4% on average, compared to calibrating the film without the presence of MRI. This perturbation can be corrected by applying a linear correction to the pixel values. Additionally, Gafchromic film did not introduce any imaging artifacts in any of the MR images acquired. PACS number: 87.50.cm.

Effect of free-breathing on left ventricular rotational mechanics in healthy subjects and patients with duchenne muscular dystrophy.

Cardiovascular magnetic resonance imaging exams can be performed during free‐breathing. This may be especially important for boys with Duchenne muscular dystrophy (DMD) given their frequently limited breath‐hold abilities. The impact of the respiratory compensation method on quantitative measurements of left ventricular (LV) rotational mechanics is incompletely understood. The purpose of this study was to evaluate differences in LV rotational mechanics acquired during breath‐holding (BH), free‐breathing with averaging (AVG), and free‐breathing with respiratory bellows gating (BEL).

Left ventricular twist and shear in patients with primary mitral regurgitation

To evaluate the relationship between left ventricular (LV) twist, shear, and twist‐per‐volume and the severity of mitral regurgitation (MR). Primary MR is a valvular disorder that induces LV dysfunction. There exist several measures of LV rotational mechanics, but it remains unclear which measure of LV dysfunction best accords with the severity of MR. We hypothesized that LV systolic twist‐per‐volume slope would decrease with increasing severity of MR because of both decreases in rotational mechanics and increases in stroke volumes.

Tagged MRI based cardiac motion modeling and toxicity evaluation in breast cancer radiotherapy.

Recent research showed radiation for breast cancer can increase heart risks (1, 2). In Ref. (2), it has been noted that for every Gy of radiation a women’s heart risk rises 7.4%. However, the correlation between radiation dose and heart tissue damage is still an open problem. A more accurate model of heart damage will significantly improve the heart safety for patients underwent radiotherapy. Modern radiation treatment planning systems (TPS) use computed tomography (CT) images for dose calculation and evaluation. For evaluation of heart toxicity from radiotherapy, the dose-volume histogram (DVH), which is generated by overlying radiation dose distribution on heart delineations in CT images, is widely used. However, there are three major factors that deteriorate the accuracy of TPS-calculated heart dose distribution. First conventional CT is a fundamentally static imaging modality without the capability to capture and depict the cardiac motion. Instead, heart is usually blurred in CT images due to the motion artifacts. Second, without special contrast dye, CT provides limited contrast between blood in heart chambers and the surrounding myocardium. The heart region in TPS is actually a mixture of myocardium and blood, although only the radiation dose to the myocardium is accountable for heart risks. Finally, there is significant intra- and inter-fractional heart motion. As heart beats involuntarily during and between radiation treatments, myocardium deforms and moves non-rigidly against the fixed radiation beam so that the static dose distribution calculated in CT based TPS does not reflect the accurate radiation dose distribution in heart. There is also concern on the choice of the heart function for the evaluation of radiation damage. Based on radiation beam geometry, only part of the heart will receive clinically significant level of radiation during breast cancer treatment. It is possible that the global heart function remains stable temporarily while cells in the irradiated part of the myocardium lose part or all of their functions. In this case, regional heart function, which can be derived from regional heart wall motion and strain analysis, is a better indication of heart damage corresponding to radiation dose. Although cardiac MRI is widely used in radiology for the diagnosis of heart disease, its application in radiation treatment planning is limited. For multiple reasons, it is not practical to use MRI directly for radiation treatment planning of breast cancer patients. However, via multimodality deformable image registration (DIR) between MRI and CT, MRI images may play a more critical role in the evaluation of the heart damage from whole breast radiation. Tagged MRI (tMRI) (3) is a relatively new imaging protocol that has been implemented in the detection and diagnosis of regional heart functional loss. tMRI methods record regional heart wall motion information as they create identifiable landmark bands (tags) in the myocardium to establish dense point to point correspondence between images. ECG-gated tMRI image sets can be acquired at different phases of the cardiac cycle using the corresponding pulse sequence. The 4D (3D plus time) cardiac motion model can be retrieved by image registration between tMRIs at different phases. In the following sessions, we use tMRI as an example to explain how additional heart function information in MRI is retrieved. It is our objective to demonstrate the additional information retrieved from MRI can help the evaluation and protection of heart risks for breast cancer patients, and we want to discuss the possibility of using MRI to establish a more accurate correlation between regional heart functional loss and radiation dose.

Depth dose perturbation by a hydrogel fiducial marker in a proton beam

The purpose of this study was to evaluate proton depth dose perturbation caused by a radio‐opaque hydrogel fiducial marker. Electronic proton stopping powers in the hydrogel were calculated for energies 0.5–250 MeV, and Monte Carlo simulations were generated of hydrogel vs. gold markers placed at various water phantom depths in a generic proton beam. Across the studied energy range, the gel/water stopping power ratio was 1.0146 to 1.0160. In the Monte Carlo simulation, the hydrogel marker caused no discernible perturbation of the proton percent depth‐dose (PDD) curve. In contrast, the gold marker caused dose reductions of as much as 20% and dose shadowing regions as long as 6.5 cm. In contrast to gold markers, the radio‐opaque hydrogel marker causes negligible proton depth dose perturbation. This factor may be taken into consideration for image‐guided proton therapy at facilities with suitable imaging modalities. PACS number: 87.55.Qr

Off-Resonance Insensitive Complementary SPAtial Modulation of Magnetization (ORI-CSPAMM) for Quantification of Left Ventricular Twist

To evaluate Off Resonance Insensitive Complementary SPAtial Modulation of Magnetization (ORI‐CSPAMM) and Fourier Analysis of STimulated echoes (FAST) for the quantification of left ventricular (LV) systolic and diastolic function and compare it with the previously validated FAST+SPAMM technique.

Intra- and Interscan Reproducibility Using Fourier Analysis of STimulated Echoes (FAST) for the Rapid and Robust Quantification of Left Ventricular Twist

To assess the intra‐ and interscan reproducibility of LV twist using FAST. Assessing the reproducibility of the measurement of new MRI biomarkers is an important part of validation. Fourier Analysis of STimulated Echoes (FAST) is a new MRI tissue tagging method that has recently been shown to compare favorably with conventional estimates of left ventricular (LV) twist from cardiac tagged images, but with significantly reduced user interaction time.

Quantitative assessment of systolic and diastolic left ventricular twist using Fourier Analysis of STimulated echoes (FAST) and CSPAMM

Summary The FAST method for measuring left ventricular twist has been expanded to semi-automatically measure torsion, peak systolic twist rate, peak diastolic untwisting rate, time to peak twist, and duration of untwisting using FAST+CSPAMM. Background Alterations in left ventricular (LV) twist are important for many pathologies including aortic stenosis[1], diastolic dysfunction[2] and aging[3]. LV twist is a measure of the rotation of the apex relative to the base of the heart. Employing a previously validated method, Fourier Analysis of STimulated echoes (FAST)[4], which measures object rotation directly in Fourier space, we performed quantitative analysis of LV twist, torsion, twisting rates, time to peak twist, and duration of untwisting in thirteen (n=13) healthy volunteers using SPAMM and CSPAMM[5] tagged images. FAST analysis typically requires 2-3 minutes of user interaction time. Methods A spoiled gradient echo pulse sequence was modified to support CSPAMM and used to acquire short-axis images in healthy volunteers (n=13) at the LV base and apex with the following parameters: 360-300x300280mm FOV, 5-6mm slice thickness, 192x144 acquisition matrix, 501 Hz/pixel receiver bandwidth, TE/ TR=3.5-3.7/4.7-6.5ms, 10mm tag spacing, 7-8 ky-lines per segment, 3/4 partial Fourier imaging, 14-16 cardiac phases, 15° FA and two averages for SPAMM imaging, and non-linearly ramped imaging flip angles (initial angle 22°) for CSPAMM[6]. Estimates of LV rotation at basal and apical slice levels for all frames were obtained from the rotation of the stimulated echo and stimulated anti-echo about the FID in Fourier Space using the FAST method[4]. Bland-Altman analysis, linear regression, paired t-test, and Wilcoxon signed-rank test were used to compare FAST+CSPAMM to a previously validated FAST+SPAMM technique. Only data obtained during the first 500ms was used for comparison of SPAMM and CSPAMM due to tag fading. P<0.01 was considered statistically significant. Results The comparison of FAST+CSPAMM to FAST+SPAMM for twist, torsion, twisting rates, time of peak twist and duration of untwisting are summarized in Table 1. Table 2 provides peak rotation and time to peak rotation for systole and diastole for the apical and basal slices, which are important metrics in aortic stenosis[1] and aging[3]. No significant differences were found for twist, torsion, or twist rate measurements made with FAST+CSPAMM to FAST+SPAMM using the paired ttest or the Wilcoxon signed-rank test. Bland-Altman analysis and linear regression demonstrate excellent agreement between techniques (Table 1). These values match well with literature[7]. Conclusions The FAST+CSPAMM is a semi-automated method that provides a quick and quantitative assessment of systolic and diastolic left ventricular twist, torsion, and twisting rates. When FAST is used in conjunction with CSPAMM duration of untwisting can be measured, which may provide further insight into left ventricular diastolic dysfunction.

Complementary radial tagging for improved myocardial tagging contrast

To develop and evaluate complementary radial tagging (CRT) for improved myocardial tagging contrast.