Paul Dasari

MRI investigation of the linkage between respiratory motion of the heart and markers on patient's abdomen and chest: Implications for respiratory amplitude binning list-mode PET and SPECT studies

Respiratory motion of the heart impacts the diagnostic accuracy of myocardial-perfusion emission-imaging studies. Amplitude binning has come to be the method of choice for binning list-mode based acquisitions for correction of respiratory motion in PET and SPECT. In some subjects respiratory motion exhibits hysteretic behavior similar to damped non-linear cyclic systems. The detection and correction of hysteresis between the signals from surface movement of the patients body used in binning and the motion of the heart within the chest remains an open area for investigation. This study reports our investigation in nine volunteers of the combined MRI tracking of the internal respiratory motion of the heart using Navigators with stereo-tracking of markers on the volunteers chest and abdomen by a visual-tracking system (VTS). The respiratory motion signals from the internal organs and the external markers were evaluated for hysteretic behavior analyzing the temporal correspondence of the signals. In general, a strong, positive correlation between the external marker motion (AP direction) and the internal heart motion (SI direction) during respiration was observed. The average ± standard deviation in the Spearmans ranked correlation coefficient (ρ) over the nine volunteer studied was 0.92±0.1 between the external abdomen marker and the internal heart, and 0.87±0.2 between the external chest marker and the internal heart. However despite the good correlation on average for the nine volunteers, in three studies a poor correlation was observed due to hysteretic behavior between inspiration and expiration for either the chest marker and the internal motion of the heart, or the abdominal marker and the motion of the heart. In all cases we observed a good correlation of at least either the abdomen or the chest with the heart. Based on this result, we propose the use of marker motion from both the chest and abdomen regions when estimating the internal heart motion to detect and address hysteresis when binning list-mode emission data.

Digital anthropomorphic phantoms of non-rigid human respiratory and voluntary body motion for investigating motion correction in emission imaging

The development of methods for correcting patient motion in emission tomography has been receiving increased attention. Often the performance of these methods is evaluated through simulations using digital anthropomorphic phantoms, such as the commonly used extended cardiac torso (XCAT) phantom, which models both respiratory and cardiac motion based on human studies. However, non-rigid body motion, which is frequently seen in clinical studies, is not present in the standard XCAT phantom. In addition, respiratory motion in the standard phantom is limited to a single generic trend. In this work, to obtain a more realistic representation of motion, we developed a series of individual-specific XCAT phantoms, modeling non-rigid respiratory and non-rigid body motions derived from the magnetic resonance imaging (MRI) acquisitions of volunteers. Acquisitions were performed in the sagittal orientation using the Navigator methodology. Baseline (no motion) acquisitions at end-expiration were obtained at the beginning of each imaging session for each volunteer. For the body motion studies, MRI was again acquired only at end-expiration for five body motion poses (shoulder stretch, shoulder twist, lateral bend, side roll, and axial slide). For the respiratory motion studies, an MRI was acquired during free/regular breathing. The magnetic resonance slices were then retrospectively sorted into 14 amplitude-binned respiratory states, end-expiration, end-inspiration, six intermediary states during inspiration, and six during expiration using the recorded Navigator signal. XCAT phantoms were then generated based on these MRI data by interactive alignment of the organ contours of the XCAT with the MRI slices using a graphical user interface. Thus far we have created five body motion and five respiratory motion XCAT phantoms from the MRI acquisitions of six healthy volunteers (three males and three females). Non-rigid motion exhibited by the volunteers was reflected in both respiratory and body motion phantoms with a varying extent and character for each individual. In addition to these phantoms, we recorded the position of markers placed on the chest of the volunteers for the body motion studies, which could be used as external motion measurement. Using these phantoms and external motion data, investigators will be able to test their motion correction approaches for realistic motion obtained from different individuals. The non-uniform rational B-spline data and the parameter files for these phantoms are freely available for downloading and can be used with the XCAT license.

Correction of hysteretic respiratory motion in SPECT myocardial perfusion imaging: Simulation and patient studies

Purpose: Amplitude‐based respiratory gating is known to capture the extent of respiratory motion (RM) accurately but results in residual motion in the presence of respiratory hysteresis. In our previous study, we proposed and developed a novel approach to account for respiratory hysteresis by applying the Bouc–Wen (BW) model of hysteresis to external surrogate signals of anterior/posterior motion of the abdomen and chest with respiration. In this work, using simulated and clinical SPECT myocardial perfusion imaging (MPI) studies, we investigate the effects of respiratory hysteresis and evaluate the benefit of correcting it using the proposed BW model in comparison with the abdomen signal typically employed clinically. Methods: The MRI navigator data acquired in free‐breathing human volunteers were used in the specially modified 4D NCAT phantoms to allow simulating three types of respiratory patterns: monotonic, mild hysteresis, and strong hysteresis with normal myocardial uptake, and perfusion defects in the anterior, lateral, inferior, and septal locations of the mid‐ventricular wall. Clinical scans were performed using a Tc‐99m sestamibi MPI protocol while recording respiratory signals from thoracic and abdomen regions using a visual tracking system (VTS). The performance of the correction using the respiratory signals was assessed through polar map analysis in phantom and 10 clinical studies selected on the basis of having substantial RM. Results: In phantom studies, simulations illustrating normal myocardial uptake showed significant differences (P < 0.001) in the uniformity of the polar maps between the RM uncorrected and corrected. No significant differences were seen in the polar map uniformity across the RM corrections. Studies simulating perfusion defects showed significantly decreased errors (P < 0.001) in defect severity and extent for the RM corrected compared to the uncorrected. Only for the strong hysteretic pattern, there was a significant difference (P < 0.001) among the RM corrections. The errors in defect severity and extent for the RM correction using abdomen signal were significantly higher compared to that of the BW (severity = −4.0%, P < 0.001; extent = −65.4%, P < 0.01) and chest (severity = −4.1%, P < 0.001; extent = −52.5%, P < 0.01) signals. In clinical studies, the quantitative analysis of the polar maps demonstrated qualitative and quantitative but not statistically significant differences (P = 0.73) between the correction methods that used the BW signal and the abdominal signal. Conclusions: This study shows that hysteresis in respiration affects the extent of residual motion left in the RM‐binned data, which can impact wall uniformity and the visualization of defects. Thus, there appears to be the potential for improved accuracy in reconstruction in the presence of hysteretic RM with the BW model method providing a possible step in the direction of improvement.

The effect of time-of-flight and point spread function modeling on 82 Rb myocardial perfusion imaging of obese patients

BackgroundThe effect of time-of-flight (TOF) and point spread function (PSF) modeling in image reconstruction has not been well studied for cardiac PET. This study assesses their separate and combined influence on 82Rb myocardial perfusion imaging in obese patients.MethodsThirty-six obese patients underwent rest-stress 82Rb cardiac PET. Images were reconstructed with and without TOF and PSF modeling. Perfusion was quantitatively compared using the AHA 17-segment model for patients grouped by BMI, cross-sectional body area in the scanner field of view, gender, and left ventricular myocardial volume. Summed rest scores (SRS), summed stress scores (SSS), and summed difference scores (SDS) were compared.ResultsTOF improved polar map visual uniformity and increased septal wall perfusion by up to 10%. This increase was greater for larger patients, more evident for patients grouped by cross-sectional area than by BMI, and more prominent for females. PSF modeling increased perfusion by about 1.5% in all cardiac segments. TOF modeling generally decreased SRS and SSS with significant decreases between 2.4 and 3.0 (P < .05), which could affect risk stratification; SDS remained about the same. With PSF modeling, SRS, SSS, and SDS were largely unchanged.ConclusionTOF and PSF modeling affect regional and global perfusion, SRS, and SSS. Clinicians should consider these effects and gender-dependent differences when interpreting 82Rb perfusion studies.Spanish AbstractAntecedentesEl efecto de los algoritmos de reconstrucción “time of flight” (TOF) y “point spread function” (PSF) en la reconstrucción de imágenes no ha sido bien estudiado para el PET cardiaco. Este estudio evalúa su influencia en por separado y combinado en los estudios de imagen de perfusión miocárdica con 82Rb en pacientes obesos.MétodosTreinta y seis pacientes obesos fueron sometidos a un PET cardiaco 82Rb en estrés y en reposo. Las imágenes fueron reconstruidas con y sin TOF y PSF. La perfusión fue comparada cuantitativamente utilizando el modelo segmentario AHA17 para pacientes agrupados por IMC, área corporal transversal in el campo de vista del escáner, sexo y volumen ventricular izquierdo miocárdico. Los puntajes sumados de reposo (SRS), los puntajes sumados de estrés (SSS) y el puntaje diferencial sumado (SDS) fueron comparados.ResultadosEl TOF mejoró la uniformidad visual del mapa polar e incrementó la perfusión de la pared septal hasta un 10%. Este incremento fue mayor para pacientes más grandes, más evidentemente en pacientes agrupados por área transversal que por IMC, y siendo más prominente en mujeres. El PSF aumentó la perfusión por cerca de 1.5% en todos los segmentos cardiacos. El TOF generalmente disminuyó el SRS y el SSS con disminuciones significativas entre 2.4 y 3 (P < .05), lo cual podría afectar la estratificación por riesgo; el SDS permanece igual. Con el modelamiento PSF, el SRS, el SSS y el SDS no presentaron cambios.ConclusiónEl TOF y el PSF afecta a la perfusión regional y global, el SRS y el SSS. Los clínicos deberían considerar estos efectos y las diferencias dependientes de sexo cuando se interpretan los estudios de perfusión con 82Rb.Chinese Abstract背景飞行时间(TOF)和点扩散函数(PSF)建模对于心脏 PET 成像重建的影响尚未完善建立。本研究评估其单独以及联合使用对肥胖病人行铷 82 心肌灌注成像的影响。方法36 个肥胖病人接受静息-负荷的铷 82 心脏 PET 成像扫描。图像分别在有无 TOF 和 PSF 建模的情况下被重建。病人按照 BMI、扫描仪视野下横断面的体表面积、性别和左室容积进行分组,采用 AHA 17节段模型量化对比灌注情况,比较静息灌注总积分(SRS),负荷灌注总积分(SSS)和灌注总积分差值(SDS)。结果TOF 改进了靶心图的视觉一致性, 间隔壁的灌注增加了10%。这种增加表现为: 体型越大的病人增加越大, 以横断面体表面积分组的病人比用 BMI 分组的病人增加更明显,女性比男性增加更突出。在所有的心脏节段中, PSF 建模增加了约 1.5% 的灌注。TOF 建模总体上显著降低了SRS和 SSS(在 2.4 和 3.0 之间, P < .05), 这会影响风险分层; SDS 保持不变。利用 PSF 建模, SRS, SSS 和 SDS 在很大程度上保持不变。结论TOF 和 PSF 建模影响局部和整体灌注、SRS 以及 SSS。当阅读铷 82 灌注图像时, 临床医生应该考虑这些因素的影响以及性别导致的不同。French AbstractContexteL’effet de la modélisation du temps de vol (TOF) et de la fonction d’étalement ponctuel (PSF) pour la reconstruction d’images n’a pas été bien étudiée pour la TEP en cardiologie. Cette étude évalue l’influence séparée et combinée des ces deux facteurs sur la perfusion myocardique par imagerie au 82Rb chez les patients obèses.MéthodesTrente-six patients obèses ont été soumis à une étude TEP repos-effort au 82Rb au repos. Les images ont été reconstruites avec et sans modélisation TOF et PSF. Les résultats de la perfusion myocardique a été comparée quantitativement en utilisant le modèle de 17 segments de l’American Heart Association (AHA). Les patients ont été groupés selon l’index de leur masse corporelle (IMC), et selon leur dimension corporelle transversale dans le champ de vision du scanner, sexe et volume myocardique ventriculaire gauche. Les score de perfusion myocardique au repos (SRS), après effort (SSS) et les scores différentiels (SDS) ont été comparés.RésultatsTOF améliore l’uniformité visuelle de la carte polaire et augmente la perfusion de la paroi septale de 10%. Cette augmentation est plus importante chez les patients de grande taille et plus apparente chez les patients groupés selon leur dimension corporelle transversale zone plutôt que par l’IMC, et plus élevée chez les femmes. La modélisation PSF augmente la perfusion d’environ 1,5% dans tous les segments cardiaques. La modélisation TOF diminue significativement les scores SRS et le SSS de 2,4 et 3,0 points (P < 0,05), ce qui peut changer la stratification; le score SDS est dans l’ensemble inchangé. Avec la modélisation PSF, SRS, SSS et SDS sont largement inchangés.ConclusionLa modélisation TOF et PSF affectent la perfusion régionale et globale, SRS et SSS. Les cliniciens devraient tenir compte de ces effets et des différences entre les sexes lors de l’interprétation 82Rb études de perfusion.

Creation of 3D Digital Anthropomorphic Phantoms which Model Actual Patient Non-rigid Body Motion as Determined from MRI and Position Tracking Studies of Volunteers

Patient motion can cause artifacts, which can lead to difficulty in interpretation. The purpose of this study is to create 3D digital anthropomorphic phantoms which model the location of the structures of the chest and upper abdomen of human volunteers undergoing a series of clinically relevant motions. The 3D anatomy is modeled using the XCAT phantom and based on MRI studies. The NURBS surfaces of the XCAT are interactively adapted to fit the MRI studies. A detailed XCAT phantom is first developed from an EKG triggered Navigator acquisition composed of sagittal slices with a 3 x 3 x 3 mm voxel dimension. Rigid body motion states are then acquired at breath-hold as sagittal slices partially covering the thorax, centered on the heart, with 9 mm gaps between them. For non-rigid body motion requiring greater sampling, modified Navigator sequences covering the entire thorax with 3 mm gaps between slices are obtained. The structures of the initial XCAT are then adapted to fit these different motion states. Simultaneous to MRI imaging the positions of multiple reflective markers on stretchy bands about the volunteers chest and abdomen are optically tracked in 3D via stereo imaging. These phantoms with combined position tracking will be used to investigate both imaging-data-driven and motion-tracking strategies to estimate and correct for patient motion. Our initial application will be to cardiacperfusion SPECT imaging where the XCAT phantoms will be used to create patient activity and attenuation distributions for each volunteer with corresponding motion tracking data from the markers on the body-surface. Monte Carlo methods will then be used to simulate SPECT acquisitions, which will be used to evaluate various motion estimation and correction strategies.

The Effect of Time-of-Flight and Point Spread Function Modeling on Quantitative Cardiac PET of Large Patients: Phantom Studies

The purpose of this paper was to evaluate the effects of time-of-flight (TOF) and point spread function (PSF) modeling on quantitation accuracy in cardiac positron emission tomography (PET) of large patients. Medium and large anthropomorphic cardiac torso phantoms with three anatomical configurations were used in simulating medium and large patients with: 1) arms inside the field of view (FOV); 2) breast for female anatomy; and 3) arms outside the FOV. The effects of TOF and PSF modeling were assessed under two experimental conditions, with and without mismatch in CT attenuation-correction (CTAC) maps. The PET data were reconstructed using analytical and iterative algorithms which included TOF and PSF in combination with six incremental post-reconstruction smoothing filter widths. Polar maps were created by sampling the left ventricle cardiac insert and used for further analyses. The quantitation accuracy of global and regional activity estimates were quantified with contrast recovery coefficient and coefficient of variation. Relative activity bias in the polar maps was compared between the mismatched CTAC reconstructions and their reference matched CTAC reconstructions. The results show that the global and regional quantitation accuracy generally improves with both TOF and PSF modeling; however, with PSF modeling there is an increased variability. For both phantoms, the best quantitation accuracy with least variability was achieved for reconstruction with TOF, PSF modeling, and small post-reconstruction smoothing filter. The relative degree of improvement in quantitation accuracy and uniformity was higher with TOF for the large phantom than the medium phantom. Mismatch artifacts in CTAC maps affected the quantitation accuracy; TOF had a modest effect in reducing the errors, while PSF modeling had little effect.

Influence of time-of-flight and point spread function modeling in myocardial perfusion imaging of large patients

Using a medium and large sized anthropomorphic cardiac-torso phantoms we studied the effects of TOF and PSF to evaluate Rb-82 cardiac imaging of large patients. The reconstructed images were analyzed for recovery coefficient and RMS deviations on the polar plot data. Modelling PSF resulted in higher recovery coefficients while the reconstructions with TOF yielded reduced RMS deviations compared to non-TOF counterparts.

Toward a framework for high resolution parametric respiratory motion modelling

A framework to facilitate the realization of dynamic MRI data with simultaneously increased temporal and spatial resolution is proposed. Deformable registration to a reference frame of spatially sparse, high temporally resolved two dimensional sagittal slices acquired sequentially across a volunteers lateral dimension serve as a subset of incomplete observation of full three dimensional vector fields. Registration of an averaged, re-binned, single respiratory cycle with full spatial sampling serves as a basis for estimation of full three dimensional vector fields derived from the sparse subset. The inverse of the estimated full 3D vector fields from sparse measurements allows propagation of a high resolution, re binned static volume with breathing modes derived from the sparse dynamic data. Proof of concept experiments are undertaken with the anthropomorphic XCAT phantom. A quantitative evaluation of full vector fields derived from sparse samples in comparison to their ground truth results in a mean error of the order of 1mm. A qualitative assessment of the motion of a propagated high resolution static MRI volume is presented.

Interactive generation of digital anthropomorphic phantoms from XCAT shape priors

In SPECT imaging, patient respiratory and body motion can cause artifacts that degrade image quality. Developing and evaluating motion correction algorithms are facilitated by simulation studies where a numerical phantom and its motion are precisely known, from which image data can be produced. Previous techniques to test motion correction methods generated XCAT phantoms modeled from MRI studies and motion tracking but required manually segmenting the major structures within the whole upper torso, which can take 8 hours to perform. Additionally, segmentation in two dimensional MRI slices and interpolating into three dimensional shapes can lead to appreciable interpolation artifacts as well as requiring expert knowledge of human anatomy in order to identify the regions to be segmented within each slice. We propose a new method that mitigates the long manual segmentation times for segmenting the upper torso. Our interactive method requires that a user provide only an approximate alignment of the base anatomical shapes from the XCAT model with an MRI data. Organ boundaries from aligned XCAT models are warped with displacement fields generated from registering a baseline MR image to MR images acquired during pre-determined motions, which amounts to automated segmentation each organ of interest. With our method we can show the quality of segmentation is equal that of expert manual segmentation does not require a user who is an expert in anatomy, and can be completed in minutes not hours. In some instances, due to interpolation artifacts, our method can generate higher quality models than manual segmentation.

Dense motion propogation from sparse samples for free breathing respiratory motion modelling

The development of models of respiration for motion correction in diagnostic imaging have often utilized dynamic 4D data, on which plausible respiratory motion patterns can be estimated or validated. To date, dynamic 4D MRI has often been used, its attraction lying in its zero radiation burden and large field of view. However, limitations in scanner technology produces poor contrast images when such volumetric data are acquired at high speed (> 1s). Therefore, in this latest work we provide the first demonstration that sparse sampling of dynamic MRI may be used as an alternative approach to successfully build high contrast, high resolution 4D models of free-breathing respiratory motion. This is achieved by constrained articulation of a high contrast/ high spatial resolution single static volume. The articulation is based on estimating, in the eigen domain, complete 4D motion vector fields from sparsely-sampled free-breathing dynamic MRI data, but constrained by an eigenbasis that characterizes a corresponding average 4D single respiratory cycle dataset. We demonstrate that this approach can provide equivalent motion vector fields compared to fully sampled 4D dynamic data, whilst preserving the corresponding high resolution/ high contrast inherent in the corresponding static image volume. Validation is performed on three 4D MRI volunteer datasets using 8 extracted slices from a fast 4D acquisition (0.7sec per volume). The estimated deformation fields from sparse sampling are compared to the fully sampled equivalents, resulting in an rms error of the order of 2mm, validating the approach. We also present exemplar 4D high contrast, high resolution articulated volunteer datasets using this methodology. This approach facilitates greater freedom in the acquisition of free breathing respiratory motion sequences which may be used to inform motion modeling methods in a range of imaging modalities.