Modeling and suppression of respiration-related physiological noise in echo-planar functional magnetic resonance imaging using global and one-dimensional navigator echo correction.

A major source of noise in functional magnetic resonance imaging (fMRI) arises from modulations in the local magnetic field in the head due to motion of the subject's chest through the respiratory cycle, and this physiologic noise can nullify the gains in statistical power expected by the use of higher magnetic fields for fMRI. In particular, fMRI data acquired using echo-planar imaging (EPI) are very sensitive to these spatially and temporally varying respiration-induced frequency offsets. In this study, accurate 3D magnetic field maps in the head were measured and used to determine the frequency offsets at the two extremes of the respiratory cycle. From these maps, spatially dependent frequency variations from about -1.0 Hz to +1.5 Hz were measured in the brain through the respiratory cycle. Simulations of a typical axial EPI fMRI experiment acquired in the presence of this measured field variation were performed, demonstrating regional image intensity variations between 1 and 5% in single pixel time series. The inadequacy of either global or 1D navigator echo corrections to measure and suppress respiratory-induced noise in fMRI time series is demonstrated. The nature of the spatial variations observed suggests that 2D approaches should be considered.     

Design and application of finite impulse response digital filters

The finite impulse response (FIR) digital filter is a spatial domain filter with a frequency domain representation. The theory of the FIR filter is presented and techniques are described for designing FIR filters with known frequency response characteristics. Rational design principles are emphasized based on characterization of the imaging system using the modulation transfer function and physical properties of the imaged objects. Bandpass, Wiener, and low-pass filters were designed and applied to ²⁰¹T1 myocardial images. The bandpass filter eliminates low-frequency image components that represent background activity and high-frequency components due to noise. The Wiener, or minimum mean square error filter sharpens the image while also reducing noise. The Wiener filter illustrates the power of the FIR technique to design filters with any desired frequency reponse. The lowpass filter, while of relative limited use, is presented to compare it with a popular elementary smoothing filter.This study was supported in part by National Institutes of Health Grants No. HL17646 and HL13851.     

Determination of the optimum filter function for SPECT imaging

An observer study was performed in order to evaluate several filters used in SPECT imaging. The filters were applied to the simulated projection data of a uniform activity density cylinder which contained a cold, spherical lesion, 2 cm in diameter. The data incorporated the effects of the detector and scatter response functions, photon attenuation, and noise. Reconstructed transaxial images were used in 2AFC and ROC observer studies testing lesion detectability. In the 2AFC experiment, the Hanning filter scored lowest and did not show a optimum cutoff frequency. The Butterworth filter performed better and showed a well-defined optimum cutoff frequency at 0.15 cycles/pixel. The Metz filter performed as well as the optimum Butterworth but did not show an optimum power factor. In the ROC study, a high power Metz filter demonstrated an ROC curve of lower Az index and different shape from a lower power Metz filter and the optimum Butterworth filter.     

A parallel approach to STAP implementation for fMRI data

PURPOSE: To exploit the capabilities of parallel processing in applying the space-time adaptive processing (STAP) algorithm, previously explored on a small scale for functional magnetic resonance imaging (fMRI) applications, to conventional size fMRI data sets. MATERIALS AND METHODS: STAP is a two-dimensional filter that is able to locate fMRI activations in both space and frequency. It is applied here for the construction of brain activation maps in fMRI using Visual Age C, incorporating Engineering and Scientific Subroutine Library (ESSL) functions, compiled in 64-bit, and executed on an IBM SP supercomputer. RESULTS: Computer simulations incorporating actual MRI noise indicate that STAP, incorporated using the method of steepest descent, is feasible on conventional size data sets and exhibits an improvement in detecting activations over the more traditional cross correlation method of fMRI analysis when the response is unknown. CONCLUSION: STAP is feasible on traditional size fMRI data sets and useful in elucidating spatial and temporal connectivity.     

fMRI of human olfaction at the individual level: Interindividual variability

Purpose:

To determine whether the range of normal variation of human olfactory functional magnetic resonance imaging (fMRI) activations in healthy single subjects is compatible with the detection of atypical patterns.

Materials and Methods:

In an event‐related olfactory experiment, the variability of fMRI activation in six bilateral olfactory areas known to be affected in neurodegenerative diseases was measured in a region of interest (ROI) analysis in terms of intensity, localization, and overlap on 51 subjects. fMRI measurements were compared against measurements from a visual experiment performed on 25 subjects.

Results:

Olfaction induced activations with low intensity, high variability, and a 4‐fold lower contrast‐to‐noise ratio (CNR) than vision. Even in the best case (piriform cortex), mean pairwise activation overlap was still less than 40%. None of the olfactory ROIs showed significant activation for all subjects at the permissive threshold of P < 0.001. A gender‐dependent significantly stronger activation was found in the bilateral piriform cortex of male subjects.

Conclusion:

Linking t‐statistics and CNR showed that for all olfactory ROIs, CNR is either near or below the estimated threshold of 0.73 found to be necessary to obtain significant activations. In our experimental conditions the low reliability of olfactory activations should prompt major reservations over using fMRI of human olfaction as a diagnostic tool in single subjects. J. Magn. Reson. Imaging 2013;37:92–100. © 2012 Wiley Periodicals, Inc.     

Use of finite-memory Wiener filters in scintigram processing

Scintigraphic images are subject to two forms of imperfection: blurring due to the components of the detector (collimator, photomultiplier, analog processing devices) and noise resulting from radioactive disintegration. We use Wiener filters operating within a limited area of the spatial domain in processing these images. After defining statistical models of ideal image and degrading processes, we study the effects of the parameters on filter features. Finally, we present examples demonstrating filter performance.     

Comparison of temporal filtering methods for dynamic contrast MRI myocardial perfusion studies.

Dynamic contrast myocardial perfusion studies may benefit from methods that speed up the acquisition. Unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD), and a similar linear interpolation method have been shown to be effective at reducing the number of phase encodes needed for cardiac wall motion studies by using interleaved sampling and temporal filtering. Here such methods are evaluated in cardiac dynamic contrast studies, with particular regard to the effects of the choice of filter and the interframe motion. Four different filters were evaluated using a motion-free canine study. Full k-space was acquired and then downsampled to allow for a measure of truth. The different filters gave nearly equivalent images and quantitative flow estimates compared to full k-space. The effect of respiratory motion on these schemes was graphically depicted, and the performance of the four temporal filters was evaluated in seven human subjects with respiratory motion present. The four filters provided images of similar quality. However, none of the filters were effective at eliminating motion artifacts. Motion registration methods or motion-free acquisitions may be necessary to make these reduced FOV approaches clinically useful.     

Improved interpretation of gated cardiac images by use of digital filters

The authors describe a digital filter that greatly enhances the quality of gated cardiac blood-pool images. Spatial filtering is accomplished with a minimum-mean-square-error (Wiener) filter incorporating measured camera blur and Poisson noise statistics. A low-pass temporal filter is then applied to each pixel, with the cutoff frequency determined from measurements of frequency spectra in 20 patients. This filter was evaluated in routine clinical use for nearly one year and found to significantly improve chamber definition, delineate wall motion abnormalities better, and reduce noise. To quantitatively assess the effect of the filter on image interpretation, four experienced observers evaluated wall motion in a series of mathematically simulated left ventricular images. ROC analysis revealed that accuracy in assessing wall motion was significantly greater with the filtered images.     

Variation of noise in multi-run functional MRI using generalized autocalibrating partially parallel acquisition (GRAPPA)

Purpose:

To investigate the noise variation in multi‐run functional MRI (fMRI) scans using generalized autocalibrating partially parallel acquisition (GRAPPA), with a focus on the cause of this variation.

Materials and Methods:

A phantom was continuously scanned for 10 runs using echo‐planar imaging (EPI) combined with GRAPPA to simulate a multi‐run fMRI exam. The variation of noise between runs was examined for different GRAPPA acceleration factors. The noise variation was also evaluated in a real fMRI experiment with human subjects at an acceleration factor of two. The cause of noise variation was explored by offline reconstruction using different GRAPPA weights and numerical simulation of GRAPPA reference scans.

Results:

It was found that the noise distribution in the image is stable within a run but may vary randomly from run to run. The variation of noise was also observed in fMRI experiments with human subjects. The variation can be significantly reduced if all the images from individual runs are reconstructed using the same reference scan data.

Conclusion:

Both phantom experiments and human data showed that the noise pattern may change in different fMRI runs. The variation is mainly caused by the random noise in separate reference scans for GRAPPA in each run. J. Magn. Reson. Imaging 2012;462‐470. © 2011 Wiley Periodicals, Inc.     

Functional MR study of a motor task and the Tower of London task at 1.0 T

Introduction The use of functional magnetic resonance imaging (fMRI) for clinical applications and basic neuroscience is constantly increasing. The discussion about minimum performance requirement for a correct implementation of fMRI is still open, and one of the critical points is the magnetic field strength. We tested the feasibility of fMRI at 1.0 T during motor and cognitive tasks.Methods Fourteen healthy subjects were scanned during a motor task and 12 while performing the Tower of London task. In the activated areas, the percentage signal change due to BOLD (blood oxygenation level dependent) contrast was analysed. To check basic image quality of the acquisition system we measured quality indices in a temporal series of images of a phantom.Results Motor and cognitive brain activations matched previous results obtained at higher field strengths. The mean percentage change over subjects in the motor task was in the range 1.3–2.6% for the primary motor area and 0.8–6.7% for the cerebellum. In the cognitive task, the mean percentage change over subjects was 0.7–1.2% for a frontal area and 0.6–2.8% for a parietal area. The percentage noise of the phantom temporal series was less than 0.4%. Percentage changes and signal to noise ratio, although lower than that obtained with high-field systems, allowed activation maps to be obtained in all subjects.Conclusion Our results replicate previous fMRI results demonstrating reproducible motor-related brain activations and extend the field to a complex cognitive task, thus providing evidence of the safety for routine clinical use of 1-T equipment.A.B. and O.R. contributed equally to the realization of the study and to the drafting of the paper.     

Enhancement of SPECT images by Fourier filtering the projection image set

Tomographic images from rotating gamma camera systems are often difficult to interpret because of poor contrast and high noise levels. A method is presented for improving the quality of these images by Fourier filtering the projection image set prior to reconstruction. A two-dimensional circularly symmetric Gaussian function is used as the spatial frequency filter. This filter can be optimized to enhance contrast and suppress noise in the projection image set in a straightforward and simple manner from the power spectra of representative projections. Preprocessing of the projections makes it possible to use a ramp reconstruction filter. The resulting tomographic sections show a dramatic improvement in image quality.     

Spinal fMRI during proprioceptive and tactile tasks in healthy subjects: activity detected using cross-correlation, general linear model and independent component analysis

Introduction  Functional MRI (fMRI) of the spinal cord is able to provide maps of neuronal activity. Spinal fMRI data have been analyzed in previous studies by calculating the cross-correlation (CC) between the stimulus and the time course of every voxel and, more recently, by using the general linear model (GLM). The aim of this study was to compare three different approaches (CC analysis, GLM and independent component analysis (ICA)) for analyzing fMRI scans of the cervical spinal cord. Methods  We analyzed spinal fMRI data from healthy subjects during a proprioceptive and a tactile stimulation by using two model-based approaches, i.e., CC analysis between the stimulus shape and the time course of every voxel, and the GLM. Moreover, we applied independent component analysis, a model-free approach which decomposes the data in a set of source signals. Results  All methods were able to detect cervical cord areas of activity corresponding to the expected regions of neuronal activations. Model-based approaches (CC and GLM) revealed similar patterns of activity. ICA could identify a component correlated to fMRI stimulation, although with a lower statistical threshold than model-based approaches, and many components, consistent across subjects, which are likely to be secondary to noise present in the data. Conclusions  Model-based approaches seem to be more robust for estimating task-related activity, whereas ICA seems to be useful for eliminating noise components from the data. Combined use of ICA and GLM might improve the reliability of spinal fMRI results.     

DTI和fMRI在正常成人视觉系统的联合应用研究

目的:联合应用BOLD-fMRI及DTI两种技术,用量化的指标研究正常成人视觉皮质激活区域的弥散特征以及与之相连视放射的弥散特性之间的关系,评价此种方法在显示脑区连接方面的价值。材料和方法:10名正常志愿者行视觉刺激下的fMRI扫描,应用闪烁的棋盘方格作为任务刺激,同时扫描层厚与位置相同的DTI图像。用SPM99处理fMRI图像,DTVⅡ软件处理DTI图像。将脑的激活图叠加于FA或DEC图,分别测量视放射以及视觉皮质激活区域的FA值,并将二者进行比较。结果:10例均见双侧初级视觉皮质V1以及V2区的激活;5例伴有V5区即视觉运动区(MT区)的激活,其中3例可见外侧膝状体(LGN)的激活信号。视放射FA均值与激活区的FA均值比较有显著性差异(P<0.001)。将激活图叠加于DEC图后,直观显示了前后走行的视放射纤维连接激活的LGN与激活的视皮质。结论:联合fMRI与DTI方法发现BOLD信号主要位于具有各向同性特征的灰质区域,更有助于BOLD的定位;可以直观显示不同脑区之间的纤维连接,有助于研究脑结构与功能之间的关系。     

Optimization of Butterworth filter for brain SPECT imaging

A method has been described to optimize the cutoff frequency of the Butterworth filter for brain SPECT imaging. Since a computer simulation study has demonstrated that separation between an object signal and the random noise in projection images in a spatial-frequency domain is influenced by the total number of counts, the cutoff frequency of the Butterworth filter should be optimized for individual subjects according to total counts in a study. To reveal the relationship between the optimal cutoff frequencies and total counts in brain SPECT study, we used a normal volunteer and^99mTc hexamethyl-propyleneamine oxime (HMPAO) to obtain projection sets with different total counts. High quality images were created from a projection set with an acquisition time of 300-seconds per projection. The filter was optimized by calculating mean square errors from high quality images visually inspecting filtered reconstructed images. Dependence between total counts and optimal cutoff frequencies was clearly demonstrated in a nomogram. Using this nomogram, the optimal cutoff frequency for each study can be estimated from total counts, maximizing visual image quality. The results suggest that the cutoff frequency of Butterworth filter should be determined by referring to total counts in each study.     

Theoretical considerations for evaluating the degree of random-periodicity of radiographic noise.

Radiographic noise properties have been evaluated using the Wiener spectrum. However, this approach is not appropriate for periodic noise for two reasons. One is that it takes infinite values at the spatial frequencies of periodic noise. The other is that when adopting a numerical integration, it allows unstable values at each spatial frequency, depending on the integral region. Introducing three types of spectra (W(1), W(2), and W(3)) in connection with the Wiener spectrum, we propose a practical approach to evaluation of periodic noise. Radiographic images sometimes contain noise that is not totally random and not perfectly periodic. Therefore, using the Wiener spectrum and the W(1) spectrum, we also propose two factors for evaluation of the degree of random-periodicity of noise containing periodic signals.     

Utility of fixed-quantity analysis in evaluating image noise and resolution in multi-slice CT

We analyzed the spatial frequency of images generated with multi-slice CT (Z-axis) and estimated the definition and noise characteristics. The noise characteristics were calculated with Wiener spectra from multi planar reconstruction (MPR) images, and, in the evaluation of definition characteristics, the modulation transfer function (MTF) was calculated using section sensitivity profile (SSP) with the base method. The Wiener spectra showed a tendency for noise to decrease from a low frequency domain as the effective slice thickness increased. In addition, Wiener spectra were not much different in helical pitch because effective slice thickness became thicker with increases in helical pitch. Examination results suggest that quantitative analysis of the characteristics of image noise and resolution in multi-slice CT images may provide an optical parameter for improving the quality of images in clinical data.     

Assessment of coronary artery stents using 16-slice MDCT angiography: evaluation of a dedicated reconstruction kernel and a noise reduction filter

To compare the effect of different reconstruction kernels and a noise-reducing postprocessing filter on the delineation of coronary artery stents in 16-slice CT-angiography. Ten patients with coronary stents (seven LAD, five RCX and three RCA) were examined with a 16-slice MDCT using standard acquisition parameters. Images were reconstructed using a medium soft (B30f) and a dedicated, edge-enhancing kernel (B46f). Additional postprocessing with an edge-preserving filter was performed on B46f images to reduce the image noise. In multiplanar reformations (MPRs) along and perpendicular to the stent axis, intraluminal attenuation values and the visible lumen diameter were measured. Image noise was measured in the subcutaneous fat using a region of interest (ROI) technique. Arterial enhancement in the aorta was 275.1 HU. Attenuation in the stent lumen was 390.4, 340.0 and 346.8 HU in MPRs derived from B30f, original B46 and postprocessed B46f images. The mean noise level was 20.4, 35.0 and 24.9 HU respectively. The visible lumen diameter was significantly greater in B46f and postprocessed B46f images (2.17 and 2.16 mm), compared to 1.93 mm in B30f images (p<0.01). Edge-enhancing reconstruction kernels increase the visible stent lumen, but also increase image noise. Dedicated postprocessing filters can reduce the introduced noise without a loss of spatial resolution.     

Comparison of fMRI coregistration results between human experts and software solutions in patients and healthy subjects

Functional magnetic resonance imaging (fMRI) performed by echo-planar imaging (EPI) is often highly distorted, and it is therefore necessary to coregister the functional to undistorted anatomical images, especially for clinical applications. This pilot study provides an evaluation of human and automatic coregistration results in the human motor cortex of normal and pathological brains. Ten healthy right-handed subjects and ten right-handed patients performed simple right hand movements during fMRI. A reference point chosen at a characteristic anatomical location within the fMRI sensorimotor activations was transferred to the high resolution anatomical MRI images by three human fMRI experts and by three automatic coregistration programs. The 3D distance between the median localizations of experts and programs was calculated and compared between patients and healthy subjects. Results show that fMRI localization on anatomical images was better with the experts than software in 70% of the cases and that software performance was worse for patients than healthy subjects (unpaired t-test: P = 0.040). With 45.6 mm the maximum disagreement between experts and software was quite large. The inter-rater consistency was better for the fMRI experts compared to the coregistration programs (ANOVA: P = 0.003). We conclude that results of automatic coregistration should be evaluated carefully, especially in case of clinical application.     

颈动脉狭窄或闭塞患者数字记忆的功能磁共振成像研究

目的:探讨左侧颈内动脉/大脑中动脉狭窄和闭塞患者与正常人数字工作记忆的激活脑区差异及其记忆障碍的脑加工机制。方法:对12例左侧颈动脉狭窄/闭塞患者(其中左侧颈内动脉狭窄4例,左侧颈内动脉闭塞2例,左侧大脑中动脉狭窄5例,左侧大脑中动脉闭塞1例,24例正常对照组进行数字记忆任务的功能磁共振成像,采用AFNI软件进行数据分析和脑功能区活动图像。结果:左侧颈内动脉/大脑中动脉狭窄和闭塞患者的反应时间较对照组显著延长(P0.05),正确率较对照组明显降低(P0.05)。脑激活图显示数字工作记忆的编码期、保持期、提取期患者组的脑激活明显少于正常对照组,而且脑激活的左侧半球优势减弱,右侧半球出现代偿性激活,其中提取期最为明显。结论:左侧颈动脉狭窄/闭塞患者数字记忆任务的脑激活区存在损害,功能磁共振成像研究能为早期诊断认知障碍提供影像学依据。     

Optimal preprocessing Butterworth-Wiener filter for Tl-201 myocardial SPECT

The optimal frequency characteristic of Butterworth-Wiener filters (BWF) for improving the image quality of 201Tl myocardial SPECT was determined by a phantom experiment. Thirty two projection images of the phantom containing 11.1 MBq of 201Tl with 4 different cold lesions were collected during a 180 degree arc of a gamma camera. A set of the projection images were processed with each of 27 different BWFs, and SPECT images were reconstructed by Shepp-Logan filtered backprojection. The SPECT images were evaluated for their diagnostic ability to visibly detect the cold lesions by four nuclear medicine physicians. The lesion contrasts were used as an adjunctive tool to determine the optimum filter. The optimal combination of the parameters determining BWF characteristics (for the data of about 100 count/pixel at the myocardium) is: 1) cutoff of 0.25/pixel, 2) FWHM of 3 pixels, 3) noise/signal ratio of 0.02. FWHM and noise/signal ratio affected lesion contrast much less than cutoff frequency. Clinical myocardial SPECT images processed with the optimal BWF showed less noise and sharper delineation of the myocardium.