双探头SPECT心肌灌注显像中位移伪影的实验研究

目的 探讨双探头SPECT心肌灌注显像时位移伪影的影像特征和识别方法。方法 将心脏模型置于检查床上，与受检患者的心脏方向一致。在图像采集过程中，模型依次沿相当于患者左右、头尾和前后方向分别在不同起始点、对不同帧数作一定距离的位移。结果位移伪影的共同特点是表现为室壁放射性分布不均匀，“热区”与“冷区”交替出现，在短轴上最早出现，且表现最为明显；伪影进一步发展会在水平长轴和垂直长轴上表现为心尖附近放射性稀疏或缺损，出现与相邻室壁伴行且形态相近的“伴影”。结论位移伪影主要表现为室壁放射性分布不均，“热区”与“冷区”交替出现，在短轴图像上易于早期发现。     

运动引起心肌APECT断层伪影的实验研究

探讨心肌断层采集过程中体位移动的方向、幅度、时限对断层影像的影响。方法　在探头旋转到 45°、90°、135°时 ,将心脏模型依次沿x、y和z轴方向分别移动± 5mm、± 10mm、± 2 0mm。其图像分别与无移动者对比 ,并由 3名医师判别伪影。结果经统计学处理。结果　在移动 5mm时无伪影。移动 10mm以上出现伪影 ,且距离愈大愈明显。在短轴上呈典型的三角形分布且交替出现“冷”、“热”结节。在水平及垂直长轴上 ,表现为前间隔和前侧壁、前壁和下壁的对称性变薄内收 ,心尖拉长。结论　位移≥ 10mm时 ,出现具有特征性的可识别伪影     

18F-FDG PET/CT伪影和不常见生理性摄取分析

目的 探讨^18F-脱氧葡萄糖（FDG）PET/CT显像各种伪影的影像学表现及其产生的原理.方法 回顾性分析^18F-FDG PET/CT检查患者的图像,根据伪影产生的原因进行分类,同时对临床不常见的生理性摄取进行分析.结果 伪影分为自身因素和设备技术因素伪影,自身因素所致伪影中以呼吸运动伪影和高密度物质伪影最为常见;设备因素伪影中以截断伪影、注射点外漏和放射性污染最为常见.不常见的生理性摄取包括：子宫内膜摄取、乳腺摄取和脂肪摄取.结论 PET显像伪影影像学表现可分为“热区”或“冷区”.不常见的生理性摄取主要表现为“热区”.伪影产生原因中以CT应用于PET显像后物理学因素多见.不常见的生理性摄取与检查技术有关.     

运动位移对SPECT心肌灌注显像结果的影响

目的 探讨运动位移对SPECT心肌灌注显像结果的影响.方法 收集健康体格检查者201 Tl心肌灌注显像资料6例,利用计算机软件对原始图像进行多组模拟位移.重建各种位移和无位移状态下的心肌断层图像,利用定量计算软件自动获得各种位移状态和无位移状态下左心室各壁段的血流灌注分数.比较各种位移状态和无位移状态下的图像及其血流灌注分数的差异.分析不同位移状态对心肌灌注显像图像质量及定量计算结果的影响.采用SPSS 12.0软件进行数据处理.结果 在各种位移方式中,1帧和1个像素的所有位移对图像质量及定量计算结果无明显影响,随着位移帧数和(或)位移距离(像素数)的增加,图像逐渐出现放射性稀疏、缺损以及变形,并出现"拖尾"、"伴影"等伪影特征.向上移动时,主要影响左心室下后壁;向下移动时,主要影响前壁及下后壁;向右移动时,主要影响间壁、心尖部、前壁及下后壁;向左移动时,主要影响问壁及下后壁.其中,沿x轴移动时心肌血流灌注分数的改变大于沿y轴移动时(t=2.848,P＜0.01),向下移动的改变大于向上移动(t=2.941,P＜0.01),向右移动的改变大于向左移动(t=6.598,P＜0.01).结论 大于1帧和1个像素的位移可对心肌灌注显像图质量及定量计算结果产生不同程度的影响,不同方向及不同幅度的位移对左心室各壁段的影响具有不同特征.     

用钡造影剂模型研究18F-FDG符合线路显像中基于CT的过度衰减校正

目的通过模型实验探讨钡造影剂（简称钡剂）在基于CT衰减校正（AC）技术的^18F-脱氧葡萄糖（FDG）符合线路显像中产生伪影的规律。方法将不同质量浓度（0．05、0．1、0．2、0．3、0．4、0．5、1．0、2．0、3．0kg／L）、相同体积（2．5m1）和相同质量浓度（0．5kg／L）、不同体积（0．5、1．0、1．5、2．0、2．5m1）的钡剂灌人乳胶指套并浸于含^18F—FDG水溶液（3．7kBq／m1）的圆柱体模型中。按常规行符合线路数据采集和重建，观察图像并作定量分析。结果模型中均匀液体部分的AC图像放射性分布均匀，非衰减校正（NAC）图像放射性分布呈中央低周边高的不均匀状态。钡剂部位在NAC图像中低于周围液体计数形成“冷区”，在AC图像中高于周围计数呈“热区”。质量浓度≥0．1kg／L的钡剂以及直径〉10mm（V银剂=0．5ml）的乳胶指套在AC图像上产生了“热区”伪影。结论基于CT的AC技术改善了^18F—FDG符合线路显像的图像质量，但钡剂的存在可能造成衰减的过度校正，形成“热区”伪影。对体内残留钡剂的患者，通过仔细阅读CT图像、AC及NAC图像，能够鉴别放 射性浓聚区是否为伪影。     

SPECT／CT图像配准不良对心肌灌注显像CT衰减校正的影响

目的探讨CT与SPECT图像配准不良对MPICT衰减校正（CTAC）的影响。方法99Tcm-MIBIMPI受检者19名,均为行健康体格检查者,其中男11名,女8名,年龄（65．3±9．6）岁。对MPI图像进行CTAC。利用仪器自带的软件对CT图像进行模拟位移：相对心脏位置进行上、下、左、右、前、后6个方向的移动,移动幅度分别为0．5、1．0、1．5、2．0、2．5、3．0、3．5、4．0和4．5em。重建不同位移状态的CTAC心肌断层图像,利用靶心图获得左心室各壁段的放射性计数百分比,比较位移前后左心室各壁段放射性计数的差异和图像差异。采用SPSS13．0软件对数据进行配对t检验。结果当CT图像的移动距离为0．5cm时,所有位移方向的CTAC图像均未见明显可识别的图像伪影。当CT图像的移动距离≥1．0CITI时,左心室各壁段出现不同程度的图像伪影以及放射性计数的改变;CT图像向上、下、左、右、前、后方向位移时,分别对心尖部,前壁和心尖部,间壁,前壁、心尖部和侧壁,侧壁和下后壁,前壁、心尖部和间壁放射性计数的影响最为显著。向下位移时左心室各壁段放射性计数的改变大于向上位移[（-9．68±8．06）％和（-2．04±1．83）％;f=6．573,P〈0．01],向右位移的改变大于向左位移[（-9．02±8．47）％和（-4．38±3．67）％;t=1．987,P〈0．05]。在左心室各壁段中,前壁、心尖部和侧壁的伪影程度明显较下后壁和间壁显著。结论CT与SPECT图像配准不良可使MPICTAC图像出现不同程度的伪影,其伪影出现的部位和严重程度与配准不良的方向和幅度密切相关。     

肾上腺肿块病变的运动规律及影响因素的临床研究

目的 探讨肾上腺肿块病变的运动规律、幅度大小及其影响因素,为实施立体定向放射治疗时确定计划靶区（PTV）提供参考依据.方法 38例转移肾上腺肿瘤病人采用B超下穿刺的方法将1～2个金标植入肾上腺肿瘤瘤体内,并在模拟定位机下对金标的运动幅度进行测量,得出其在病人身体左右（x轴）、前后（y轴）、头脚（z轴）方向的移动幅度,用多元线性回归模型分析影响因素.采用t检验对（左、右肾上腺病灶在）z轴方向上的运动幅度进行比较.结果 38例病人肾上腺内的金标在x轴方向的移动距离为0.1～0.4 cm[（0.27±0.07） cm],y轴方向为0.1～0.5 cm[（0.31±0.11） cm],z轴方向为0.5～1.2 cm[（0.87±0.21） cm].z轴方向上病灶的运动幅度可能仅受到病灶位置的影响（P=0.002）,其余方向未显示运动幅度受年龄、身高、体质量、病灶位置、大小的影响.比较z轴上不同病灶位置的运动幅度,左、右肾上腺病灶分别为（0.99±0.22） cm和（0.79±0.16） cm,右肾上腺病灶的运动幅度较左肾上腺病灶小（t=4.08,P=0.000）.结论 肾上腺肿块病变的移动受呼吸运动影响,在z轴方向上的移动距离最大,在确定PTV安全边界时应主要考虑其在z轴方向的移动.同时右肾上腺因肝脏的限制,其运动幅度较左肾上腺的要小.     

心肌显像病人所致伪影的辨析及其对策研究

重点介绍了心肌灌注显像过程中，由于病人因素导致的位移、膈肌衰减和肝脏放射性聚集等所产生的伪影的表现和校正方法。     

心肌显像病人所致伪影的辨析及其对策研究

重点介绍了在心肌灌注显像过程中,由于病人因素导致的位移、膈肌衰减和肝脏放射性聚集等所产生的伪影的表现和校正方法。     

022 SPECT心肌灌注显像的位移伪影的定量分析与新位移校正程序的评价

目的：应用单、双探头和定量灌注SPECTQPS）软件研究由模拟位移产生的缺损类型和程，探讨采用新的自动校正软件校正模拟伪影和床伪影产生的心肌灌注缺损影像的效果。方法：对８例病人进行２０１T１静息与９９Tcm－MIBISPECT双核素心肌显像的结果完全正常，用单、双探头SPECT仪对这８例病人的９９Tcm－MIBISPECT影像分为早、中和晚期进行模拟位移，每个位移距离为１、２和３个像素。对原始投影资料进行向上移动来模拟垂直方向上的位移和进行向左移动模拟横向上的位移时，分为复位和非复位位移。对１３０个临床上发生位移的病人…     

Correction of spatial distortion in MR imaging: a prerequisite for accurate stereotaxy

With magnetic resonance (MR) imaging, accurate spatial information--critical for effective stereotaxy--demands a homogeneous static field and linear gradients. Inhomogeneities and nonlinearities induced by eddy currents during the pulse sequences distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. These errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "two-dimensional phantom" displaying "pincushion" distortion in the image (i.e., x, y) plane, and the "three-dimensional phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 5 to 1-2 mm) by calculations based on modeling the distortion as a fourth order two-dimensional polynomial. Based on these corrected images, errors in the z coordinate and tilt of image planes may be corrected by adjustment of the gradient shimming currents. Such correction not only implements stereotaxy under MR guidance but also provides for the accurate transfer of anatomic/pathologic information between MR and CT images.     

Motion artifacts in subsecond conventional CT and electron-beam CT: pictorial demonstration of temporal resolution.

To visually demonstrate the effective temporal resolution of subsecond conventional (slip-ring) and electron-beam computed tomographic (CT) systems, two phantoms containing high-contrast test objects were scanned with a slip-ring CT system (effective exposure time, 0.5 second) and an electron-beam CT system (exposure time, 0.1 second). Images were acquired of each phantom at rest, during translation along the x axis at speeds of 10-100 mm/sec, and during rotation about isocenter at speeds of 0.1 and 0.5 revolution per second. Motion artifacts and loss of spatial resolution were judged to be absent, noticeable, or severe. For 0.5-second conventional CT images, motion artifacts and loss of spatial resolution were noticeable at 10 mm/sec and 0.1 revolution per second and were severe at speeds greater than or equal to 20 mm/sec and at 0.5 revolution per second. For 0.1-second electron-beam CT scans, noticeable, but not severe, motion artifacts and loss of spatial resolution occurred at speeds between 40 and 100 mm/sec and at 0.5 revolution per second. Over the range of physiologic speeds examined, the images provide visually compelling evidence of the effect of improving temporal resolution in CT.     

Rapid autocorrection using prescan navigator echoes.

Autocorrection is an adaptive motion correction algorithm that does not require an in vivo measurement of the motion record. A novel method for ensuring convergence of this algorithm when motion is severe is presented. A limited number of navigator echoes are acquired before the imaging sequence to obtain a "snapshot" of the object. Phase differences between the navigator and image k-space data are used as an estimate of motion-induced phase shifts in the image, followed by autocorrection. In phantom data a six-fold reduction in computation time compared to autocorrection alone was realized. These results indicate that this navigator/autocorrection combination may be useful for reducing motion artifacts and computation time for MR exams when motion along the image phase encoding axis is severe.     

Influence of the reference scan and scan time on the arterial phase of liver magnetic resonance imaging

The controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) technique can decrease scan time. The purpose of this study was to determine whether an arterial phase scan can be performed in 5 s using the CAIPIRINHA short-scan and a reference scanning technique. The generalized autocalibrating partially parallel acquisition (GRAPPA), the CAIPIRINHA routine (CAIPI-routine), and the CAIPIRINHA short-scanning (CAIPI-short) methods were compared. The scan time for each method was preset to 20 s, 15 s, and 10 s, respectively. The reference scan had a scan time of 5 s. A phantom study was used to compare the influence of artifacts during the reference scan. For comparison, the phantom was moved during the last 5 s. In the clinical studies of suspected chronic liver diseases, magnetic resonance imaging of the liver is usually performed while the patient is breath-hold. The motion artifacts of each method were compared. Artifacts were reduced in reference scans using the CAIPIRINHA method. At 5 s after initiation, the rate of change in the standard deviation value was within 30% compared to that of the original image. Motion artifacts due to the influence of the reference scan when a patient failed to hold their breath did not complicate image evaluation. The proportion of motion artifacts for each sequence was as follows: GRAPPA, 5.8%; CAIPI-routine, 1.9%; and CAIPI-short, 0.7%. The arterial phase can be scanned in 5 s using the CAIPI-short and reference scan techniques.     

Reduction of respiratory motion artifacts in PET imaging of lung cancer by respiratory correlated dynamic PET: methodology and comparison with respiratory gated PET.

This study proposes a new method to reduce respiratory motion artifacts in PET images of lung cancer. The method is referred to as respiratory-correlated dynamic PET (RCDPET). RCDPET enables the acquisition of 4-dimensional PET data without the need for a respiratory tracking device. In this article, we compare this method with respiratory-gated PET (RGPET). Both methods provide the ability to correct for motion artifacts and more accurately quantitate radiotracer uptake within lung lesions. Both methods were evaluated in phantom studies and 1 patient. METHODS: With RCDPET, data are acquired in consecutive 1-s time frames. A point source attached to a rigid foam block is set on the patient's abdomen and is extended into the camera field of view at the level of the lesion by means of a low-density rod. The position of this source is used to track respiratory motion through the consecutive dynamic frames. Image frames corresponding to a user-selected lesion position within the breathing cycle, in correlation with the point source position, are then identified after scanning. The sinograms of the selected image frames are summed and then reconstructed using iterative reconstruction with segmented attenuation correction. RESULTS: The results from phantom studies with both RGPET and RCDPET were within 10% agreement, for both activity quantitation and image noise levels. In a clinical application, the quantitation of the SUV(max) and the lesion's size showed a 6% and 2% difference, respectively, between RCDPET and RGPET measurements. CONCLUSION: RCDPET can be considered as a comparable, or alternative, method to RGPET in reducing the smearing effects due to respiration and improving quantitation of PET in the thorax. One advantage of RCDPET over RGPET is the ability to retrospectively reconstruct the PET data at any phase or amplitude in the breathing cycle.     

Influence of image acquisition parameters on CT artifacts and polyp depiction in spiral CT colonography: in vitro evaluation

PURPOSE: To quantify the effects of spiral computed tomographic (CT) acquisition parameters on the magnitude of three-dimensional (3D) rippling artifacts and polyp depiction. MATERIALS AND METHODS: An in vitro colon phantom was constructed with air-filled acrylic cylinders that contained synthetic polyps of 3-13 mm. The phantom was submerged in fluid and positioned at four angles of inclination relative to the z axis. Image data were acquired at collimation and pitch combinations of 3 mm and 1.67 and 5 mm and 1.6, respectively. Rippling artifacts were quantified by measuring the longitudinal variation of in-plane phantom edge width, and the influence of these artifacts on the depiction of pedunculated and sessile polyps was assessed qualitatively. RESULTS: The in-plane magnitude of the rippling artifact was a function of the angle of inclination relative to the longitudinal axis and the table increment. The through-plane periodicity of the artifact was equal to one-half the table increment. CONCLUSION: The table increment and angle of inclination of the surface of the object relative to the z axis determine the periodicity and magnitude of the rippling artifact at 3D spiral CT colonography. Although the depiction of small pedunculated polyps was not compromised, some sessile polyps were degraded by the artifact.     

The influence of flow and motion in MRI of diffusion using a modified CE-FAST sequence

Severe motion and flow artifacts are a problem in MRI of diffusion in vivo due to the application of strong magnetic field gradients. Here it is shown that image artifacts can be removed by using a modified fast-scan MRI sequence (CE-FAST) in conjunction with averaging of diffusion-weighted images. In phantom studies slow (coherent) flow (less than 1 mm s-1) in the presence of strong diffusion gradients is shown to cause signal losses in diffusion-weighted images that depend on the relative orientations of the flow direction and the diffusion gradient. On the other hand, pulsatile motions of macroscopic dimensions (e.g., 1 mm, 1 Hz, in-plane) lead to smearing and ghosting of signal intensities along the phase-encoding direction of the images. In both phantoms and rabbit brains in vivo motion artifacts were found to be reducible by averaging 8-16 images. Unfortunately, the resulting image contrast no longer represents a "true" diffusion contrast but is affected by additional signal losses due to motion averaging. All experiments were performed on a 40-cm-bore 2.35-T Bruker Medspec system.     

Compensation for effects of linear motion in MR imaging

Various compensation methods for different types of motion during MR image acquisition have been proposed. Presented here is a postprocessing scheme for eliminating artifacts due to linear, intra-slice motion of known velocity. The data for each phase encoding or "view" acquired from a moving object are shown to differ from those which would be measured from the stationary object by a phase factor which depends on the object's displacement from a reference point. This derivation is then used to propose a correction scheme for linear motion in which all phase encodings measured at different positions of the moving object are collapsed onto the same reference position. After subsequent reconstruction, the object appears perfectly "focused." By selection of different reference positions, the method permits positioning of the compensated object as desired within the field of view of the image. This property allows the method to be extended to create sequences of corrected images with smooth object motion between frames of the sequence. The basic correction scheme and its variations were tested experimentally in phantom studies with velocities as large as 8 cm/s.     

Reduction of respiratory motion artifacts in transverse relaxation rate (R2) images of the liver.

An empirical motion artifact suppression technique has been developed to reduce the respiratory motion artifacts in axial single spin-echo magnetic resonance (MR) images of the liver post-acquisition. The correction scheme is based on the observation that the dominant motion artifacts within abdominal MR images are ghosts that follow the profile and signal intensity of high signal intensity boundaries, such as those for the subcutaneous fat along the anterior abdominal wall. The technique is applied to the reduction of respiratory motion artifacts in a spin echo image series of the liver of an iron-loaded patient and of a manganese chloride phantom subject to respiratory motion. Subsequent improvements to transverse relaxation rate (R2) image analysis are then demonstrated on the motion-corrected spin echo images, illustrating the utility of the technique for application in the R2 image-based measurement and mapping of liver iron concentration.     

Band artifacts due to bulk motion.

Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase- and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period.