Arterial input functions from MR phase imaging

A method is presented for obtaining high-sensitivity arterial input functions following bolus intravenous contrast agent administration. Arterial contrast agent is monitored by phase reconstruction of single-shot echo-planar images. During bolus injections of a gadolinium (Gd) agent in a baboon, data were acquired at the mid-abdominal aorta, and magnitude and phase-shift images were reconstructed. Pair-wise image subtraction was used to minimize phase aliasing. The phase-based method is shown to have a significant potential improvement in sensitivity compared to the magnitude approach. The phase method also has a general linear response to concentration. This method may have potential utility in quantitative imaging of blood flow and contrast agent kinetics.     

Initial clinical experience with spiral CT and 3D arterial reconstruction in intracranial aneurysms and arteriovenous malformations

We studied 32 consecutive patients with known or suspected cerebrovascular abnormalities studied with spiral CT following a intravenous bolus injection of iodinated contrast medium with a power injector. Flow was 3 or 4 ml/s. In an attempt to define the appropriate delay time and scan duration a cranial angio-CT without table increment was performed on 10 patients. Enhancement was measured by manually placed regions of interest within the left middle cerebral artery and the inferior sagittal sinus. All patients except one had intraarterial angiography (DSA) for comparison. In 6 patients with an arteriovenous malformation (AVM) follow-up was possible after one and/or two embolisation procedures. These patients had plain and contrast-enhanced spiral CT. The diagnosis was aneurysm in 9 (8 berry aneurysms, one giant fusiform aneurysm), AVM in 13 (all supratentorial) and traumatic arteriovenous fistula in one. In 9 patients there were no detectable pathological vascular findings. After 3D reconstruction the size (between 5 and 28 mm), location and the relationship to the parent vessel of the aneurysms, the extent of the AVMs and the distribution of the embolisation material could be demonstrated clearly. The main feeding vessel(s), nidus and draining veins were reliably shown. The decreased extent of the AVMs after embolisation was clearly demonstrated. There was no difference in diagnosis when DSA and 3D-CT were compared by two independent radiologists. We consider arterial spiral CT with 3D reconstruction to have the potential of offering important diagnostic information for the treatment of intracranial AVMs and aneurysms.The authors would like to dedicate this paper to Prof. Dr. sc. med. R. Lehmann     

Artifacts in 3D rotational angiography: an experimental study

PURPOSE: To investigate artifacts in three-dimensional rotational angiography (3D-RA) in an experimental model and to evaluate which parameters influence their distribution. MATERIAL AND METHODS: 3D-RA was carried out in a circular vessel phantom filled with contrast medium. Two different rotational angulations were used: 160 degrees causing 64 images and 180 degrees causing 90 or 120 images. The images were transferred to one workstation for reconstruction of axial slices and then to another workstation for 3D reconstructions. The 3D reconstructions were compared with standardized threshold settings. RESULTS: The artifacts occurred where the vessel had a longer path parallel to the rotation plane and became increasingly pronounced when the threshold level was raised. The artifacts decreased in size when rotation angle and number of projections were increased. CONCLUSION: The quality of the 3D reconstructions from RA was degraded by beam-hardening and sampling artifacts. The sampling artifacts were diminished by increasing both the rotation angle and the number of projections. The distortions in the 3D reconstructions caused by beam-hardening remain to be resolved. The threshold values also had a considerable influence on the 3D reconstructions.     

Four-dimensional vascular tree reconstruction using moving grid deformation

RATIONALE AND OBJECTIVES: Thinned perforator flaps have been widely used in plastic surgery for greater survivability and decreased morbidity. However, quantitative analysis of three-dimensional (3D) blood flow direction and location has not been examined yet. Such information will benefit and guide the surgical thinning and dissection process. Toward this goal, this study was performed for 3D vascular tree reconstruction with the incorporation of temporal contrast-agent propagation information (three spatial dimensions plus one temporal dimension; ie, 4D). MATERIALS AND METHODS: A novel computational framework by adopting a moving grid deformation method is presented. To take advantage of temporal information of the bolus propagating, a sequential segmentation procedure is proposed. Moreover, the temporal evolution of the vascular tree (4D vascular tree) is reconstructed during the procedure. RESULTS: Eight anterolateral thigh perforator flaps from eight cadavers were used for this study. The age range is 60-80 years old and the gender includes four males and four females. The 3D nature of the vascular structure and 4D vascular tree evolving process are showed in comparison with maximum intensity projection images. CONCLUSION: The proposed computational framework demonstrates effectiveness in the modeling of 4D vascular tree. Furthermore, it reveals the ability to detect small vessel tree structures that are beyond the limit of image resolution.     

Visualization of intracranial arteriovenous malformation using multivariate analysis

Intracranial arteriovenous malformations(AVMs) are composed of native deformed vessels where arteries and veins are directly connected in the absence of capillary vessels, and have a complicated three-dimensional(3D) shape. The purpose of this study was to define precisely the location of AVM areas for stereotactic irradiation. The proposed method used a sequence of dynamic X-ray CT images with contrast and opaque media. We analyzed these images by a principal component analysis(PCA) method and extracted component images: artery, vein, and background images. Then we made a fusion image with these three images and displayed the AVM areas. Results showed that the proposed method could identify the physiological location of these AVMs areas. Furthermore, we reconstructed 3D fusion images using two-dimensional component images extracted by the PCA method.     

Three-dimensional rotational angiography of transplanted renal arteries: influence of an extended angle of rotation on beam-hardening artifacts

PURPOSE: To investigate whether three-dimensional rotational angiography (3D-RA) of the transplant renal artery performed with an extended angle of rotation can reduce beam-hardening artifacts in 3D reconstructed images without image quality being lost or side effects to the transplanted kidney being increased. MATERIAL AND METHODS: 3D-RA with a C-arm rotation of 180 degrees was performed consecutively in 12 renal transplanted patients with suspicion of renal artery stenosis. A 1.7-mm balloon occlusion catheter was placed using the crossover technique and this was compared to a protocol with 160 degrees rotation and a traditional 1.4-mm catheter in 10 patients. The occurrence of beam-hardening artifacts was registered and the effects of the reduced contrast load on image quality and of arterial occlusion on renal function were assessed. RESULTS: The extended angle of rotation, from 160 degrees to 180 degrees, reduced the beam-hardening artifacts. Artifacts were observed in 4/11 patients (36%) in the study group and in all 10 (100%) of the controls. There was no statistical difference regarding image quality between the two protocols. Renal function was equally affected in both protocols. CONCLUSION: 3D-RA with an extended C-arm rotation reduced the beam-hardening artifacts. Image quality was not reduced despite the reduced contrast medium load. The different protocols had no effect on patient outcome.     

Time-resolved projection angiography after bolus injection of contrast agent

A method for MR angiography after bolus injection of a normal dose (0.1 mmol/kg) of contrast agent is presented. Projection angiograms are acquired with a non-slice selective Snapshot FLASH sequence with a time resolution of 1 s per image or better. Typically 40 to 60 images are acquired consecutively after bolus injection of a contrast agent. The signal from vessels can be separated from background by postprocessing based on the observed temporal evolution of the signal intensities during bolus passage. The subsecond projection MR-DSA is a reliable and robust technique to produce high resolution anatomical images of the vascular system avoiding the necessity of exact timing of the contrast agent bolus. It also supplies functional information about the hemodynamics in the observed region including perfusion.     

MR digital subtraction angiography of cerebral arteriovenous malformations

BACKGROUND AND PURPOSE: Although phase-contrast MR angiography provides some information regarding hemodynamics of cerebral arteriovenous malformations (AVMs), most conventional MR angiographic techniques have not been helpful in this respect. We attempted to determine the value of MR digital subtraction angiography (DSA) in assessing AVM hemodynamics. METHODS: We developed an MR DSA technique by combining rapid thick-section T1-weighted imaging with a bolus injection of contrast material. The temporal resolution was 0.56 to 0.61 seconds per scan. MR DSA images obtained from 14 patients with AVMs were reviewed. Anatomic depiction of each component of the AVM was rated using a four-point grading scale (excellent = 3, good = 2, fair = 1, poor = 0) to compare conventional vs MR angiograms. RESULTS: We were able to obtain serial images in which passage of contrast material was evident within the AVM, although the sequence we used allowed images to be obtained in only one projection. The average score for feeders, nidi, and drainers was 1.6, 2.4, and 2.3, respectively, with an overall average of 2.1. CONCLUSION: The spatial resolution of our technique may fall below the level needed for identification of small vascular components of an AVM. Additionally, the limited slab may restrict application of the technique to assessment of large or very small AVMs. MR DSA, however, can show the hemodynamics of AVMs and may serve as a supplement to conventional MR imaging in the diagnosis of cerebral AVMs.     

Examination of scan parameters that influence image quality in three-dimensional rotational angiography(3D-RA)

Because the imaging qualities of three-dimensional rotational angiography(3D-RA)are influenced by various imaging parameters, it is difficult to obtain high-quality images from 3D-RA. In this study, we compared two methods of 3D-RA, the propeller rotation technique(PRT)and the roll rotation technique(RRT)by changing image intensifier(I.I.)sizes(5, 7 and 9 inches)using a test chart and handmade phantom. The results of this study demonstrated that one of the factors determining the image quality of 3D-RA was spatial resolution. Therefore, it was important to choose an optimum I.I. size that was similar in size to collimating the region of interest(ROI)in clinical use. Another factor influencing image quality was radiographic condition, especially the setting of tube voltage. This factor was indispensable in obtaining good image contrast, but the use of high-voltage exposure was one of the reasons for lower image contrast. Therefore, if image contrast was insufficient, the image qualities of 3D-RA became worse with increasing tube voltage because the tube voltage in this study was automatically changed according to scanning method and I.I. size. In addition, because the spatial resolution of PRT was similar to that of RRT, we thought it better to use PRT because the data acquisition time(scan time)of this technique was 4 seconds shorter than that of RRT, whereas, if PRT was used, it was necessary to set a suitable rate of injection of contrast medium because the setting of the tube voltage of PRT was 10 kV higher than that of RRT. In conclusion, to improve the image qualities of 3D-RA, we considered it necessary to obtain sufficient image contrast not influenced by high tube voltage and to choose an optimum I.I. size suitable for the spatial resolution of ROI.     

三维增强磁共振肺动脉血管造影诊断肺动脉血栓栓塞

目的 探讨三维增强ＭＲ肺动脉血管造影（３Ｄ ｃｏｎｔｒａｓｔ－ｅｎｈａｎｃｅｄ ＭＲ ｐｕｌｍｏｎａｒｙ ａｎｇｉｏｇｒａｐｈｙ,３Ｄ ＣＥＭＲＰＡ）诊断肺动脉血栓栓塞的价值。方法 肺动脉栓塞患者１２例,共行１７次３Ｄ ＣＥＭＲＰＡ,其中４例５次于３Ｄ ＣＥＭＲＰＡ当天进行了选择性肺动脉ＤＳＡ。静脉注射钆喷替酸葡甲胺０．２ｍｍｏｌ／ｋｇ后,屏气获得２４层肺血管连续冠状面像,以最大强度投影法（ＭＩＰ     

Intracerebral arteriovenous malformations: evaluation with selective MR angiography and venography

Magnetic resonance (MR) angiography and spin-echo methods were used to evaluate intracerebral arteriovenous malformations (AVMs) in 10 patients. Spin-echo images obtained with flow presaturation demonstrated the nidus of the AVM in all cases, but it was difficult to determine feeding vessels. These vessels were directly visualized with three-dimensional MR angiography; their presence could be indirectly determined by means of selective presaturation of individual vessels, which resulted in a marked decrease in signal within the portion of the AVM supplied by that vessel. Vascular supplies from the internal carotid artery and anterior, middle, or posterior cerebral arteries were detected in all cases, but in three large malformations it was not possible to demonstrate small feeding vessels. MR angiograms were also helpful for further defining the nidus. The combination of MR angiographic and spin-echo methods provides information useful for therapeutic planning not provided by either technique alone.     

A micro-tomography method based on X-ray diffraction enhanced imaging for the visualization of micro-organs and soft tissues.

Diffraction enhanced imaging (DEI) is one of the phase contrast imaging methods using the monochromatic X-ray from synchrotron, which provides information on the out-of-plane angular deviation of X-ray. DEI allows the investigation of micro-structures inside weakly absorbing samples at high spatial resolution without serious radiation exposure. Tomographic techniques can be applied readily to phase contrast images. The combination of DEI and tomography allows for a reconstruction of refractive index gradient distribution inside weakly absorbing samples with micrometer resolution, particularly suited for the 3D observation of micro-organisms and tissues. The existing phase contrast tomography methods based on DEI use phase contrast images as projections, such images contain not only the phase information, but also the absorption information. A novel method (DEI in the tomography mode) was developed to greatly increase the proportion of refraction information in phase contrast images by computing the difference between the two sets of images acquired at different angles of the rocking curve to adopt the projections with a complete set (2pi) for reconstruction. The reconstructed images of cochlea of a guinea pig showed the spatial structures and the micro-features inside the samples. The new method reveals higher spatial resolution compared to the conventional phase contrast tomography methods and is more suitable to the investigation of micro-structures of micro-organisms and tissue materials.     

Selective contrast-enhanced MR angiography.

In this study the feasibility of intraarterial contrast administration was investigated. Its use for navigation and treatment evaluation during MR-guided intravascular interventions was explored in phantom and animal experiments. An injection protocol was developed, which accounts for sequence parameters and vessel flow rate. Tracking a bolus of contrast agent was useful to verify the catheter tip position and to assess flow conditions. Compared to intravenous contrast-enhanced magnetic resonance angiography (CE-MRA), selective contrast administration permitted a strongly reduced dose. In two-dimensional (2D) acquisitions overlap of vessels was prevented. Injection and acquisition were easily and accurately synchronized in selective 3D CE-MRA, and a high contrast concentration could be maintained during the entire acquisition. Selective injection is useful in the course of an intervention, to facilitate navigation, provide information on flow conditions, and to evaluate treatment progress repeatedly.     

Comparison of the extent of delayed-enhancement cardiac magnetic resonance imaging with and without phase-sensitive reconstruction at 3.0 T

OBJECTIVES: To evaluate phase-sensitive reconstructed images versus magnitude images generated by an inversion recovery pulse sequence for the determination of myocardial infarct size in delayed-enhancement cardiac magnetic resonance (DE-CMR) at 3 T. MATERIALS AND METHODS: Thirty patients were examined at 3 T and DE images were obtained 10 minutes after contrast agent administration using a phase-sensitive breath-hold segmented inversion recovery gradient echo sequence. From magnitude and phase images, the percentage of hyperenhanced myocardium was expressed. Contrast-to-noise ratio (CNR) measurements were performed in hyperenhanced and normal myocardium. RESULTS: We observed excellent correlation and concordance between hyperenhanced myocardium determined on phase-sensitive reconstructed and magnitude images. The mean CNR values were significantly higher in phase-sensitive reconstructed images compared with magnitude images (10.5 +/- 5.4 vs. 6.1 +/- 4.8; P < 0.001). CONCLUSIONS: DE-CMR with phase-sensitive reconstruction at 3.0 T provides similar results to magnitude images, but with a significantly greater CNR between infarcted and normal myocardium.     

Contrast-enhanced MRA for investigation of cerebral arteriovenous malformations

We evaluated contrast-enhanced MRA (enhanced 3-D fast gradient-echo [efgre3d] with spectral inversion recovery) for identification of 15 intracranial arteriovenous malformations (AVMs) in 14 patients. Demonstration of the feeding arteries was classified as good for 16 examinations on maximum-intensity projections and multiprojection volume reconstruction images. The nidus was seen well in all patients. Definition of the draining veins was good or fair except for one poor result. Therapeutic effects were clearly demonstrated in three follow-up series. Contrast-enhanced MRA using efgre3d is useful for delineation of AVMs and for follow-up after treatment.     

Double-reference cross-correlation algorithm for separation of the arteries and veins from 3D MRA time series

PURPOSE: To present a novel postprocessing technique for artery/vein separation and background suppression from contrast-enhanced time-resolved magnetic resonance angiography datasets in order to improve the diagnosis of vessel pathologies and arteriovenous fistulas. MATERIALS AND METHODS: Ten normal, two pathologic datasets of the brain, and one hand angiography dataset were postprocessed. Cross-correlation maps between the signal time course of every voxel in the dataset and selected arterial and venous references regions of interest (ROIs) were obtained; these maps were subsequently nonlinearly transformed to obtain two indices representing the likelihood of a voxel belonging to a vessel category. Red-green-blue (RGB) color encoding was utilized to depict synthetic arteriogram and venogram images in a single diagnostically meaningful image. RESULTS: The technique enabled correct visual separation of vessels on various datasets, as evaluated by two expert neuroradiologists, and also highlighted characteristics of flow in arteriovenous fistulas. A quantitative comparison with existing techniques showed better separation performance on 3 out of 10 normal datasets and higher stability to acquisition characteristics and contrast agent bolus dispersion. CONCLUSION: This method can be helpful in the diagnosis of vascular diseases in subjects where bolus dispersion makes it difficult to discriminate between arteries and veins with standard methods (subtraction or correlation analysis).     

Time resolved contrast enhanced intracranial MRA using a single dose delivered as sequential injections and highly constrained projection reconstruction (HYPR CE)

Time‐resolved contrast‐enhanced magnetic resonance angiography of the brain is challenging due to the need for rapid imaging and high spatial resolution. Moreover, the significant dispersion of the intravenous contrast bolus as it passes through the heart and lungs increases the overlap between arterial and venous structures, regardless of the acquisition speed and reconstruction window. An innovative technique is presented that divides a single dose contrast into two injections. Initially a small volume of contrast material (2–3 mL) is used to acquiring time‐resolved weighting images with a high frame rate (2 frames/s) during the first pass of the contrast agent. The remaining contrast material is used to obtain a high resolution whole brain contrast‐enhanced (CE) magnetic resonance angiography (0.57 × 0.57 × 1 mm³) that is used as the spatial constraint for Local Highly Constrained Projection Reconstruction (HYPR LR) reconstruction. After HYPR reconstruction, the final dynamic images (HYPR CE) have both high temporal and spatial resolution. Furthermore, studies of contrast kinetics demonstrate that the shorter bolus length from the reduced contrast volume used for the first injection significantly improves the arterial and venous separation. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Bolusoptimierte CT-Angiographie

This paper compares different contrast injection techniques for optimizing vessel contrast in CT angiography (CTA). The optimal vessel contrast shall be defined as constant strong enhancement confined to the scanning interval. This "plateau enhancement" guarantees high-quality CTA images and should therefore be approximated during every CTA examination by an appropriate contrast injection protocol. With well-established injection techniques such as the standard bolus technique (constant uniphasic contrast bolus for all patients) or adjustment of the scan delay, considerable individual differences in the arterial enhancement can be observed, and a nondiagnostic examination or an inefficient use of contrast agent might be the result in a particular patient. Therefore, two sophisticated mathematical models have recently been developed for analyzing the individual enhancement characteristics. These models can be exploited to predict the arterial enhancement for any given intravenous contrast bolus in any patient and to optimize the contrast bolus in order to approach the ideal "plateau enhancement." These techniques have to prove their effectiveness in larger clinical series.     

Cerebral blood flow: assessment with dynamic contrast-enhanced T2*-weighted MR imaging at 1.5 T

The authors assessed regional cerebral blood flow dynamics with magnetic resonance (MR) imaging enhanced with gadolinium diethylenetriaminepentaacetic acid (DTPA). After bolus administration of Gd-DTPA, rapid T2*-weighted gradient-echo images were acquired. Image acquisition time ranged from 2 to 3 seconds. The signal intensity (SI) of brain tissue and blood vessels markedly decreased during the first pass of contrast agent through the brain due to the local field inhomogeneity caused by the concentrated paramagnetic contrast agent. The method was used in 18 subjects with no cerebrovascular disease and 32 patients with stroke, vascular stenosis, arteriovenous malformation, and cerebral neoplasm. Comparison with intracranial angiography was performed in three patients and with single-photon emission computed tomography of blood flow in four. The change in T2* relaxation rate was approximately linearly related to the dose of contrast agent. The SI change increased as the echo time was lengthened. Regions in cerebral infarcts, metastases, and arteriovenous malformations showed different enhancement patterns than those of edema around a lesion and of normal brain tissue. Abnormal circulation times in patients with vascular stenoses were demonstrated. The method provides information about cerebral blood flow dynamics not available from conventional MR imaging and MR angiography.