RESUMO
PURPOSE: Current breast DCE-MRI strategies provide high sensitivity for cancer detection but are known to be insufficient in fully capturing rapidly changing contrast kinetics at high spatial resolution across both breasts. Advanced acquisition and reconstruction strategies aim to improve spatial and temporal resolution and increase specificity for disease characterization. In this work, we evaluate the spatial and temporal fidelity of a modified data-driven low-rank-based model (known as MOCCO, model consistency condition) compressed-sensing (CS) reconstruction compared to CS with temporal total variation with radial acquisition for high spatial-temporal breast DCE MRI. METHODS: Reconstruction performance was characterized using numerical simulations of a golden-angle stack-of-stars breast DCE-MRI acquisition at 5-second temporal resolution. Specifically, MOCCO was compared with CS total variation and conventional SENSE reconstructions. The temporal model for MOCCO was prelearned over the source data, whereas CS total variation was performed using a first-order temporal gradient sparsity transform. RESULTS: The MOCCO reconstruction was able to capture rapid lesion kinetics while providing high image quality across a range of optimal regularization values. It also recovered kinetics in small lesions (1.5 mm) in line-profile analysis and error images, whereas g-factor maps showed relatively low and constant values with no significant artifacts. The CS-TV method demonstrated either recovery of high spatial resolution with reduced temporal accuracy using large regularization values, or recovery of rapid lesion kinetics with reduced image quality using low regularization values. CONCLUSION: Simulations demonstrated that MOCCO with radial acquisition provides a robust imaging technique for improving temporal fidelity, while maintaining high spatial resolution and image quality in the setting of bilateral breast DCE MRI.
Assuntos
Meios de Contraste , Interpretação de Imagem Assistida por Computador , Artefatos , Mama/diagnóstico por imagem , Imageamento por Ressonância MagnéticaRESUMO
PURPOSE: To evaluate the incidence and severity of potentially thrombus mimicking, flow-induced misallocation artifacts in a clinical setting. Two-point "Dixon" fat-water separation methods, with bipolar readout gradients, may suffer from flow-induced fat-water misallocation artifacts. If these artifacts occur within blood vessels, they may mimic thrombus. MATERIALS AND METHODS: Two-point Dixon coronal and axial images acquired in 102 consecutive patients were retrospectively evaluated for the presence of flow-induced artifacts in arteries and veins. Artifacts were graded on a 3-point scale (none, mild, severe) by two independent readers. Interreader agreement was evaluated with kappa statistics. RESULTS: Reader 1 reported 63 artifacts in 46 (45%) of the cases (severe in 19 cases, 18.6%). Reader 2 reported 51 artifacts in 43 (42.2%) of the cases (severe in 18 cases, 17.6%). Misallocation of fat and water was apparent in all datasets with severe artifacts, whereas variable signal intensity changes in water and fat images were observed in mild artifacts. Interreader agreement was good for artifacts appearing in coronal images (κ = 0.7) and fair for artifact appearance in axial images (κ = 0.24). CONCLUSION: Our study shows a high incidence of flow-induced mild and severe artifacts in a two-point Dixon method with bipolar readout gradients. This artifact should not be misinterpreted as intravascular thrombus. LEVEL OF EVIDENCE: 3 J. Magn. Reson. Imaging 2017;45:229-236.
Assuntos
Artefatos , Angiografia por Ressonância Magnética/métodos , Trombose/diagnóstico por imagem , Trombose/epidemiologia , Adolescente , Adulto , Idoso , Idoso de 80 Anos ou mais , Reações Falso-Positivas , Feminino , Humanos , Incidência , Masculino , Pessoa de Meia-Idade , Variações Dependentes do Observador , Reprodutibilidade dos Testes , Fatores de Risco , Sensibilidade e Especificidade , Índice de Gravidade de Doença , Wisconsin/epidemiologia , Adulto JovemRESUMO
PURPOSE: Two-point fat-water separation methods are increasingly being used for chest and abdominal MRI and have recently been introduced for use in MR angiography of the lower extremities. With these methods, flowing spins can accumulate unintended phase shifts between the echo times. The purpose of this study is to demonstrate that these phase shifts can lead to inaccurate signals in the water and fat images. THEORY AND METHODS: In vitro experiments were conducted at 1.5T and 3.0T using a stenosis-mimicking phantom and a computer-controlled pump to image a range of physiologically relevant velocities. RESULTS: In the phantom images acquired using bipolar readout gradients, fat-water signal inaccuracies were visible in regions of flow, with increasing severity as the flow rate was increased. Additionally, similar effects were observed in regions of high flow in clinical chest and liver exams. In the phantom images, the effect was eliminated by using a dual-pass method without bipolar readout gradients. CONCLUSION: When using fat-water separation methods with bipolar readout gradients, phase shifts caused by the motion of spins can lead to signal inaccuracies in the fat and water images. These artifacts can be mitigated by using approaches that do not use bipolar readout gradients.
Assuntos
Artefatos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Angiografia por Ressonância Magnética/métodos , Imagens de Fantasmas , Doenças Vasculares/diagnóstico , Doenças Vasculares/fisiopatologia , Tecido Adiposo , Algoritmos , Velocidade do Fluxo Sanguíneo/fisiologia , Água Corporal , Constrição Patológica/diagnóstico , Constrição Patológica/fisiopatologia , Meios de Contraste , Desenho de Equipamento , Humanos , Perna (Membro)/irrigação sanguínea , Angiografia por Ressonância Magnética/instrumentação , Sensibilidade e Especificidade , Razão Sinal-RuídoRESUMO
PURPOSE: To develop and evaluate a method for volumetric contrast-enhanced MRI of the liver, with high spatial and temporal resolutions, for combined dynamic imaging and MR angiography (MRA) using a single injection of contrast agent. METHODS: An interleaved variable density (IVD) undersampling pattern was implemented in combination with a real-time-triggered, time-resolved, dual-echo 3D spoiled gradient echo sequence. Parallel imaging autocalibration lines were acquired only once during the first time frame. Imaging was performed in 10 subjects with focal nodular hyperplasia (FNH) and compared with their clinical MRI. The angiographic phase of the proposed method was compared with a dedicated MR angiogram acquired during a second injection of contrast. RESULTS: A total of 21 FNH, three cavernous hemangiomas, and 109 arterial segments were visualized in 10 subjects. The temporally resolved images depicted the characteristic arterial enhancement pattern of the lesions with a 4-s update rate. Images were graded as having significantly higher quality compared with the clinical MRI. Angiograms produced from the IVD method provided noninferior diagnostic assessment compared with the dedicated MR angiogram. CONCLUSION: Using an undersampled IVD imaging method, we have demonstrated the feasibility of obtaining high spatial and temporal resolution dynamic contrast-enhanced imaging and simultaneous MRA of the liver.
Assuntos
Hemangioma Cavernoso do Sistema Nervoso Central/patologia , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Neoplasias Hepáticas/patologia , Angiografia por Ressonância Magnética/métodos , Meglumina/análogos & derivados , Compostos Organometálicos , Adulto , Algoritmos , Meios de Contraste/administração & dosagem , Interpretação Estatística de Dados , Feminino , Humanos , Aumento da Imagem/métodos , Masculino , Meglumina/administração & dosagem , Compostos Organometálicos/administração & dosagem , Reprodutibilidade dos Testes , Tamanho da Amostra , Sensibilidade e EspecificidadeRESUMO
PURPOSE: To demonstrate the feasibility of direct virtual coil (DVC) in the setting of 4D dynamic imaging used in multiple clinical applications. THEORY AND METHODS: Three dynamic imaging applications were chosen: pulmonary perfusion, liver perfusion, and peripheral MR angiography (MRA), with 18, 11, and 10 subjects, respectively. After view-sharing, the k-space data were reconstructed twice: once with channel-by-channel (CBC) followed by sum-of-squares coil combination and once with DVC. Images reconstructed using CBC and DVC were compared and scored based on overall image quality by two experienced radiologists using a five-point scale. RESULTS: The CBC and DVC showed similar image quality in image domain. Time course measurements also showed good agreement in the temporal domain. CBC and DVC images were scored as equivalent for all pulmonary perfusion cases, all liver perfusion cases, and four of the 10 peripheral MRA cases. For the remaining six peripheral MRA cases, DVC were scored as slightly better (not clinically significant) than the CBC images by Radiologist A and as equivalent by Radiologist B. CONCLUSION: For dynamic contrast-enhanced MR applications, it is clinically feasible to reduce image reconstruction time while maintaining image quality and time course measurement using the DVC technique.
Assuntos
Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Armazenamento e Recuperação da Informação/métodos , Angiografia por Ressonância Magnética/métodos , Interface Usuário-Computador , Algoritmos , Estudos de Viabilidade , Humanos , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
PURPOSE: To demonstrate the feasibility of performing single breathhold, noncardiac gated, ultrafast, high spatial-temporal resolution whole chest MR pulmonary perfusion imaging in humans. MATERIALS AND METHODS: Eight subjects (five male, three female) were scanned with the proposed method on a 3 Tesla clinical scanner using a 32-channel phased-array coil. Seven (88%) were healthy volunteers, and one was a patient volunteer with sarcoidosis. The peak lung enhancement phase for each subject was scored for gravitational effect, peak parenchymal enhancement and severity of artifacts by three cardiothoracic radiologists independently. RESULTS: All studies were successfully performed by MR technologists without any additional training. Mean parenchymal signal was very good, measuring 0.78 ± 0.13 (continuous scale, 0 = "none" â 1 = "excellent"). Mean level of motion artifacts was low, measuring 0.13 ± 0.08 (continuous scale, 0 = "none" â 1 = "severe"). CONCLUSION: It is feasible to perform single breathhold, noncardiac gated, ultrafast, high spatial-temporal resolution whole chest MR pulmonary perfusion imaging in humans.
Assuntos
Artefatos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Angiografia por Ressonância Magnética/métodos , Circulação Pulmonar/fisiologia , Sarcoidose/patologia , Sarcoidose/fisiopatologia , Adulto , Algoritmos , Velocidade do Fluxo Sanguíneo/fisiologia , Suspensão da Respiração , Estudos de Viabilidade , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Adulto JovemRESUMO
For MR applications such as contrast-enhanced MR angiography, it is desirable to achieve simultaneously high spatial and temporal resolution. The current clinical standard uses view-sharing methods combined with parallel imaging; however, this approach still provides limited spatial and temporal resolution. To improve on the clinical standard, we present an interleaved variable density (IVD) sampling method that pseudorandomly undersamples each individual frame of a 3D Cartesian ky-kz plane combined with parallel imaging acceleration. From this dataset, time-resolved images are reconstructed with a method that combines parallel imaging with a multiplicative constraint. Total acceleration factors on the order of 20 are achieved for contrast-enhanced MR angiography of the lower extremities, and improvements in temporal fidelity of the depiction of the contrast bolus passage are demonstrated relative to the clinical standard.
Assuntos
Algoritmos , Artérias/anatomia & histologia , Meios de Contraste , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Angiografia por Ressonância Magnética/métodos , Interpretação Estatística de Dados , Humanos , Reprodutibilidade dos Testes , Tamanho da Amostra , Sensibilidade e EspecificidadeRESUMO
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 mm(3) ) 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.
Assuntos
Artérias Cerebrais/anatomia & histologia , Veias Cerebrais/anatomia & histologia , Meios de Contraste/administração & dosagem , Gadolínio/administração & dosagem , Aumento da Imagem/métodos , Angiografia por Ressonância Magnética/métodos , Algoritmos , Humanos , Interpretação de Imagem Assistida por Computador/métodos , Injeções Intravenosas , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
OBJECTIVE: The purpose of our study was to determine if tubal patency can be assessed by MR hysterosalpingography (HSG) using a clinically available MR angiographic sequence (3D time-resolved imaging of contrast kinetics [TRICKS]). This capability would enhance the value of MRI in women with infertility. CONCLUSION: MR HSG effectively shows tubal patency and can be considered when both conventional HSG and standard MRI are necessary for the evaluation of women with infertility, such as in women with suspected uterine anomalies or extrauterine disease.
Assuntos
Algoritmos , Doenças das Tubas Uterinas/diagnóstico , Tubas Uterinas/patologia , Imageamento Tridimensional/métodos , Infertilidade Feminina/diagnóstico , Angiografia por Ressonância Magnética/métodos , Adulto , Meios de Contraste , Doenças das Tubas Uterinas/complicações , Feminino , Gadolínio DTPA , Humanos , Histerossalpingografia/métodos , Aumento da Imagem/métodos , Infertilidade Feminina/etiologia , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
Conventional bolus-chase acquisition generates peripheral runoff images using a single injection of the contrast material. Low spatial resolution, small slice coverage and venous contamination are major problems especially in the distal stations. A technique is presented herein in which whole-body magnetic resonance angiography is performed using a dual-contrast-injection four-station acquisition protocol. Bolus sharing was performed between two stations: the abdomen and calf stations share the first bolus injection, while the thorax and thigh stations share the second bolus injection. The combination of variable density sampling and elliptical centric acquisition order was applied to the abdomen and thorax stations. The scan time was extended to generate high spatial resolution arterial phase images with broad slice coverage for the calf and thigh stations. The feasibility of this technique was demonstrated using phantom and in vivo human volunteer studies.
Assuntos
Meios de Contraste/administração & dosagem , Gadolínio DTPA/administração & dosagem , Perna (Membro)/irrigação sanguínea , Angiografia por Ressonância Magnética/métodos , Imagem Corporal Total , Humanos , Imageamento Tridimensional , Imagens de FantasmasRESUMO
OBJECTIVE: Currently, dynamic contrast-enhanced (DCE) breast magnetic resonance imaging (MRI) prioritizes spatial resolution over temporal resolution given the limitations of acquisition techniques. The purpose of our intrapatient study was to assess the ability of a novel high spatial and high temporal resolution DCE breast MRI method to maintain image quality compared with the clinical standard-of-care (SOC) MRI. MATERIALS AND METHODS: Thirty patients, each demonstrating a focal area of enhancement (29 benign, 1 cancer) on their SOC MRI, consented to undergo a research DCE breast MRI on a second date. For the research DCE MRI, a method (DIfferential Subsampling with Cartesian Ordering [DISCO]) using pseudorandom k-space sampling, view sharing reconstruction, 2-point Dixon fat-water separation, and parallel imaging was used to produce images with an effective temporal resolution 6 times faster than the SOC MRI (27 vs 168 seconds, respectively). Both the SOC and DISCO MRI scans were acquired with matching spatial resolutions of 0.8 × 0.8 × 1.6 mm. Image quality (distortion/artifacts, resolution, fat suppression, lesion conspicuity, perceived signal-to-noise ratio, and overall image quality) was scored by 3 radiologists in a blinded reader study. RESULTS: Differences in image quality scores between the DISCO and SOC images were all less than 0.8 on a 10-point scale, and both methods were assessed as providing diagnostic image quality in all cases. DISCO images with the same high spatial resolution, but 6 times the effective temporal resolution as the SOC MRI scans, were produced, yielding 20 postcontrast time points with DISCO compared with 3 for the SOC MRI, over the same total time interval. CONCLUSIONS: DISCO provided comparable image quality compared with the SOC MRI, while also providing 6 times faster effective temporal resolution and the same high spatial resolution.
Assuntos
Neoplasias da Mama/diagnóstico por imagem , Meios de Contraste , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética/métodos , Adulto , Idoso , Artefatos , Mama/diagnóstico por imagem , Feminino , Humanos , Pessoa de Meia-Idade , Reprodutibilidade dos Testes , Razão Sinal-RuídoRESUMO
BACKGROUND AND PURPOSE: Three-dimensional phase-contrast (3DPC) is limited by long imaging times, limited coverage, flow artifacts, and the need to perform multiple additional 2D examinations (2DPC) to measure flow. A highly undersampled 3D radial acquisition (isotropic-voxel radial projection imaging [PCVIPR]) makes it possible to increase the product of volume coverage and spatial resolution by a factor of 30 for the same imaging time as conventional Cartesian 3DPC. This provides anatomic information over a large volume with high isotropic resolution and permits retrospective measurement of average flow rates throughout the volume. METHODS: PCVIPR acquires a reference and three flow-encoded acquisitions for each VIPR projection. Complex difference images were formed by combining information from all flow directions. Following retrospective definition of planes perpendicular to selected vessels, volume flow rates were determined by using phase-difference information. The accuracy of average flow measurement was investigated in a phantom and in six volunteers. Anatomic PCVIPR images acquired in three patients and three volunteers by using a 384(3) matrix were compared with conventional Cartesian 3DPC. RESULTS: The flow validation produced R2 = 0.99 in vitro and R2 = 0.97 in vivo. PCVIPR produced minimal streak and pulsatile flow artifacts. PCVIPR produced far higher resolution and volume coverage in comparable imaging times. The highest acceleration factors relative to 3DPC were achieved by using gadolinium-contrast material. Ultimately, acceleration factors are limited by signal-to-noise ratio. CONCLUSION: PCVIPR rapidly provides isotropic high-resolution angiographic images and permits retrospective measurement of average flow rate throughout the volume without the need to prescribe multiple 2D acquisition planes.
Assuntos
Angiografia/métodos , Artéria Basilar/diagnóstico por imagem , Artérias Cerebrais/diagnóstico por imagem , Imageamento Tridimensional , Adulto , HumanosRESUMO
RATIONALE AND OBJECTIVES: To investigate changes in lung microstructure in healthy adult subjects with no smoking history using diffusion-weighted 3He MRI. MATERIALS AND METHODS: Diffusion magnetic resonance imaging using hyperpolarized helium 3 (3He) was applied to healthy volunteers to explore the dependence of lung microstructural changes with age, reflected by changes in the apparent diffusion coefficient (ADC) of 3He in lung air spaces. Data from three sites (University of Virginia (UVa), N = 25; University of Wisconsin (UW), N = 8; University of Nottingham (UN), N = 11) were combined in pooled analysis, including a total of N = 44 subjects (age range, 18-69 years; average age, 41.7 +/- 16.7 years). RESULTS: ADC was found to depend on age at all three sites (UW, R = +0.95, P = .0003; UVa, R = +0.74, P < .0001; UN, R = +0.96, P < .0001). Increases in mean ADCs with age appeared similar across sites (UW, +0.0017 cm2 s(-1) y(-1); UVa, +0.0015 cm2 s(-1) y(-1); pooled, +0.0015 cm2 s(-1) y(-1); P = .71). In a regional analysis performed on UW data, the increase in ADC affected all regions of the lung, but the apical and middle regions showed a greater increase compared with the base of the lung. CONCLUSION: Results suggest the observed age dependence of the ADC may be caused by changes in lung microstructure that increase alveolar volume during the aging process.
Assuntos
Envelhecimento/fisiologia , Imagem de Difusão por Ressonância Magnética , Hélio , Pulmão/fisiologia , Adolescente , Adulto , Idoso , Envelhecimento/patologia , Feminino , Humanos , Isótopos , Pulmão/anatomia & histologia , Masculino , Pessoa de Meia-Idade , Capacidade de Difusão PulmonarRESUMO
BACKGROUND AND PURPOSE: Abnormal flow of CSF through the foramen magnum has been implicated in the pathogenesis of clinical deficits in association with Chiari I malformation. The purpose of this study was to test the hypothesis that peak CSF velocities in the foramen magnum are increased in patients with Chiari I malformations. METHODS: Eight adult patients with symptomatic Chiari I malformations and 10 adult volunteers were studied with cardiac gated, phase-contrast MR imaging in the axial plane at the foramen magnum. The spatial uniformity of flow velocity in the foramen magnum was assessed at 14 time frames within the R-R interval. The velocity in each of the voxels at each of the time frames was calculated, and the peak systolic and diastolic velocities were tabulated for the patients and controls. RESULTS: For the normal volunteers, the CSF velocities in the subarachnoid space were relatively uniform throughout the subarachnoid space at each of the time frames. Peak systolic velocity ranged from 1.2 to 3.3 cm/s, and peak diastolic velocity ranged from 1.6 to 4.5 cm/s. In symptomatic patients with Chiari I, velocities in the foramen magnum did not appear as uniform throughout the subarachnoid space in the phase-contrast images. Peak systolic velocities ranged from 1.8 to 4.8 cm/s, and peak diastolic velocities ranged from 2.5 to 5.3 cm/s. Peak systolic velocity was significantly higher (P =.01) in the patients than in the control volunteers. CONCLUSION: Patients with Chiari I malformations have significant elevations of peak systolic velocity in the CSF in the foramen magnum.
Assuntos
Malformação de Arnold-Chiari/diagnóstico , Líquido Cefalorraquidiano/fisiologia , Diástole/fisiologia , Forame Magno/patologia , Aumento da Imagem , Imageamento por Ressonância Magnética , Sístole/fisiologia , Adulto , Malformação de Arnold-Chiari/líquido cefalorraquidiano , Artefatos , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Valores de Referência , Reologia , Espaço Subaracnóideo/patologia , Siringomielia/líquido cefalorraquidiano , Siringomielia/diagnósticoRESUMO
Time-resolved contrast-enhanced magnetic resonance angiography (CE-MRA) provides contrast dynamics in the vasculature and allows vessel segmentation based on temporal correlation analysis. Here we present an automated vessel segmentation algorithm including automated generation of regions of interest (ROIs), cross-correlation and pooled sample covariance matrix analysis. The dynamic images are divided into multiple equal-sized regions. In each region, ROIs for artery, vein and background are generated using an iterative thresholding algorithm based on the contrast arrival time map and contrast enhancement map. Region-specific multi-feature cross-correlation analysis and pooled covariance matrix analysis are performed to calculate the Mahalanobis distances (MDs), which are used to automatically separate arteries from veins. This segmentation algorithm is applied to a dual-phase dynamic imaging acquisition scheme where low-resolution time-resolved images are acquired during the dynamic phase followed by high-frequency data acquisition at the steady-state phase. The segmented low-resolution arterial and venous images are then combined with the high-frequency data in k-space and inverse Fourier transformed to form the final segmented arterial and venous images. Results from volunteer and patient studies demonstrate the advantages of this automated vessel segmentation and dual phase data acquisition technique.
Assuntos
Algoritmos , Vasos Sanguíneos/anatomia & histologia , Gadolínio , Interpretação de Imagem Assistida por Computador/métodos , Angiografia por Ressonância Magnética/métodos , Reconhecimento Automatizado de Padrão/métodos , Análise de Variância , Meios de Contraste , Interpretação Estatística de Dados , Humanos , Aumento da Imagem/métodos , Análise de Regressão , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Estatística como AssuntoRESUMO
PURPOSE: To investigate the properties of HYPR (HighlY constrained back PRojection) processing-the temporal fidelity and the improvements of spatial/temporal resolution-for contrast-enhanced MR angiography in a pilot study of the lower extremities in healthy volunteers. MATERIALS AND METHODS: HYPR processing with a radial three-dimensional (3D) stack-of-stars acquisition was investigated for contrast-enhanced MR angiography of the lower extremities in 15 healthy volunteers. HYPR images were compared with control images acquired using a fast, multiphase, 2D Cartesian method to verify the temporal fidelity of HYPR. HYPR protocols were developed for achieving either a high frame update rate or a minimal slice thickness by adjusting the acquisition parameters. HYPR images were compared with images obtained using 3D TRICKS, a widely used protocol in dynamic 3D MRA. RESULTS: HYPR images showed good temporal agreement with 2D control images. In comparison with TRICKS, HYPR stack-of-stars demonstrated higher spatial and temporal resolution. High radial undersampling factors for each time frame were permitted, typically approximately 50 to 100 compared with fully sampled radial imaging. CONCLUSION: In this feasibility study, HYPR processing has been demonstrated to improve the spatial or temporal resolution in peripheral CE-MRA.
Assuntos
Processamento de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Perna (Membro)/irrigação sanguínea , Angiografia por Ressonância Magnética/métodos , Artefatos , Meios de Contraste , Humanos , Projetos PilotoRESUMO
Highly constrained back-projection (HYPR) is a data acquisition and reconstruction method that provides very rapid frame update rates and very high spatial resolution for a time series of images while maintaining a good signal-to-noise ratio and high image quality. In this study we used simulations to evaluate the temporal and spatial characteristics of images produced using the HYPR algorithm. The simulations demonstrate that spatial accuracy is well maintained in the images and the temporal changes in signal intensity are represented with high fidelity. The waveforms representing signal intensity as a function of time obtained from regions-of-interest placed in simulated objects track the true curves very well, with variations from the truth occurring only when objects with very different temporal behavior are very close to each other. However, even when objects with different temporal characteristics are touching, their influences on each other are small.
Assuntos
Aumento da Imagem/métodos , Processamento de Imagem Assistida por Computador/métodos , Angiografia por Ressonância Magnética/métodos , Algoritmos , Simulação por Computador , Meios de Contraste , Imagens de Fantasmas , Fatores de TempoRESUMO
PURPOSE: To investigate the utility of nonuniform angular spacing of projections in a three-dimensional (3D) hybrid undersampled projection reconstruction (PR) acquisition for contrast-enhanced (CE) time-resolved carotid imaging. MATERIALS AND METHODS: Carotid CE magnetic resonance angiography (CE-MRA) was performed on seven healthy volunteers using a time-resolved hybrid sequence that combined undersampled PR acquisition in-plane and Cartesian slice encoding through-plane. The undersampling streak artifact comes mainly from the superior-inferior (S/I) direction in carotid imaging and is suppressed by nonuniform distribution of the projections. Phantom and volunteer studies were performed to demonstrate its efficacy. RESULTS: The undersampling streak artifact was significantly suppressed through a nonuniform distribution of the projection angles with more projections aligned along the S/I direction. The hybrid PR sequence combined with nonuniform distribution of the projection angles provided time-resolved images of the carotid arteries with high temporal resolution (two seconds per frame) and high spatial resolution (1.0 x 1.0 x 1.5 mm(3)) simultaneously. CONCLUSION: High-resolution dynamic imaging of the carotid arteries is feasible with the use of a hybrid undersampled PR acquisition. Undersampling streak artifact can be suppressed significantly through nonuniform distribution of the projections.