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OBJECTIVES: To evaluate the reproducibility of liver R2* measurements between a 2D cardiac ECG-gated and a 3D breath-hold liver CSE-MRI acquisition for liver iron quantification. METHODS: A total of 54 1.5 T MRI exams from 51 subjects (18 women, 36 men, age 35.2 ± 21.8) were included. These included two sub-studies with 23 clinical MRI exams from 19 patients identified retrospectively, 24 participants with known or suspected iron overload, and 7 healthy volunteers acquired prospectively. The 2D cardiac and the 3D liver R2* maps were acquired in the same exam. Either acquisitions were reconstructed using a complex R2* algorithm that accounts for the presence of fat and residual phase errors due to eddy currents. Data were analyzed using colocalized ROIs in the liver. RESULTS: Linear regression analysis demonstrated high Pearson's correlation and Lin's concordance coefficient for the overall study and both sub-studies. Bland-Altman analysis also showed good agreement, except for a slight increase of the mean R2* value above ~ 400 s-1. The Kolmogorow-Smirnow test revealed a non-normal distribution for (R2* 3D-R2* 2D) values from 0 to 600 s-1 in contrast to the 0-200 s-1 and 0-400 s-1 subpopulations. Linear regression analysis showed no relevant differences other than the intercept, likely due to only 7 measurements above 400 s-1. CONCLUSIONS: The results demonstrate that R2*-measurements in the liver are feasible using 2D cardiac R2* maps compared to 3D liver R2* maps as the reference. Liver R2* may be underestimated for R2* > 400 s-1 using the 2D cardiac R2* mapping method.
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Sobrecarga de Hierro , Hierro , Adolescente , Adulto , Femenino , Humanos , Sobrecarga de Hierro/diagnóstico por imagen , Hígado/diagnóstico por imagen , Imagen por Resonancia Magnética , Masculino , Persona de Mediana Edad , Reproducibilidad de los Resultados , Estudios Retrospectivos , Adulto JovenRESUMEN
OBJECTIVES: Ferumoxytol is an ultra-small superparamagnetic iron oxide (USPIO) agent that is taken up by splenic tissue. This study describes our initial institutional experience of ferumoxytol-enhanced MRI (feMRI) for differentiating intrapancreatic splenules (IPS) from other pancreatic lesions. METHODS: In this retrospective study, patients with computed tomographic imaging that identified small enhancing lesions in the tail of the pancreas subsequently underwent feMRI for further characterization. The feMRI protocol included T2-weighted (T2w) imaging with and without fat suppression (FS), R2* mapping, diffusion-weighted imaging (DWI), and T1-weighted (T1w) imaging with FS, prior to contrast injection. Immediately after slow intravenous infusion with 3 mg/kg body weight ferumoxytol, T1w was repeated. Delayed imaging with all sequences were obtained 24-72 h after ferumoxytol administration. RESULTS: Seven patients underwent feMRI. In two patients, the pancreatic lesions were presumed as pancreatic neuroendocrine tumor (PNET) from feMRI and in the remaining 5 IPS. One of the two patients with PNET was symptomatic for NET. In another symptomatic patient with pathologically proven duodenal NET and suspected PNET, the pancreatic lesion was proven to be an IPS on feMRI. IPS demonstrated strong negative enhancement in feMRI on T2w and increased R2* values consistent with splenic tissue, while the presumed PNETs did not enhance. T2w FS was helpful on the pre-contrast images to identify IPS, while R2* did on post-contrast images. Neither DWI nor T1w contributed to differentiating PNETs from IPS. CONCLUSIONS: This study demonstrates the potential utility of feMRI as a helpful adjunct diagnostic tool for differentiating IPS from other pancreatic lesions. Further studies in larger patient cohorts are needed.
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Óxido Ferrosoférrico , Neoplasias , Medios de Contraste , Imagen de Difusión por Resonancia Magnética , Humanos , Imagen por Resonancia Magnética , Estudios RetrospectivosRESUMEN
BACKGROUND AND PURPOSE: Intrinsic T1-hyperintense signal has recently been reported in the deep gray nuclei on brain MR imaging after multiple doses of gadolinium-based contrast agents. Most reports have included adult patients and excluded those undergoing radiation or chemotherapy. We investigated whether T1 shortening is also observed in children and tried to determine whether radiochemotherapy is a risk factor for this phenomenon. MATERIALS AND METHODS: In this single-center retrospective study, we reviewed clinical charts and images of all patients 18 years of age or younger with ≥4 gadobenate dimeglumine-enhanced MRIs for 6 years. Seventy-six children (mean age, 9.3 years; 60 unconfounded by treatment, 16 with radiochemotherapy) met the selection criteria (>4 MR imaging examinations; mean, 8). T1 signal intensity ratios for the dentate to pons and globus pallidus to thalamus were calculated and correlated with number of injections, time interval, and therapy. RESULTS: Among the 60 children without radiochemotherapy, only 2 had elevated T1 signal intensity ratios (n = 20 and 16 injections). Twelve of the 16 children with radiochemotherapy showed elevated signal intensity ratios. Statistical analysis demonstrated a significant signal intensity ratio change for the number of injections (P < .001) and amount of gadolinium (P = .008), but not for the interscan time interval (P = .35). There was a significant difference in the average signal intensity ratio change between those with and without radiochemotherapy (P < .001). Chart review revealed no new neurologic deficits in any patients, related to their underlying conditions and prior surgeries. CONCLUSIONS: Compared with published adult series, children show a similar pattern of T1 hyperintense signal changes of the dentate and globus pallidus after multiple gadobenate dimeglumine injections. The T1 signal changes in children may have a later onset but are accelerated by radiochemotherapy.
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Encéfalo/efectos de los fármacos , Medios de Contraste/efectos adversos , Imagen por Resonancia Magnética/métodos , Meglumina/análogos & derivados , Compuestos Organometálicos/efectos adversos , Adolescente , Encéfalo/diagnóstico por imagen , Encéfalo/efectos de la radiación , Neoplasias Encefálicas/tratamiento farmacológico , Neoplasias Encefálicas/radioterapia , Quimioradioterapia , Niño , Femenino , Humanos , Masculino , Meglumina/efectos adversos , Estudios RetrospectivosRESUMEN
OBJECTIVES: Objectives of this study were to compare radial time-resolved phase contrast magnetic resonance imaging (4D Flow-MRI) with perivascular ultrasound (pvUS) and to explore a porcine model of acute pre-hepatic portal hypertension (PHTN). METHODS: Abdominal 4D Flow-MRI and pvUS in portal and splenic vein, hepatic and both renal arteries were performed in 13 pigs of approximately 60 kg. In six pigs, measurements were repeated after partial portal vein (PV) ligature. Inter- and intra-reader comparisons and statistical analysis including Bland-Altman (BA) comparison, paired Student's t tests and linear regression were performed. RESULTS: PvUS and 4D Flow-MRI measurements agreed well; flow before partial PV ligature was 322 ± 30 ml/min in pvUS and 297 ± 27 ml/min in MRI (p = 0.294), and average BA difference was 25 ml/min [-322; 372]. Inter- and intra-reader results differed very little, revealed excellent correlation (R 2 = 0.98 and 0.99, respectively) and resulted in BA differences of -5 ml/min [-161; 150] and -2 ml/min [-28; 25], respectively. After PV ligature, PV flow decreased from 356 ± 50 to 298 ± 61 ml/min (p = 0.02), and hepatic arterial flow increased from 277 ± 36 to 331 ± 65 ml/min (p = n.s.). CONCLUSION: The successful in vivo comparison of radial 4D Flow-MRI to perivascular ultrasound revealed good agreement of abdominal blood flow although with considerable spread of results. A model of pre-hepatic PHTN was successfully introduced and acute responses monitored. KEY POINTS: ⢠Radial 4D Flow-MRI in the abdomen was successfully compared to perivascular ultrasound. ⢠Inter- and intra-reader testing demonstrated excellent reproducibility of upper abdominal 4D Flow-MRI. ⢠A porcine model of acute pre-hepatic portal hypertension was successfully introduced. ⢠4D Flow-MRI successfully monitored acute changes in a model of portal hypertension.
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Velocidad del Flujo Sanguíneo/fisiología , Hipertensión Portal/diagnóstico , Circulación Hepática/fisiología , Angiografía por Resonancia Magnética/métodos , Vena Porta/diagnóstico por imagen , Ultrasonografía/métodos , Animales , Modelos Animales de Enfermedad , Arteria Hepática/diagnóstico por imagen , Arteria Hepática/fisiopatología , Hipertensión Portal/fisiopatología , Masculino , Vena Porta/fisiopatología , Reproducibilidad de los Resultados , PorcinosRESUMEN
OBJECTIVES: To compare 3D-inversion-recovery balanced steady-state free precession (IR-bSSFP) non-contrast-enhanced magnetic resonance angiography (MRA) with 3D-contrast-enhanced MRA (CE-MRA) for assessment of renal artery stenosis (RAS) using digital subtraction angiography (DSA) as the reference standard. METHODS: Bilateral RAS were surgically created in 12 swine. IR-bSSFP and CE-MRA were acquired at 1.5 T and compared to rotational DSA. Three experienced cardiovascular radiologists evaluated the IR-bSSFP and CE-MRA studies independently. Linear regression models were used to calibrate and assess the accuracy of IR-bSSFP and CE-MRA, separately, against DSA. The coefficient of determination and Cohen's kappa coefficient were also generated. RESULTS: Calibration of the three readers' RAS grading revealed R(2) values of 0.52, 0.37 and 0.59 for NCE-MRA and 0.48, 0.53 and 0.71 for CE-MRA. Inter-rater agreement demonstrated Cohen's kappa values ranging from 0.25 to 0.65. Distal renal artery branch vessels were visible to a significantly higher degree with NCE-MRA compared to CE-MRA (p < 0.001). Image quality was rated excellent for both sequences, although image noise was higher with CE-MRA (p < 0.05). In no cases did noise interfere with image interpretation. CONCLUSIONS: In a well-controlled animal model of surgically induced RAS, IR-bSSFP based NCE-MRA and CE-MRA accurately graded RAS with a tendency for stenosis overestimation, compared to DSA. KEY POINTS: ⢠IR-bSSFP and CE-MRA are accurate methods for diagnosis of renal artery stenosis ⢠IR-bSSFP and CE-MRA demonstrate excellent agreement with DSA ⢠Both IR-bSSFP and CE-MRA have a tendency to overestimate renal artery stenosis.
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Angiografía de Substracción Digital/métodos , Angiografía por Resonancia Magnética/métodos , Obstrucción de la Arteria Renal/patología , Animales , Medios de Contraste , Modelos Animales de Enfermedad , Imagenología Tridimensional/métodos , Arteria Renal/patología , Obstrucción de la Arteria Renal/diagnóstico , Reproducibilidad de los Resultados , PorcinosRESUMEN
BACKGROUND AND PURPOSE: The chronic cerebrospinal venous insufficiency hypothesis raises interest in cerebrospinal venous blood flow imaging, which is more complex and less established than in arteries. For accurate assessment of venous flow in chronic cerebrospinal venous insufficiency diagnosis and research, we must account for physiologic changes in flow patterns. This study examines day-to-day flow variability in cerebrospinal veins by use of 4D MR flow and contrast-enhanced MRA under typical, uncontrolled conditions in healthy individuals. MATERIALS AND METHODS: Ten healthy volunteers were scanned in a test-retest fashion by use of a 4D flow MR imaging technique and contrast-enhanced MRA. Flow parameters obtained from phase contrast-vastly undersampled isotropic projection reconstruction and contrast-enhanced MRA scoring measurements in the head, neck, and chest veins were analyzed for internal consistency and interscan reproducibility. RESULTS: Internal consistency was satisfied at the torcular herophili, with an input-output difference of 2.2%. Percentages of variations in flow were 20.3%, internal jugular vein; 20.4%, azygos vein; 6.8%, transverse sinus; and 5.1%, common carotid artery. Retrograde flow was found in the lower internal jugular vein (4.8%) and azygos vein (7.2%). Contrast-enhanced MRA interscan κ values for the internal jugular vein (left: 0.474, right: 0.366) and azygos vein (-0.053) showed poor interscan agreement. CONCLUSIONS: Phase contrast-vastly undersampled isotropic projection reconstruction blood flow measurements are reliable and highly reproducible in intracranial veins and in the common carotid artery but not in veins of the neck (internal jugular vein) and chest (azygos vein) because of normal physiologic variation. Retrograde flow normally may be observed in the lower internal jugular vein and azygos vein. Low interrater agreement in contrast-enhanced MRA scans was observed. These findings have important implications for imaging diagnosis and experimental research of chronic cerebrospinal venous insufficiency.
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Venas Cerebrales/anatomía & histología , Venas Cerebrales/fisiología , Gadolinio , Interpretación de Imagen Asistida por Computador/métodos , Imagenología Tridimensional/métodos , Angiografía por Resonancia Magnética/métodos , Compuestos Organometálicos , Médula Espinal/irrigación sanguínea , Adulto , Algoritmos , Anisotropía , Velocidad del Flujo Sanguíneo/fisiología , Medios de Contraste , Femenino , Humanos , Aumento de la Imagen/métodos , Masculino , Reproducibilidad de los Resultados , Tamaño de la Muestra , Sensibilidad y EspecificidadRESUMEN
Accurate, noninvasive measurements of liver fat content are needed for the early diagnosis and quantitative staging of nonalcoholic fatty liver disease. Chemical shift-based fat quantification methods acquire images at multiple echo times using a multiecho spoiled gradient echo sequence, and provide fat fraction measurements through postprocessing. However, phase errors, such as those caused by eddy currents, can adversely affect fat quantification. These phase errors are typically most significant at the first echo of the echo train, and introduce bias in complex-based fat quantification techniques. These errors can be overcome using a magnitude-based technique (where the phase of all echoes is discarded), but at the cost of significantly degraded signal-to-noise ratio, particularly for certain choices of echo time combinations. In this work, we develop a reconstruction method that overcomes these phase errors without the signal-to-noise ratio penalty incurred by magnitude fitting. This method discards the phase of the first echo (which is often corrupted) while maintaining the phase of the remaining echoes (where phase is unaltered). We test the proposed method on 104 patient liver datasets (from 52 patients, each scanned twice), where the fat fraction measurements are compared to coregistered spectroscopy measurements. We demonstrate that mixed fitting is able to provide accurate fat fraction measurements with high signal-to-noise ratio and low bias over a wide choice of echo combinations.
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Hígado Graso/diagnóstico , Aumento de la Imagen/métodos , Imagenología Tridimensional/métodos , Imagen por Resonancia Magnética/métodos , Algoritmos , Agua Corporal , Humanos , Interpretación de Imagen Asistida por Computador , Método de Montecarlo , Fantasmas de ImagenRESUMEN
PURPOSE: To demonstrate the feasibility of PC-VIPR (Phase Contrast Vastly undersampled Imaging with Projection Reconstruction) for the depiction and hemodynamic analysis of hepatic and splanchnic vessels in patients with portal hypertension. MATERIALS AND METHODS: Twenty-four cirrhotic patients (55.9 ± 10.4 years) were scanned using 5-point PC-VIPR for high spatial resolution imaging with large volume coverage at 3 Tesla (T) using a 32-channel body coil. Vessel segmentation and hemodynamic visualization included color-coded three-dimensional (3D) streamlines and particle traces. Segmentation quality was compared with contrast-enhanced multi-phase liver imaging. Flow pattern analysis was performed in consensus of three readers. The MELD score was calculated to estimate disease severity and was correlated to image quality. RESULTS: Good to excellent visualization quality was achieved in 23/24 cases. All arterial vessels and 144/168 vessels of the portal venous (PV) circulation were unambiguously identified. No correlation with the MELD score was found. Eight of 148 vessels of the PV circulation demonstrated reverse (hepatofugal) flow. Hepatofugal flow in small tributaries to PV flow were present in three cases despite hepatopetal flow in the PV. CONCLUSION: This feasibility study demonstrates the feasibility of PC-VIPR for simultaneous morphological and hemodynamic assessment of the hepatic and splanchnic vasculature in cirrhosis and portal hypertension. Future studies with quantitative analyses are warranted.
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Arteria Hepática/patología , Arteria Hepática/fisiopatología , Hipertensión Portal/patología , Hipertensión Portal/fisiopatología , Imagenología Tridimensional/métodos , Circulación Esplácnica , Velocidad del Flujo Sanguíneo , Estudios de Factibilidad , Femenino , Humanos , Angiografía por Resonancia Magnética/métodos , Masculino , Persona de Mediana Edad , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
OBJECTIVE: To compare articular cartilage signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and thickness measurements on a 1.5 T and a 3.0 T magnetic resonance (MR) scanner using three-dimensional spoiled gradient recalled echo (3D-SPGR) and two 3D steady-state free precession (SSFP) sequences. METHODS: Both knees of five volunteers were scanned at 1.5 T and at 3.0 T using a transmit-receive quadrature extremity coil. Each examination consisted of a sagittal 3D-SPGR sequence, a sagittal fat suppressed 3D-SSFP (FS-SSFP) sequence, and a sagittal Dixon 3D-SSFP sequence. For quantitative analysis, we compared cartilage SNR and CNR efficiencies, as well as average cartilage thickness measurements. RESULTS: For 3D-SPGR, cartilage SNR efficiencies at 3.0 T increased compared to those at 1.5 T by a factor of 1.83 (range: 1.40-2.09). In comparison to 3D-SPGR, the SNR efficiency of FS-SSFP increased by a factor of 2.13 (range: 1.81-2.39) and for Dixon SSFP by a factor of 2.39 (range: 1.95-2.99). For 3D-SPGR, CNR efficiencies between cartilage and its surrounding tissue increased compared to those at 1.5 T by a factor of 2.12 (range: 1.75-2.47), for FS-SSFP by a factor 2.11 (range: 1.58-2.80) and for Dixon SSFP by a factor 2.39 (range 2.09-2.83). Average cartilage thicknesses of load bearing regions were not different at both field strengths or between sequences (P>0.05). Mean average cartilage thickness measured in all knees was 2.28 mm. CONCLUSION: Articular cartilage imaging of the knee on a 3.0 T MR scanner shows increased SNR and CNR efficiencies compared to a 1.5 T scanner, where SSFP-based techniques show the highest increase in SNR and CNR efficiency. There was no difference between average cartilage thickness measurements performed at the 1.5 T and 3.0 T scanners or between the three different sequences.
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Cartílago Articular/anatomía & histología , Articulación de la Rodilla/anatomía & histología , Imagen por Resonancia Magnética/métodos , Adulto , Cartílago Articular/fisiología , Femenino , Fémur/anatomía & histología , Humanos , Procesamiento de Imagen Asistido por Computador/métodos , Articulación de la Rodilla/fisiología , Masculino , Soporte de PesoRESUMEN
OBJECTIVE: Treatment of iatrogenic femoral artery pseudoaneurysms with thrombin injection has been reported as an efficacious and safe procedure. The major risk of this procedure is distal limb ischemia from thrombosis, resulting from thrombin escape. The cumulative average dose of thrombin reported in the literature is approximately 1100 U per patient. Minimizing the thrombin dose may reduce the risks of the procedure. This study reports our experience with low-dose thrombin injection for the treatment of pseudoaneurysms. MATERIALS AND METHODS: Twenty-three patients with 26 postcatheterization femoral pseudoaneurysms were administered thrombin injection with color-flow Doppler sonographic guidance. Pseudoaneurysm volume ranged from 1 to 41 cm(3) with an average of 6.7 cm(3) and a median of 4 cm(3). Two patients received therapeutic doses of IV heparin for anticoagulation. When possible, the neck of the pseudoaneurysm was occluded during the injection to promote stagnation and prevent thrombin leakage. Sonographic follow-up was routinely performed after 24 hr. RESULTS: An average dose of 192 U of thrombin was used (range, 50-450 U), and time to coagulation ranged from 10 to 60 sec. All 26 pseudoaneurysms were successfully thrombosed, although one required repeated treatment because of recanalization noted at 1-day follow-up. There were no complications. CONCLUSION: Doses of thrombin at an average of fivefold lower than previously reported were effective in the treatment of 26 iatrogenic femoral pseudoaneurysms, even in the presence of anticoagulation. This experience shows that a much smaller dose of a potentially dangerous medication can achieve the same efficacy as previously used higher doses.
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Aneurisma Falso/tratamiento farmacológico , Cateterismo Cardíaco , Arteria Femoral , Trombina/administración & dosificación , Anciano , Anciano de 80 o más Años , Aneurisma Falso/diagnóstico por imagen , Relación Dosis-Respuesta a Droga , Femenino , Arteria Femoral/diagnóstico por imagen , Arteria Femoral/efectos de los fármacos , Humanos , Enfermedad Iatrogénica , Inyecciones , Masculino , Persona de Mediana Edad , Resultado del Tratamiento , Ultrasonografía Doppler en ColorRESUMEN
Important advances in rapid magnetic resonance (MR) imaging technology and its application to cardiovascular imaging have been made during the past decade. High-field-strength clinical magnets, high-performance gradient hardware, and ultrafast pulse sequence technology are rapidly making the vision of a comprehensive "one-stop shop" cardiac MR imaging examination a reality. This examination is poised to have a significant effect on the management of coronary artery disease by means of assessment of wall motion with tagging and pharmacologic stress testing, evaluation of the coronary microvasculature with perfusion imaging, and direct visualization of the coronary arteries with MR coronary angiography. This article reviews current state-of-the-art pulse sequence technology and its application to the evaluation of ischemic heart disease by means of MR tagging with dobutamine stress testing, MR perfusion imaging, and MR coronary angiography. Cutting edge areas of research in coil design and exciting new areas of metabolic and oxygen level-dependent imaging are also explored.
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Imagen por Resonancia Magnética/métodos , Isquemia Miocárdica/diagnóstico , Isquemia Miocárdica/fisiopatología , Cardiotónicos , Medios de Contraste , Dobutamina , Humanos , Angiografía por Resonancia Magnética , Imagen por Resonancia Magnética/instrumentaciónRESUMEN
Cardiac magnetic resonance imaging is a rapidly emerging field that has seen tremendous advances in the past decade. Central to the development of effective imaging strategies has been the advent of high-performance gradient hardware and the exploitation of their speed characteristics through specialized pulse sequences well suited for cardiac imaging. These advances have facilitated unprecedented acquisition times that now approach echocardiographic frame rates, while maintaining excellent image quality. This article provides a detailed overview of advanced pulse sequence technology and approaches currently taken to maximize speed performance and image quality. In particular, segmented K-space techniques that include single-echo and multiecho spoiled gradient-echo imaging as well as steady-state free precession imaging are discussed. Finally, spiral and fast spin-echo techniques are explored. Examples of common applications of these pulse sequences are presented.
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Imagen Eco-Planar/métodos , Cardiopatías/diagnóstico , Intensificación de Imagen Radiográfica/métodos , Femenino , Análisis de Fourier , Humanos , Imagen por Resonancia Magnética/métodos , Masculino , Sensibilidad y EspecificidadRESUMEN
PURPOSE: To determine whether myocardial arterial perfusion and oxygen concentration can be quantified simultaneously from the same images by using spin labeling and the blood oxygenation level-dependent (BOLD) effect with fast spin-echo (SE) imaging. MATERIALS AND METHODS: A T2-weighted fast SE pulse sequence was written to image isolated, arrested, blood-perfused rabbit hearts (n = 6) at 4.7 T. Perfusion images with intensity in units of milliliters per minute per gram that covered the entire left ventricle with 0.39 x 0.39 x 3.00-mm resolution were obtained in less than 15 minutes with a 32-fold reduction in imaging time from that of a previous study. Estimates of oxygen concentration were made from the same images acquired for calculation of perfusion images. RESULTS: Estimates of regional myocardial oxygen content could be made from the perfusion images; this demonstrated the feasibility of three-dimensional calculation of regional oxygen consumption, which requires concomitant measurement of both oxygen content and flow. Fast SE imaging was shown to be as sensitive to hemoglobin desaturation as standard SE imaging. Perfusion abnormalities and oxygen deficits were easily identified and verified qualitatively with gadopentetate dimeglumine on both perfusion and BOLD images obtained after coronary arterial ligation. CONCLUSION: T2-weighted fast SE imaging combined with perfusion-sensitive spin labeling can be used to measure myocardial arterial perfusion and oxygen concentration. This provides the groundwork for calculation of regional myocardial oxygen consumption.
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Circulación Coronaria/fisiología , Procesamiento de Imagen Asistido por Computador/instrumentación , Imagen por Resonancia Magnética/instrumentación , Miocardio/metabolismo , Consumo de Oxígeno/fisiología , Animales , Imagen Eco-Planar/instrumentación , Hemoglobinas/análisis , Masculino , Infarto del Miocardio/fisiopatología , ConejosRESUMEN
Interleaved echo-planar imaging (EPI) is susceptible to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. Most EPI applications rely on "reference scans" to measure delays, and post-processing algorithms are used to correct these errors. Unfortunately, delay estimates made with most reference scan techniques are object dependent, since they are biased by magnetic field inhomogeneities and chemical shift. The current work describes the effects of field inhomogeneities and their influence on system time delay estimation. Subsequently, a new, object-independent "balanced" reference method using two readout echo trains is proposed for time delay measurements.
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Imagen Eco-Planar/métodos , Procesamiento de Imagen Asistido por Computador/métodos , Algoritmos , Artefactos , Encéfalo/anatomía & histología , Humanos , Modelos Anatómicos , Valores de ReferenciaRESUMEN
To investigate the effects of water exchange on quantification of perfusion, data were acquired in isolated hearts (n = 11) and used to develop a model of exchange. Myocardial T1 was measured 3 times/sec during step changes in concentration of intravascular (polylysine-gadolinium-diethylene-triamine-pentaacetic acid) and extracellular (gadoteridol) agents. For the intravascular agent, the change in 1/T1 (deltaR1) was lower than predicted by fast exchange (2.7+/-0.5 vs. 7.8 sec(-1), respectively), and suggested an intra-extravascular exchange rate of 3 Hz. For the extracellular agent, contrast kinetics were similar to those of similarly sized molecules (wash-in time constant 38+/-5 sec), and the data suggested fast interstitial-cellular exchange. Modeling showed that perfusion is underestimated for both agents if exchange is ignored, although the relationships of measured to actual perfusion were monotonic. We conclude that myocardial water exchange strongly affects first-pass enhancement but that ignoring the effects of exchange may still provide reasonable estimates of regional perfusion differences.
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Agua Corporal/metabolismo , Medios de Contraste , Circulación Coronaria , Imagen por Resonancia Magnética , Miocardio/metabolismo , Animales , Gadolinio , Gadolinio DTPA , Compuestos Heterocíclicos , Técnicas In Vitro , Compuestos Organometálicos , Polilisina/análogos & derivados , ConejosRESUMEN
Cardiac magnetic resonance imaging requires high temporal resolution to resolve motion and contrast uptake with low total scan times to avoid breathing artifacts. While spoiled gradient echo (SPGR) imaging is robust and reproducible, it is relatively inefficient and requires long breath-holds to acquire high time resolution movies of the heart. Echo planar imaging (EPI) is highly efficient with excellent signal-to-noise ratio (SNR) behavior; however, it is particularly difficult to use in the heart because of its sensitivity to chemical shift, susceptibility, and motion. EPI may also require reference scans, which are used to measure hardware delays and phase offsets that cause ghosting artifacts; these reference scans are more difficult and less reliable in the heart. Consequently, a hybrid EPI/SPGR sequence is proposed for application to rapid cardiac imaging. A detailed optimization of SNR and echo train length for multi-echo sequences is presented. It is shown that significant reductions in total scan time are possible while maintaining good image quality. This will allow complete motion sampling of the entire heart in one to three breath-holds, necessary for MR cardiac dobutamine stress testing. Improved speed performance also permits sampling of three to six slices every heartbeat for bolus injection perfusion studies.
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Imagen Eco-Planar/métodos , Corazón/anatomía & histología , Imagen por Resonancia Magnética/métodos , Contracción Miocárdica , HumanosRESUMEN
Interleaved echo-planar imaging (EPI) is an ultrafast imaging technique important for applications that require high time resolution or short total acquisition times. Unfortunately, EPI is prone to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. In this work, it is shown mathematically and experimentally that system time delays are orientation dependent, resulting from anisotropic physical gradient delays. This analysis characterizes the behavior of time delays in oblique coordinates, and a new ghosting artifact caused by anisotropic delays is described. "Compensation blips" are proposed for time delay correction. These blips are shown to remove the effects of anisotropic gradient delays, eliminating the need for repeated reference scans and postprocessing corrections. Examples of phantom and in vivo images are shown.
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Encéfalo/anatomía & histología , Imagen Eco-Planar , Algoritmos , Anisotropía , Artefactos , Humanos , Procesamiento de Imagen Asistido por Computador , Modelos Teóricos , Fantasmas de ImagenRESUMEN
BACKGROUND: Microvascular obstruction within an area of myocardial infarction indicates worse functional recovery and a higher risk of postinfarction complications. After prolonged coronary occlusion, contrast-enhanced MRI identifies myocardial infarction as a hyperenhanced region containing a hypoenhanced core. Because the time course of microvascular obstruction after infarction/reperfusion is unknown, we examined whether microvascular obstruction reaches its full extent shortly after reperfusion or shows significant progression over the following 2 days. METHODS AND RESULTS: Seven dogs underwent 90-minute balloon occlusion of the left anterior descending coronary artery (LAD) followed by reflow. Gadolinium-DTPA-enhanced MRI performed at 2, 6, and 48 hours after reperfusion was compared with radioactive microsphere blood flow (MBF) measurements and myocardial staining to define microvascular obstruction (thioflavin S) and infarct size (triphenyltetrazolium chloride, TTC). The MRI hypoenhanced region increased 3-fold during 48 hours after reperfusion (3.2+/-1.8%, 6.7+/-4.4%, and 9.9+/-3.2% of left ventricular mass at 2, 6, and 48 hours, respectively, P<0.03) and correlated well with microvascular obstruction (MBF <50% of remote region, r=0.99 and thioflavin S, r=0.93). MRI hyperenhancement also increased (21.7+/-4.0%, 24.3+/-4.6%, and 28.8+/-5.1% at 2, 6, and 48 hours, P<0.006) and correlated well with infarct size by TTC (r=0.92). The microvascular obstruction/infarct size ratio increased from 13.0+/-4.8% to 22.6+/-8.9% and to 30.4+/-4.2% over 48 hours (P=0.024). CONCLUSION: The extent of microvascular obstruction and the infarct size increase significantly over the first 48 hours after myocardial infarction. These results are consistent with progressive microvascular and myocardial injury well beyond coronary occlusion and reflow.
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Circulación Coronaria , Microcirculación/patología , Infarto del Miocardio/patología , Infarto del Miocardio/fisiopatología , Reperfusión Miocárdica , Miocardio/patología , Análisis de Varianza , Animales , Presión Sanguínea , Perros , Gadolinio DTPA , Frecuencia Cardíaca , Imagen por Resonancia Magnética , Microcirculación/fisiopatología , Factores de TiempoRESUMEN
Adiabatic pulses, although useful in generating uniform spin nutation in the presence of inhomogeneous B1 fields, are limited for NMR imaging applications due to the lack of slice-selective excitation capability. Selective excitation techniques using gradient modulation have been introduced; however, present methods require either a minimum of two excitations or eight adiabatic segments. Here, a scheme is presented that allows single-shot, arbitrary flip-angle, and slice-selective excitation with only four adiabatic half-passage segments. The technique is demonstrated via computer simulation and experimental tests on a phantom. Furthermore, issues associated with the implementation of these gradient-modulated adiabatic pulses are discussed.
Asunto(s)
Imagen por Resonancia Magnética , Espectroscopía de Resonancia Magnética , Simulación por Computador , Análisis de Fourier , Humanos , Fantasmas de Imagen , Sensibilidad y EspecificidadRESUMEN
Cardiac echo-planar imaging suffers invariably from regions of severe distortion and T*2 decay in the myocardium. The purpose of this work was to perform local measurements of T*2 and field inhomogeneities in the myocardium and to identify the sources of focal signal loss and distortion. Field inhomogeneity maps and T*2 were measured in five normal volunteers in short-axis slices spanning from base to apex. It was found that T*2 ranged from 26 ms (SD = 7 ms, n = 5) to 41 ms (SD = 11 ms, n = 5) over most of the heart, and peak-to-peak field inhomogeneity differences were 71 Hz (SD = 14 Hz, n = 5). In all hearts, regions of severe signal loss were consistently adjacent to the posterior vein of the left ventricle; T*2 in these regions was 12 ms (SD = 2 ms, n = 5), and the difference in resonance frequency with the surrounding myocardium was 70-100 Hz. These effects may be caused by increased magnetic susceptibility from deoxygenated blood in these veins.