Quantification of pulmonary perfusion with MR imaging: recent advances

Recent advances in magnetic resonance pulmonary perfusion imaging are reviewed, focusing on magnetic resonance perfusion imaging using gadolinium contrasts agents or spin labeling of blood using naturally flowing spins as the source of intravascular signal. These recent developments in magnetic resonance imaging have made it possible to analyze data quantitatively which holds significant potential for clinical imaging of lung perfusion and opens windows to functional MR imaging of the lung. We believe that fast magnetic resonance functional imaging will play an important role in the assessment of pulmonary function and the pulmonary disease process.     

MRI of the pulmonary parenchyma

Imaging of the pulmonary parenchyma represents a unique challenge for MRI. Limited signal is caused by low proton density, susceptibility artifacts, and physiological motion (cardiac pulsation, respiration). Recently, further improvements in MRI techniques have widened the potential for investigations of pulmonary parenchymal disease. These include very short echo times, ultrafast turbo-spin-echo acquisitions, projection reconstruction technique, breathhold imaging, ECG triggering, contrast agents (perfusion imaging, aerosols), sodium imaging, hyperpolarized noble gas imaging, and oxygen enhancement. By using widely available techniques, MRI is helpful in the assessment of (a) acute alveolitic processes in chronic infiltrative lung disease, (b) detection and characterization of pulmonary nodules, (c) detection, characterization, and follow-up of pneumonia, (d) differentiation of obstructive atelectasis from non-obstructive atelectasis and infarctions, and (e) measurements of lung water content. Chronic bronchitis, bronchiectasis, and emphysema are not readily assessable by routine MRI techniques. More sophisticated techniques are under investigation for MR imaging of pulmonary ventilation and perfusion. They represent the beginning of functional MR imaging of the lung which will be established in the future.     

MR imaging of lung cancer

Since publication of the Radiologic Diagnostic Oncology Group Report in 1991, the clinical application of pulmonary magnetic resonance (MR) imaging to patients with lung cancer has been limited. Computed tomography has been much more widely available for staging of lung cancer in clinical situations. Currently, ventilation and perfusion scintigraphy is the only modality that demonstrates pulmonary function while 2-[fluorine-18]-fluoro-2-deoxy-D-glucose positron emission tomography is the only modality that reveals biological glucose metabolism of lung cancer. However, recent advancements in MR imaging have made it possible to evaluate morphological and functional information in lung cancer patients more accurately and quantitatively. Pulmonary MR imaging may hold significant potential to substitute for nuclear medicine examinations. In this review, we describe recent advances in MR imaging of lung cancer, focusing on (1) characterization of solitary pulmonary nodules; (2) differentiation from secondary change; evaluation of (3) medastinal invasion, (4) chest wall invasion, (5) lymph node metastasis, and (6) distant metastasis; and (7) pulmonary functional imaging. We believe that further basic studies, as well as clinical applications of newer MR techniques, will play an important role in the management of patients with lung cancer.     

Pulmonary ventilation-perfusion MR imaging in clinical patients

The purpose of this study was to evaluate the feasibility of comprehensive magnetic resonance (MR) assessment of pulmonary perfusion and ventilation in patients. Both oxygen-enhanced ventilation MR images and first-pass contrast-enhanced perfusion MR images were obtained in 16 patients with lung diseases, including pulmonary embolism, lung malignancy, and bulla. Inversion recovery single-shot fast spin-echo images were acquired before and after inhalation of 100% oxygen. The overall success rate of perfusion MR imaging and oxygen-enhanced MR imaging was 94% and 80%, respectively. All patients with pulmonary embolism showed regional perfusion deficits without ventilation abnormality on ventilation-perfusion MR imaging. The results of the current study indicate that ventilation-perfusion MR imaging using oxygen inhalation and bolus injection of MR contrast medium is feasible for comprehensive assessment of pulmonary ventilation-perfusion abnormalities in patients with lung diseases.     

Diffusion and perfusion MRI of the lung and mediastinum

With ongoing technical improvements such as multichannel MRI, systems with powerful gradients as well as the development of innovative pulse sequence techniques implementing parallel imaging, MRI has now entered the stage of a radiation-free alternative to computed tomography (CT) for chest imaging in clinical practice. Whereas in the past MRI of the lung was focused on morphological aspects, current MRI techniques also enable functional imaging of the lung allowing for a comprehensive assessment of lung disease in a single MRI exam.Perfusion imaging can be used for the visualization of regional pulmonary perfusion in patients with different lung diseases such as lung cancer, chronic obstructive lung disease, pulmonary embolism or for the prediction of postoperative lung function in lung cancer patients. Over the past years diffusion-weighted MR imaging (DW-MRI) of the thorax has become feasible with a significant reduction of the acquisition time, thus minimizing artifacts from respiratory and cardiac motion. In chest imaging, DW-MRI has been mainly suggested for the characterization of lung cancer, lymph nodes and pulmonary metastases.In this review article recent MR perfusion and diffusion techniques of the lung and mediastinum as well as their clinical applications are reviewed.     

Ultrafast MR grid-tagging sequence for assessment of local mechanical properties of the lungs.

While MR imaging with tagged magnetization has shown great utility in the study of muscle mechanics, the evaluation of pulmonary mechanics has long been hindered by the technical difficulties in MR imaging of lung parenchyma. In this study, a fast MR grid-tagging technique is described for dynamic assessment of regional pulmonary deformation. The method is based on a fast FLASH sequence with short TR and short TE. Tagging was achieved by using double DANTE pulse train or inversion pulses. Our results show that this technique is able to detect changes of the tagging grid caused by physiological deformation of the lung. Quantitative analysis of the data shows that this method is capable of assessing local pulmonary mechanics. The application of this technique could improve our understanding of ventilatory control, and thus provide a unique metric for assessing pulmonary disorders. Magn Reson Med 45:24-28, 2001.     

Lung morphology: fast MR imaging assessment with a volumetric interpolated breath-hold technique: initial experience with patients

PURPOSE: To prospectively evaluate the clinical feasibility of magnetic resonance (MR) imaging of the lungs with fast volumetric interpolated three-dimensional (3D) gradient-recalled-echo (GRE) sequences and to compare this examination with standard computed tomography (CT) in patients with lung abnormalities. MATERIALS AND METHODS: Twenty-five patients with different lung abnormalities were examined with 3D GRE MR imaging. The small pulmonary nodules in seven, TNM stage of large intrapulmonary tumors in eight, and benign bronchial disease in five patients were evaluated. MR imaging-based diagnoses were compared with diagnoses made at CT and at discharge from the hospital. Contingency tables and the McNemar test were used to evaluate the significance of differences between MR imaging- and CT-based diagnoses. RESULTS: The MR imaging- and CT-based diagnoses were identical in 24 of 25 patients. In the remaining patient, clinical findings confirmed the accuracy of the MR imaging finding of pleural empyema. Ten of 15 solid pulmonary nodules smaller than 10 mm in diameter were detected at MR imaging (P >.1). Tumor stages at MR imaging and CT were identical, but lymph node stages at the two examinations differed in two of eight patients owing to overestimation of lymph node size at MR imaging (P >.2). In the five patients with bronchiectasis, MR imaging depicted 26 of 33 affected lung segments; differences between MR imaging and CT findings of bronchial dilatation (P >.05) and bronchial wall thickening (P >.2) were not significant. Peribronchial fibrosis was overestimated at MR imaging owing to image artifacts (P <.05). CONCLUSION: Study results confirmed the feasibility of fast breath-hold 3D GRE MR imaging of the lung.     

MRT der Lungenperfusion

With technical improvements in gradient hardware and the implementation of innovative k-space sampling techniques, such as parallel imaging, the feasibility of pulmonary perfusion MRI could be demonstrated in several studies. Dynamic contrast-enhanced 3D gradient echo sequences as used for time-resolved MR angiography have been established as the preferred pulse sequences for lung perfusion MRI. With these techniques perfusion of the entire lung can be visualized with a sufficiently high temporal and spatial resolution. In several trials in patients with acute pulmonary embolism, pulmonary hypertension and airway diseases, the clinical benefit and good correlation with perfusion scintigraphy have been demonstrated. The following review article describes the technical prerequisites, current post-processing techniques and the clinical indications for MR pulmonary perfusion imaging using MRI.     

Pulmonary magnetic resonance angiography.

Early attempts to image the pulmonary vasculature with spin-echo magnetic resonance (MR) imaging were hampered by severe image degradation related to respiratory and cardiac pulsation artifact, susceptibility at interfaces between lung parenchyma and vessel wall, and poor contrast between flowing blood and intravascular filling defects of emboli. With the development of gradient-echo MR angiographic techniques some of these limitations were overcome; however, the need for multiple breath-holds and the frequent occurrence of flow-related artifacts that could simulate pulmonary emboli diminished their clinical utility. With the development of contrast-enhanced MR angiography, many of the limitations of earlier techniques were addressed. Images of both lungs with high signal-to-noise ratios and high contrast between flowing blood and pulmonary emboli could be acquired in a single breath-hold, during "first-pass" imaging with extracellular contrast agents in the coronal plane. However, subsegmental vessels could not be assessed with this approach. The technique has been refined further by imaging each lung separately in the sagittal plane; this offers higher resolution and total lung coverage and requires a shorter breath-hold. Finally, several investigators have reported preliminary data on imaging of the pulmonary vasculature with blood pool agents, exploiting respiratory triggering or navigator echoes to eliminate the need for breath-holding for the detection of pulmonary emboli.     

Pulmonary disorders: ventilation-perfusion MR imaging with animal models

PURPOSE: To demonstrate the capability of magnetic resonance (MR) imaging to assess alteration in regional pulmonary ventilation and perfusion with animal models of airway obstruction and pulmonary embolism. MATERIALS AND METHODS: Airway obstruction was created by inflating a 5-F balloon catheter into a secondary bronchus. Pulmonary emboli were created by injecting thrombi into the inferior vena cava. Regional pulmonary ventilation was assessed with 100% oxygen as a T1 contrast agent. Regional pulmonary perfusion was assessed with a two-dimensional fast low-angle shot, or FLASH, sequence with short repetition and echo times after intravenous administration of gadopentetate dimeglumine. RESULTS: Matched ventilation and perfusion abnormalities were identified in all animals with airway obstruction. MR perfusion defects without ventilation abnormalities were seen in all animals with pulmonary emboli. CONCLUSION: Ventilation and perfusion MR imaging are able to provide regional pulmonary functional information with high spatial and temporal resolution. The ability of MR imaging to assess both the magnitude and regional distribution of pulmonary functional impairment could have an important effect on the evaluation of lung disease.     

Contrast-enhanced MRI of the lung

The lung has long been neglected by MR imaging. This is due to unique intrinsic difficulties: (1) signal loss due to cardiac pulsation and respiration; (2) susceptibility artifacts caused by multiple air-tissue interfaces; (3) low proton density. There are many MR strategies to overcome these problems. They consist of breath-hold imaging, respiratory and cardiac gating procedures, use of short repetition and echo times, increase of the relaxivity of existing spins by administration of intravenous contrast agents, and enrichment of spin density by hyperpolarized noble gases or oxygen. Improvements in scanner performance and frequent use of contrast media have increased the interest in MR imaging and MR angiography of the lung. They can be used on a routine basis for the following indications: characterization of pulmonary nodules, staging of bronchogenic carcinoma, in particular assessment of chest wall invasion; evaluation of inflammatory activity in interstitial lung disease; acute pulmonary embolism, chronic thromboembolic pulmonary hypertension, vascular involvement in malignant disease; vascular abnormalities. Future perspectives include perfusion imaging using extracellular or intravascular (blood pool) contrast agents and ventilation imaging using inhalation of hyperpolarized noble gases, of paramagnetic oxygen or of aerosolized contrast agents. These techniques represent new approaches to functional lung imaging. The combination of visualization of morphology and functional assessment of ventilation and perfusion is unequalled by any other technique.     

Oxygen-enhanced MR imaging of mice lungs.

Inhaled molecular oxygen has been widely used in humans to evaluate pulmonary ventilation using MRI. MR imaging has recently played a greater role in examining the morphologic and physiologic characteristics of mouse models of lung disease where structural changes are highly correlated to abnormalities in respiratory function. The motivation of this work is to develop oxygen-enhanced MR imaging for mice. Conventional human MR techniques cannot be directly applied to mouse imaging due to smaller dimensions and faster cardiac and respiratory physiology. This study examines the development of oxygen-enhanced MR as a noninvasive tool to assess regional ventilation in spontaneously breathing mice. An optimized cardiac-triggered, respiratory-gated fast spin-echo imaging sequence was developed to address demands of attaining adequate signal from the parenchyma, maintaining practical acquisition times, and compensating for rapid physiological motion. On average, a 20% T1-shortening effect was observed in mice breathing 100% oxygen as compared to air. The effect of ventilation was shown as a significant signal intensity increase of 11% to 16% in the mouse parenchyma with 100% oxygen inhalation. This work demonstrates that adequate contrast and resolution can be achieved using oxygen-enhanced MR to visualize ventilation, providing an effective technique to study ventilation defects in mice.     

肺部疾病影像诊断分析

目的 评价X线胸片、CT及磁共振成像对几种常见肺疾患的诊断准确率及选择诊断方法的考虑。方法347例肺疾患包括肺炎97例、肺结核85例、肺癌108例、支气管扩张50例,以及其他7例,均经至少一种影像技术检查,其中,149例经手术病理证实,198例经临床验证。结果X线胸片对肺炎与肺结核的诊断准确率较高,分别为95％和80％,而CT对肺癌与支气管扩张的诊断准确率较高,分别为92．5％和100％。结论不同影像技术的诊断准确性随肺疾患的病种不同而异,这对合理选择诊断方法具有实际指导意义。通常,肺部炎症和结核多数以X线平片就能确诊,当平片上影像与临床不符或形态不典型,以及单靠平片难以确定时,则需借助于CT或核磁进一步观察病灶内结构及其与周围组织的关系,以获得准确诊断。     

Scintigraphic and MR perfusion imaging in preoperative evaluation for lung volume reduction surgery: pilot study results

PURPOSE: To compare MR perfusion imaging with perfusion scintigraphy in the evaluation of patients with pulmonary emphysema being considered for lung volume reduction surgery. PATIENTS AND METHODS: Six patients with pulmonary emphysema and two normal individuals were evaluated by MR perfusion imaging, perfusion scintigraphy, and selective bilateral pulmonary angiography. MR images were obtained with an enhanced fast gradient recalled echo with three-dimensional Fourier transformation technique (efgre 3D) (6.3/1.3; flip angle, 30 degrees; field of view, 45-48 cm; matrix, 256 x 160). The presence or absence of perfusion defects in each segment was evaluated by two independent observers. RESULTS: Using angiography as the gold standard, the sensitivity, specificity, and accuracy of MR perfusion imaging in detecting focal perfusion abnormalities were 90%, 87%, and 89%, respectively, while those of perfusion scintigraphy were 71%, 76%, and 71%, respectively. The diagnostic accuracy of MR perfusion imaging was significantly higher than that of scintigraphy (p<0.001, McNemar test). There was good agreement between two observers for MR perfusion imaging (kappa statistic, 0.66) and only moderate agreement for perfusion scintigraphy (kappa statistic, 0.51). CONCLUSION: MR perfusion imaging is superior to perfusion scintigraphy in the evaluation of pulmonary parenchymal perfusion in patients with pulmonary emphysema.     

Whole-body MR imaging: evaluation of patients for metastases

PURPOSE: To compare the results of whole-body magnetic resonance (MR) imaging with staging based on computed tomographic (CT), dedicated MR imaging, and nuclear scintigraphic results as standard of reference. MATERIALS AND METHODS: Fifty-one patients with known malignant tumors were included in the study. Patients were placed on a rolling table platform capable of moving the patient rapidly through the isocenter of the magnet bore. The thorax and the abdomen were imaged by using fast breath-hold T2-weighted sequences in the transverse plane. After intravenous administration of a paramagnetic contrast agent, three-dimensional gradient-echo data sets were collected in five stations and covered the body from the skull to the knees. Location and size of cerebral, pulmonary, hepatic, and osseous metastases were documented by two experienced radiologists. Whole-body MR imaging findings were compared with results obtained at skeletal scintigraphy, CT, and dedicated MR imaging. RESULTS: The mean examination time for whole-body MR imaging was 14.5 minutes. All cerebral, pulmonary, and hepatic metastases greater than 6 mm in diameter could be identified with whole-body MR imaging. Small pulmonary metastases were missed with MR imaging, which did not change therapeutic strategies, but MR imaging depicted a single hepatic metastasis that was missed with CT. Skeletal scintigraphy depicted osseous metastases in 21 patients, whereas whole-body MR imaging revealed osseous metastases in 24 patients. The additional osseous metastases seen with MR imaging were confirmed at follow-up examinations but did not result in a change in therapy. Whole-body MR imaging performed on a per-patient basis revealed sensitivity and specificity values of 100%. CONCLUSION: Whole-body MR imaging for the evaluation of metastases compared well with the reference techniques for cerebral, pulmonary, and hepatic lesions. Whole-body MR imaging was more sensitive in the detection of hepatic and osseous metastases than were the reference techniques.     

MR Imaging of Congenital Heart Diseases in Adolescents and Adults

Echocardiography and catheterization angiography suffer certain limitations in the evaluation of congenital heart diseases in adults, though these are overcome by MRI, in which a wide field-of view, unlimited multiplanar imaging capability and three-dimensional contrast-enhanced MR angiography techniques are used. In adults, recently introduced fast imaging techniques provide cardiac MR images of sufficient quality and with less artifacts. Ventricular volume, ejection fraction, and vascular flow measurements, including pressure gradients and pulmonary-to-systemic flow ratio, can be calculated or obtained using fast cine MRI, phase-contrast MR flow-velocity mapping, and semiautomatic analysis software. MRI is superior to echocardiography in diagnosing partial anomalous pulmonary venous connection, unroofed coronary sinus, anomalies of the pulmonary arteries, aorta and systemic veins, complex heart diseases, and postsurgical sequelae. Biventricular function is reliably evaluated with cine MRI after repair of te ralogy of Fallot, and Senning''s and Mustard''s operations. MRI has an important and growing role in the morphologic and functional assessment of congenital heart diseases in adolescents and adults.     

MR imaging of the pulmonary vasculature—an update

Although the advent of multi-detector row computed tomography (CT) angiography has been at the heart of improving the diagnostic management of pulmonary vascular disease, MR technology has also moved forward. This review outlines the current state of affairs of MR techniques for the assessment of pulmonary vascular diseases such as pulmonary hypertension, pulmonary arteritis and arteriovenous malformations. It highlights the main areas of MR angiography and MR perfusion imaging and discusses novel methods, such as non-contrast enhanced direct thrombus imaging, and will discuss its merits in the context of other diagnostic modalities.     

Pulmonary arterial hypertension: diagnosis with fast perfusion MR imaging and high-spatial-resolution MR angiography--preliminary experience

PURPOSE: To determine prospectively the accuracy of a magnetic resonance (MR) perfusion imaging and MR angiography protocol for differentiation of chronic thromboembolic pulmonary arterial hypertension (CTEPH) and primary pulmonary hypertension (PPH) by using parallel acquisition techniques. MATERIALS AND METHODS: The study was approved by the institution's internal review board, and all patients gave written consent prior to participation. A total of 29 patients (16 women; mean age, 54 years +/- 17 [+/- standard deviation]; 13 men; mean age, 57 years +/- 15) with known pulmonary hypertension were examined with a 1.5-T MR imager. MR perfusion imaging (temporal resolution, 1.1 seconds per phase) and MR angiography (matrix, 512; voxel size, 1.0 x 0.7 x 1.6 mm) were performed with parallel acquisition techniques. Dynamic perfusion images and reformatted three-dimensional MR angiograms were analyzed for occlusive and nonocclusive changes of the pulmonary arteries, including perfusion defects, caliber irregularities, and intravascular thrombi. MR perfusion imaging results were compared with those of radionuclide perfusion scintigraphy, and MR angiography results were compared with those of digital subtraction angiography (DSA) and/or contrast material-enhanced multi-detector row computed tomography (CT). Sensitivity, specificity, and diagnostic accuracy of MR perfusion imaging and MR angiography were calculated. Receiver operator characteristic analyses were performed to compare the diagnostic value of MR angiography, MR perfusion imaging, and both modalities combined. For MR angiography and MR perfusion imaging, kappa values were used to assess interobserver agreement. RESULTS: A correct diagnosis was made in 26 (90%) of 29 patients by using this comprehensive MR imaging protocol. Results of MR perfusion imaging demonstrated 79% agreement (ie, identical diagnosis on a per-patient basis) with those of perfusion scintigraphy, and results of MR angiography demonstrated 86% agreement with those of DSA and/or CT angiography. Interobserver agreement was good for both MR perfusion imaging and MR angiography (kappa = 0.63 and 0.70, respectively). CONCLUSION: The combination of fast MR perfusion imaging and high-spatial-resolution MR angiography with parallel acquisition techniques enables the differentiation of PPH from CTEPH with high accuracy.     

Neue Entwicklungen in der MRT des Thorax

MRI was not used often for lung imaging due to technical and physical limitations. Recent developments have considerably improved anatomical MR imaging, and at the same time new perspectives for functional imaging emerged. They consist of functional investigations of pulmonary perfusion (contrast agents, MR angiography) and ventilation (inhaled contrast aerosols, oxygen, hyperpolarized noble gases [He-3, Xe-129] and fluorinated gases [SF6]). New parameters can be measured: homogeneity of ventilation, lung volumes, airspace size, intrapulmonary oxygen partial pressure, dynamic ventilation distribution and ventilation/perfusion ratios. MRI-inherent advantages are: lack of radiation, high spatial and temporal resolution, and a broad range of functional information. MRI of lung ventilation seems to be more sensitive in the detection of ventilation defects than scintigraphy, CT or pulmonary function tests. By combining the new strategies the radiologist will be capable to improve specificity of the investigations and to characterize lung function impairments. The joint assessment of ventilation and perfusion will play a major role in this development.     

Combined MR proton lung perfusion/angiography and helium ventilation: potential for detecting pulmonary emboli and ventilation defects.

Three-dimensional (3D) perfusion imaging allows the assessment of pulmonary blood flow in parenchyma and main pulmonary arteries simultaneously. MRI using laser-polarized (3)He gas clearly shows the ventilation distribution with high signal-to-noise ratio (SNR). In this report, the feasibility of combined lung MR angiography, perfusion, and ventilation imaging is demonstrated in a porcine model. Ultrafast gradient-echo sequences have been used for 3D perfusion and angiographic imaging, in conjunction with the use of contrast agent injections. 2D multiple-section (3)He imaging was performed subsequently by inhalation of 450 ml of hyperpolarized (3)He gas. The MR techniques were examined in a series of porcine models with externally delivered pulmonary emboli and/or airway occlusions. With emboli, perfusion deficits without ventilation defects were observed; airway occlusion resulted in matched deficits in perfusion and ventilation. High-resolution MR angiography can unambiguously reveal the location and size of the blood emboli. The combination of the three imaging methods may provide complementary information on abnormal lung anatomy and function.