MR imaging of pulmonary arterial hypertension and pulmonary emboli

Intraluminal signal in the pulmonary arteries on spin-echo, ECG-gated MR images is limited to the diastolic phase of the cardiac cycle in normal subjects. Initial experience has indicated that signal persisting during systole may be characteristic of slow blood flow associated with pulmonary arterial hypertension (PAH) or of thrombotic material secondary to pulmonary embolism. This study analyzes our cumulative experience (31 patients) with multiphasic, double spin-echo MR for assessing PAH and/or suspected pulmonary embolism. In PAH, the abnormal systolic signal showed an intensity increase from first to second echo. This pattern was observed in 92% of PAH patients, including 100% of patients with pulmonary systolic pressures greater than or equal to 80 mm Hg and 60% of patients with pressures less than 80 mm Hg. At any focus in the pulmonary arteries, such signal disappeared at some phase of the cardiac cycle. In patients with pulmonary embolism, signal from thrombus was fixed throughout the cardiac cycle and showed little or no increase in relative intensity change from first- to second-echo image. Using this guideline, MR made six confirmed positive and four confirmed negative diagnoses of proximal pulmonary embolism, while it failed to identify thrombus in the one patient with a peripheral pulmonary embolism. Intraluminal signal in the pulmonary arteries caused by PAH or pulmonary embolism can be differentiated in most instances using multiphasic, double spin-echo, ECG-gated MR. However, at its current stage of development, the procedure does not appear to be useful for the evaluation of peripheral pulmonary embolism.     

MR imaging of pulmonary emboli: an experimental study in dogs

Experimental pulmonary emboli, consisting of tantalum-labeled autologous blood clots and barium-labeled 3-mm plastic spheres that did not produce an MR signal, were introduced through the femoral vein into each of five dogs. The sensitivity of MR to detect autologous clots of known size was assessed, and the size of the clot on MR was compared with its actual size. Emboli were localized by using chest radiographs made in multiple projections. Cardiac-synchronized MR imaging was performed on a 0.35-T superconducting magnet and was followed by a 99mTc macroaggregated lung scan. All 12 blood clots larger than 3 mm, and three of 20 clots less than 3 mm in transverse diameter were correctly visualized with MR. Five of the 15 clots seen on MR were larger than 150% of the actual size. There were nine false-positive emboli on MR. Two of six plastic spheres resulted in an abnormal signal on MR. MR signal from pulmonary emboli results from the thrombus itself and probably also from slow blood flow proximal to the obstruction. MR may be of value in detecting pulmonary emboli; clinical trials to evaluate its usefulness should be carried out.     

In vivo MR imaging of pulmonary arteries of normal and experimental emboli in small animals

PURPOSE: To demonstrate the feasibility of pulmonary MRA in living rodents. MATERIALS AND METHODS: A three-dimensional (3D) gradient echo sequence was adapted to perform a time-of-flight (TOF) angiography of rat lung. Angiogram with a spatial resolution of 195 x 228 x 228 microm(3) was acquired in around 33 minutes. The method was then applied in animals before and after pulmonary embolism (PE) induction. Section of the proximal right pulmonary artery was measured and compared between the two populations. RESULTS: Good quality images were obtained with a contrast-to-noise ratio (CNR) of 9 +/- 3 in the proximal part of the pulmonary artery. Cross-section areas of the right main artery are statistically different before (3.45 +/- 0.69 mm(2)) and after induction of PE (4.3 +/- 0.86 mm(2)). CONCLUSION: This noninvasive tool permits angiogram acquisition at around 200 microm spatial resolution and objective distinction between healthy and embolized arteries.     

MR imaging of the central pulmonary arterial tree in conotruncal malformation

Successful MR imaging of the central pulmonary arterial tree was performed in four cases of conotruncal malformation and in one case of pulmonary arterial banding. Angiography performed near the time of MR imaging in three cases and 15 years earlier in two other cases did not visualize the central pulmonary arterial tree. Multiplanar imaging was necessary to fully visualize the anatomy of complex congenital defects of the pulmonary arterial tree; the choice of imaging plane varied depending on the portion of the pulmonary artery to be evaluated.     

SPECT imaging of pulmonary emboli with radiolabeled thrombus-specific imaging agents

The safe and accurate diagnosis of acute pulmonary embolism (PE) remains challenging, and many PE-related deaths still occur before the detection of PE. Current techniques detect PE as "negative images," ie, the absence of contrast or downstream perfusion. There would be advantages to obtaining "positive images" of PE, by targeting imaging agents to components that are present primarily on thromboemboli. In addition to providing alternative means of diagnosing acute PE, they would also enable acute PE to be distinguished from other types of pulmonary arterial obstruction, such as unresolved intravascular defects attributable to previous PE. Positive images of PE require imaging agents to bind onto target antigens that are present predominantly on thromboemboli. The "D dimer" regions of polymerized fibrin are present in high concentrations on thromboemboli and are sufficiently accessible to binding. (99m)Tc-lableled anti-D-dimer deimmunized monoclonal antibody Fab' fragments (DI-DD-3B6/22-80B3) bind specifically to thromboemboli, with a thrombus: blood labeling ratio that allows scintigraphic detection. Another thrombus-specific imaging agent is (99m)Tc-labeled apcitide, a synthetic peptide that binds with a high affinity and specificity to the glycoprotein IIb/IIIa receptor on the membrane of activated platelets. Both of these agents have enabled the detection of lower extremity deep vein thrombi by planar scintigraphy. However, even highly radiolabeled PEs are difficult to distinguish by planar scintigraphy from the large blood pool in the heart and lungs. The spatial and contrast resolution inherent to single-photon emission computed tomography (SPECT) scanning allow the in situ imaging of pulmonary emboli that have been bound by radiolabeled thrombus-specific imaging agents. Preliminary trials in humans with acute PE have shown that the emboli can be detected after intravenous administration of (99m)Tc-lableled anti-D dimer, followed by SPECT scanning. Although clinical results are still preliminary, it appears that imaging of pulmonary emboli with SPECT, after administration of radiolabeled thrombus-specific antibody fragments, is accurate and clinically feasible.     

MR imaging of central pontine myelinolysis

Central pontine myelinolysis is a demyelinating disorder with characteristic magnetic resonance features. Findings on T1-weighted scans include a symmetric region of low signal involving the basilar pons with sparing of the descending corticospinal tracts as well as the peripheral pontine tissues. The T2-weighted images show an area of high signal corresponding to the abnormal region identified on T1-weighted scans. Associated extrapontine demyelinating lesions are also demonstrated.     

MR imaging of primary pulmonary lymphoma

We present a patient with primary pulmonary lymphoma studied by magnetic resonance (MR), whose MR appearance was considered to be suggestive of the disorder. On T1-weighted MR images the tumour was seen as a large mass with homogeneous signal intensity, replacing the entire left lower lobe, while on T2-weighted images it was demonstrated as a homogeneous mass with a surrounding thin layer of greater signal intensity. This discrimination was not possible with computed tomography.     

The detection of pulmonary emboli by magnetic resonance imaging. Evaluation of imaging parameters

The authors studied the influence of gating, cardiac cycle, and timing of rephasing gradients upon the detection of pulmonary emboli by MRI. Serial single-section MR images were obtained in normal dogs and dogs with experimental pulmonary emboli. Images with cardiac gating, respiratory gating, and ungated images were obtained. Cardiac gating in systole and late magnetization rephasing had the best diagnostic accuracy. Gated images were clearly superior to ungated; with combined cardiac and respiratory gating, sensitivity was 82% and specificity 88%. These results are sufficiently promising to warrant further studies.     

Hydatid pulmonary emboli

A case of secondary hydatosis, initially misdiagnosed as pulmonary metastases, is presented. The dissemination of hydatid cysts within the lungs in this case was the consequence of direct rupture of a hepatic hydatid into the inferior vena cava. A brief overview of the pathophysiology of hydatid disease, including a discussion of the types of hydatid rupture (contained, communicating and direct), is presented.     

MR imaging of unilateral pulmonary artery anomalies

Magnetic resonance (MR) imaging of the pulmonary arteries was performed in seven children with unilateral pulmonary artery abnormalities. Five had congenital absence or interruption of a pulmonary artery; two had postsurgical occlusion or narrowing of a pulmonary artery. A small patent hilar pulmonary artery was identified in one of the five cases of congenital pulmonary artery interruption or absence. Occluding thrombus was identified in the right pulmonary artery of one child following pulmonary artery banding. In another case marked narrowing of the right pulmonary artery was identified secondary to migration of a pulmonary artery band. The MR findings were confirmed by angiography in all cases. Magnetic resonance appears to be a useful noninvasive modality for accurate anatomic depiction of unilateral pulmonary artery abnormalities in childhood. It may be especially valuable for diagnosis of pulmonary artery stenosis or occlusion following surgical treatment of congenital heart disease.     

Emergency MR imaging of the central nervous system

Magnetic resonance (MR) of the central nervous system has few, but important indications for use in the acute setting. This report reviews the few true current clinical indications for emergency MR imaging, including ruling out spinal cord compression, vascular dissection or dural venous sinus thrombosis. Possible indications for emergency MR, including evaluation of acute stroke symptomatology, potential meningoencephalitis or vasculitis, are also presented. Future applications for MR, including MR angiography in the setting of acute subarachnoid hemorrhage and spectroscopy in acute ischemia, are mentioned.