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AJR 2004; 183:91-98
© American Roentgen Ray Society

MR Angiography with Sensitivity Encoding (SENSE) for Suspected Pulmonary Embolism: Comparison with MDCT and Ventilation–Perfusion Scintigraphy

Yoshiharu Ohno1, Takanori Higashino1, Daisuke Takenaka2, Kouji Sugimoto1, Takeshi Yoshikawa1, Hiroya Kawai3, Masahiko Fujii1, Hiroto Hatabu1,4 and Kazuro Sugimura1

1 Department of Radiology, Kobe University Graduate School of Medicine, 7-5-2 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan.
2 Department of Radiology, Kasai Municipal Hospital, 1-13, Yokoo Hojo-chou, Kasai, Hyogo 675-2312, Japan.
3 Division of Cardiovascular and Respiratory Medicine, Kobe University Graduate School of Medicine, Chuo-ku, Kobe 650-0017, Japan.
4 Present address: Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02115.

Received October 20, 2003; accepted after revision January 14, 2004.

 
Supported by Daiichi Pharmaceutical Company.

Address correspondence to Y. Ohno (yosirad{at}kobe-u.ac.jp).


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The aim of our study was to determine the utility of time-resolved contrast-enhanced MR angiography combined with sensitivity encoding (SENSE) for patients with pulmonary embolism.

SUBJECTS AND METHODS. Forty-eight consecutive patients (26 men and 22 women; age range, 27–73 years; mean age, 55 years) with suspected pulmonary embolism underwent chest radiography, contrast-enhanced MDCT, MR angiography with SENSE, ventilation–perfusion scintigraphy, and pulmonary angiography. MR angiography with SENSE was performed using IV administration of gadolinium contrast medium with a 3D turbo field-echo pulse sequence (TR/TE, 4.0/1.2; flip angle, 30°) on a 1.5-T scanner. Capabilities of diagnosing pulmonary embolism using MR angiography (data set A), contrast-enhanced MDCT (data set B), contrast-enhanced MDCT with MR angiography (data set C), ventilation–perfusion scintigraphy (data set D), and contrast-enhanced MDCT with ventilation–perfusion scintigraphy (data set E) were determined by receiver operating characteristic analysis, using the results of pulmonary angiography as the reference standard. The diagnostic capability of each data set was analyzed on a per–vascular zone and a per-patient basis with the McNemar test.

RESULTS. Sensitivity and specificity of data set A were 83% and 97%, respectively, on a per–vascular zone basis and 92% and 94%, respectively, on a per-patient basis. Specificity and accuracy of data set A were significantly higher than those of data set D on a per-patient basis (p < 0.05).

CONCLUSION. Time-resolved MR angiography with SENSE is effective for the diagnosis of pulmonary embolism.


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
The estimated incidence of symptom-producing pulmonary embolism in the United States exceeds 600,000 cases per year [1, 2]. If untreated, pulmonary embolism has an estimated mortality rate of 30%, more than 10 times the annual mortality rate for treated pulmonary embolism (2.5%) [1, 2]. However, antemortem diagnosis is made in only 10–30% of all patients with this condition [2, 3]. Despite the introduction of helical CT for evaluation of suspected pulmonary embolism over a decade ago, the initial diagnostic study performed at many institutions for patients thought to have pulmonary embolism is still chest radiography, which is followed by ventilation–perfusion scintigraphy. The Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study [4] showed that clinical assessment combined with ventilation–perfusion scintigraphy helped to establish or exclude the diagnosis of pulmonary embolism for only a minority of patients (174/713 or 24.4%)—those with clear and concordant clinical and ventilation–perfusion scintigraphic findings. Further imaging is therefore indicated for patients with indeterminate ventilation–perfusion scintigraphic results and for those with discrepancies between the results of the ventilation–perfusion study and the findings at clinical assessment. Pulmonary angiography is then needed to help confirm or exclude pulmonary embolism. However, even if available, pulmonary angiography is generally underused by physicians because it is invasive and expensive, so there is an urgent need for objective and non-invasive diagnostic examinations [5–13].

Recent years have seen an increase in the importance and use of noninvasive radiologic diagnostic tools for pulmonary embolism such as CT and MRI techniques. Advances in CT have been mainly brought about by the advent of fast acquisition using helical scanning and MDCT systems [14–16]. Some investigators have suggested that the diagnostic capability of contrast-enhanced CT is higher than that of ventilation–perfusion scintigraphy [17]. In addition, the combination of contrast-enhanced CT and ventilation–perfusion scintigraphy has shown a high diagnostic capability for pulmonary embolism. Furthermore, contrast-enhanced pulmonary MR angiography has been proposed as a noninvasive and nonionizing alternative to contrast-enhanced CT for the evaluation of various pulmonary vascular diseases [18–23]. Finally, some investigators have found that in animal experimental studies, the diagnostic capability of conventional contrast-enhanced MR angiography for small pulmonary embolisms was equal to or better than contrast-enhanced CT [24, 25].

Although remarkable results in the detection of pulmonary embolism have been reported with contrast-enhanced MR angiography, it has not yet been adopted for clinical routines because of its major shortcomings: a limited spatial resolution or image degradation resulting from motion artifacts and overlapping pulmonary veins and an acquisition time that is longer than the pulmonary circulation time (approximately 4–5 sec). In addition, an MRI scanner is generally a hostile environment for patients who are clinically unstable, particularly if they should need life support equipment that frequently is incompatible with a highly magnetized field.

A new imaging technique, known as sensitivity encoding (SENSE), was developed to improve MRI. For a detailed explanation of the SENSE technique, the reader is referred to previous articles [26–29]. In brief, SENSE makes use of the spatially varying sensitivity of receiver coils for encoding spatial information, allowing the number of Fourier encoding steps to be reduced by increasing the distance between sampling lines in Cartesian k-space. As a consequence of the undersampled k-space, a Fourier transformation of the data first creates an aliased (i.e., folded) image for each receiver coil element. For SENSE image reconstruction, "sensitivity maps" for receiver coil elements are generated with the built-in body coil of the system. SENSE reconstruction then uses the sensitivity encoding of the array coils to unfold the aliased signal components. The reduction of the number of phase-encoding steps needed for SENSE compared with full Fourier encoding is indicated with the SENSE reduction factor R. Therefore, the data acquisition time of SENSE is reduced by 1/R compared with that of standard Fourier image. On the other hand, the signal-to-noise ratio is inversely proportional to the square root of R. Noise enhancement occurs when the geometric relationship of the coil sensitivities is not optimal. This SENSE-specific noise enhancement is indicated with the local geometry factor g. Thus, in any given SENSE image, signal-to-noise ratio (SNR) compared with that on a standard Fourier image is as follows: SNRSENSE/SNRstandard = 1/(g {surd}R) [26–29]. Therefore, SENSE makes it possible to enhance both spatial and temporal resolution of contrast-enhanced 3D pulmonary MR angiography without an increase in acquisition time, and time-resolved contrast-enhanced 3D pulmonary MR angiography combined with SENSE may improve the diagnostic accuracy of the technique for pulmonary embolism.

We hypothesized that this combined technique can be used to identify thrombi in the pulmonary artery as well as pulmonary circulation abnormalities and to diagnose pulmonary embolism more accurately than contrast-enhanced MDCT; thus, it may be an alternative to ventilation–perfusion scintigraphy. The aim of our study was therefore to determine the utility of time-resolved contrast-enhanced 3D pulmonary MR angiography combined with SENSE for evaluating patients with suspected pulmonary embolism.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Patients
Forty-eight consecutive patients (26 men and 22 women; age range, 27–73 years; mean age, 55 years) believed to have a pulmonary embolism because of risk factors, symptoms, signs, or laboratory findings underwent chest radiography, contrast-enhanced MDCT, time-resolved contrast-enhanced 3D pulmonary MR angiography combined with SENSE and ventilation–perfusion scintigraphy, and pulmonary angiography. Patients who were younger than 18 years or pregnant and patients with renal failure were excluded from our study. We followed up all patients for 1 year to exclude the possibility of pulmonary embolism. The institutional review board approved the study, and informed consent was obtained from each subject before enrollment in the study.

Time-Resolved Contrast-Enhanced Pulmonary MR Angiography with SENSE
All MRI studies were obtained with a 1.5-T superconducting magnet (Gyroscan Intera, Philips Medical Systems) using a phased array coil. Time-resolved contrast-enhanced pulmonary MR angiographic images were acquired with a 3D radiofrequency spoiled gradient-recalled echo sequence combined with the SENSE technique (TR/TE, 4.0/1.2; flip angle, 30°; matrix, 512 x 333; reconstructed matrix, 512 x 512; rectangular field of view, 380 x 342 mm). A 3D slab thickness of 110 mm was acquired with 20 partitions using overcontiguous slicing in the coronal plane and a left-to-right phase-encoded direction, resulting in an effective partition thickness of 5 mm and real phase encoding in the slice direction of 10 steps. The SENSE reduction factor (R) was 3 in left–right reduction, resulting in a 4-sec temporal resolution for each 3D data set. The total acquisition time for dynamic MRI examinations was 36 sec. To assess the sensitivity of each coil for the reconstructed SENSE images obtained from MRI data, we acquired an additional reference scan with a resolution of 4–5 mm for all examinations. In addition, we performed a spoiled gradient-recalled echo sequence with half-Fourier acquisition (4.5/2.0; flip angle, 30°) in the phase-encoding direction.

To reduce the total dose of gadolinium contrast material and achieve a sharp bolus profile for visualization of pulmonary perfusion, we administered 5 mL of gadodiamide hydrate (Omniscan, Daiichi Pharmaceutical) to all patients at 5 mL/sec via an antecubital vein with an automatic infusion system (Sonic Shot, Nemoto) followed by 20 mL of saline solution delivered at the same rate. The basic theory and application of time-resolved contrast enhanced MR angiography have been documented in a previous report [29].

Before undergoing MRI, patients were carefully instructed in breath-holding and practiced the technique to produce precisely the same degree of inspiration for each scanning series. For time-resolved contrast-enhanced MR angiography with SENSE, six images were obtained during 24 sec of breath-holding at end-inspiration.

Contrast-Enhanced MDCT
A 4-MDCT scanner (Somatom Plus4 VZ, Siemens Medical Solutions) was used for scanning. Patients were scanned caudocranially in one breath-hold, and the entire thorax was included in the scanning range. The scans were obtained with 140 kVp; 110 effective mAs; collimation, 4 x 1 mm; pitch, 6:1; reconstruction collimation, 1.25 mm; and scanning rotations, 500 msec. During scanning, 100 mL of contrast material ([iohexol] Omnipaque 300; Daiichi Pharmaceutical) was administered to patients IV via an antecubital vein at 4 mL/sec with a power injector (Auto Enhance-50, Nemoto) with an empiric scanning delay of 20 sec. With this protocol, high and uniform contrast enhancement throughout the thorax was consistently achieved in all patients.

Ventilation–Perfusion Scintigraphy
Ventilation and perfusion scintigraphy was performed according to the procedures described in the PIOPED and the Advances in New Technologies Evaluating the Localization of Pulmonary Embolism (ANTELOPE) studies [4, 30, 31]. Perfusion scintigraphy was performed using an IV administration of 185 MBq of technetium 99m–labeled macroaggregates of albumin, and ventilation scintigraphy was performed using inhalation of krypton 81m (81mKr) gas. Images were acquired with a gamma camera (e-CAM, Siemens Medical Solutions) equipped with a medium-energy all-purpose collimator. The matrix size for ventilation–perfusion scintigraphy was 128 x 128. If ventilation scintigraphy could not be performed immediately because 81mKr gas was not available, ventilation scintigraphy was performed the following day or at least within 24 hr of the perfusion scintigraphy.

Diagnosis of Pulmonary Embolism
The diagnosis of pulmonary embolism in each of the vascular zones was based on findings from pulmonary angiographic images obtained by experienced angiographers. Bilateral studies were performed on all patients, with selection of the primary view (frontal or oblique) left to the discretion of the angiographer. Additional views were obtained or subselective injections were performed when deemed necessary to make or exclude the diagnosis of pulmonary embolism. The standard angiographic criteria for diagnosis were used [32]. The locations of emboli visualized on angiography were tabulated in terms of the vascular zones. Patients in whom no pulmonary embolism was found on the initial pulmonary angiographic examination were judged to be free of pulmonary embolism after an uneventful 1 year of follow-up and negative results on pulmonary angiography. For patients with an inadequate angiographic diagnosis of pulmonary embolism, exclusion of pulmonary embolism was based on an uneventful 1-year follow-up in the absence of anticoagulation medication and low or normal results on ventilation–perfusion scintigraphy. No patient with negative pulmonary angiographic results had a thromboembolic event during the 1-year follow-up period.

Image Analysis
All images were independently evaluated on a commercially available workstation (Yokogawa) by two experienced chest radiologists who were blinded to the results of other radiologic examinations. The presence of pulmonary embolism was assessed in the central and peripheral vascular zones, and the location was determined by dividing the pulmonary vascular bed into 25 zones with a slight modification of the protocol described by Teigen et al. [33]. Central vascular zones were defined as the main pulmonary artery, right and left pulmonary arteries, anterior truncus, right interlobar artery, descending trunk on the left, and left upper lobe trunk. Peripheral vascular zones comprised all segmental arteries in both lungs. The final diagnosis of pulmonary embolism based on the findings of each technique was made separately by consensus of two reviewers.

The presence of pulmonary embolism in each vascular zone was assessed on the source images of time-resolved contrast-enhanced MR angiography. On source images, the width and level window settings were selected to allow the clearest visualization of parenchymal enhancement in each dynamic series. Diagnosis of pulmonary embolism on contrast-enhanced MR angiography was based on the detection of the vascular signs of pulmonary embolism reported in the literature [33–35] or a decreased area of perfusion within the lung parenchyma with or without filling defect in the corresponding pulmonary artery. The vascular signs of pulmonary embolism on contrast-enhanced MR angiography were a partial filling defect, defined as central or marginal intraluminal areas of low signal intensity surrounded by variable amounts of contrast medium with regular or irregular borders; a complete filling defect, defined as intraluminal areas of low signal intensities that were not surrounded by contrast medium and that occupied the entire arterial section; the railway track sign, defined as thromboembolic masses seen floating freely in the lumen, allowing the flow of blood between the wall of the vessel and the thrombus or embolus or both; and mural defects in cases of peripheral areas of low signal intensity within the pulmonary artery. For each patient, the probability of pulmonary embolism was classified as one of five categories: 0–20%, normal findings; 21–40%, very low probability of pulmonary embolism; 41–60%, low probability of pulmonary embolism; 61–80%, intermediate probability of pulmonary embolism; and 81–100%, high probability of pulmonary embolism.

On contrast-enhanced MDCT, diagnosis of pulmonary embolism was based on detection of the vascular signs of pulmonary embolism as reported in the literature [33–35] or mosaic perfusion within the lung parenchyma with or without filling defect. All CT scans were analyzed with mediastinal (width, 400–450 H; level, 35–40 H), pulmonary vascular (width, 250 H; level, 35 H), and lung parenchymal (width, 1,600 H; level, –550 H) window settings. The vascular signs of pulmonary embolism for MDCT were the same as those listed above for contrast-enhanced MR angiography. A pulmonary artery was considered to contain emboli when it manifested a definite filling defect in at least two consecutive sections. The probability classifications of pulmonary embolism on MDCT were the same as those used for contrast-enhanced MR angiography.

Ventilation–perfusion scintigraphic images were interpreted in conjunction with a chest radiographs using the revised PIOPED criteria [4, 30, 31]. Images were rated by the consensus of two reviewers using the same probability classifications as those applied to the time-resolved contrast-enhanced MR angiographic and contrast-enhanced MDCT angiographic images. All scintigraphic findings on a per–vascular zone basis were comparable to the interpretations for time-resolved contrast-enhanced MR angiography and contrast-enhanced MDCT angiography.

Statistical Analysis
To evaluate the utility of time-resolved contrast-enhanced 3D pulmonary MR angiography, both reviewers assessed the presence of pulmonary embolism in each of the vascular zones on the basis of findings from time-resolved contrast-enhanced MR angiography alone (data set A), contrast-enhanced MDCT alone (data set B), and contrast-enhanced MDCT with time-resolved MR angiography (data set C). For each data set without consensus on a per–vascular zone interpretation, a kappa statistic was evaluated for determination of observer performance. Because the p values were exploratory in nature, we made no Bonferroni adjustment. Interobserver agreement was considered slight for kappa values less than 0.21, fair for kappa values of 0.21–0.40, moderate for kappa values of 0.41–0.60, substantial for kappa values of 0.61–0.80, or almost perfect for kappa values of 0.81–1.00 [36].

To evaluate the diagnostic capability of pulmonary embolism for each data set, we performed per–vascular zone analysis of the receiver operating characteristics (ROC). Sensitivity, specificity, and accuracy of each of the data sets were analyzed with the McNemar test. We also performed a per-patient analysis of the diagnostic capability for each data set and improvement in diagnostic capability as a result of using contrast-enhanced MDCT combined with ventilation–perfusion scintigraphy and time-resolved MR angiography. Sensitivity, specificity, and accuracy of data sets A, B, and C and of ventilation–perfusion scintigraphy alone (data set D) and of contrast-enhanced MDCT with ventilation–perfusion scintigraphy (data set E) were also compared using the McNemar test. A p value of less than 0.05 was considered statistically significant for all statistical analyses.


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
All MR angiographic examinations were completed successfully without any observable adverse effects. Of the 48 patients, 12 (25%) were diagnosed as having pulmonary embolism with a total of 35 central vascular zone and 41 peripheral vascular zone pulmonary embolisms. Representative cases are shown in Figure 1A, 1B, 1C. The number of cases for each score and kappa value of each of the data sets per vascular zone are shown in Table 1. Interobserver agreements for all data sets on a per–vascular zone basis were substantial.



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  		Fig. 1A. —45-year-old woman with acute pulmonary embolism. Contrast-enhanced MDCT scans show thrombi (arrows) in anterior truncus, right interlobar artery, middle and lower lobe pulmonary arteries, left pulmonary artery, left upper lobe trunk, and descending trunk on left.

 


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  		Fig. 1B. —45-year-old woman with acute pulmonary embolism. Source images of time-resolved contrast-enhanced MR angiography using SENSE reveal thrombi (arrows) in anterior truncus, right interlobar artery, left pulmonary artery, left upper lobe trunk, and descending trunk on left, and perfusion defect (small arrowheads) in left superior segment of lower lobe. Reduced pulmonary blood flow (large arrowheads) in right middle and lower lobes indicates high probability of pulmonary embolism in right middle and lower lobe pulmonary arteries.

 


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  		Fig. 1C. —45-year-old woman with acute pulmonary embolism. Perfusion scintigraphic image shows heterogeneous perfusion defects in both lungs.

 

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  		TABLE 1 Per-Vascular Zone Ratings for Probability of Pulmonary Embolism and Kappa Values for Data Sets as Assessed by Two Radiologists

 

Per–vascular zone ROC analysis of the data sets is shown in Figure 2. ROC analysis produced an area under the curve (Az) of data set A (Az = 0.96) that was slightly larger than that of set B (Az = 0.94) and equal to that of data set C (Az = 0.96). Sensitivity, specificity, positive and negative predictive values, and accuracy of data sets per vascular zone are shown in Table 2. In overall and peripheral vascular zones, sensitivity of data set B was significantly lower than that either data set A or C (p < 0.05).



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  		Fig. 2. —Receiver operating characteristic curves of data sets per vascular zone. Areas under curves of data sets A, B, and C are 0.96, 0.94, and 0.96, respectively. Per–vascular zone diagnostic capability of data set A was slightly better than that of data set B and was equal to that of data set C. {square} = time-resolved contrast-enhanced MR angiography alone (data set A), {blacksquare} = contrast-enhanced MDCT alone (data set B), {blacksquare} = contrast-enhanced MDCT with time-resolved contrast-enhanced MR angiography (data set C).

 

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  		TABLE 2 Diagnostic Capability of Data Sets Per Vascular Zone

 

Per-patient ROC analysis of data sets in Figure 3 shows that the diagnostic capability for data set A (Az = 0.97) was higher than that for data set C (Az = 0.70). Sensitivity, specificity, positive and negative predictive values, and accuracy of data sets per patient are shown in Table 3. Specificity and accuracy of data set A were significantly higher than those of data set D (p < 0.05).



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  		Fig. 3. —Receiver operating characteristic curves of data sets per patient. Areas under curves were as follows: data set A = 0.97, B = 0.92, C = 0.97, D = 0.70, and E = 0.97. Diagnostic capability of data set A was significantly better than that of data set D (p < 0.05) and was equal to that of data sets C and E. {square} = time-resolved contrast-enhanced MR angiography alone (data set A), {circ} = contrast-enhanced MDCT alone (data set B); {blacksquare} = contrast-enhanced MDCT with time-resolved contrast-enhanced MR angiography (data set C), {blacktriangleup} = ventilation–perfusion scintigraphy (data set D), = contrast-enhanced MDCT with ventilation–perfusion scintigraphy (data set E).

 

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  		TABLE 3 Diagnostic Capability of Data Sets Per Patient

 


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
In studying the diagnostic capability of each technique in identifying patients with pulmonary embolism, our preliminary results indicate that contrast-enhanced MR angiography combined with a newly developed parallel imaging technique such as SENSE has made it possible to realize time-resolved MR angiography with simultaneous high spatial and temporal resolution. Recently, some investigators have suggested that SENSE may reduce the acquisition time of various MRI techniques such as spin-echo, fast spin-echo, gradient-echo, and echo-planar sequences [26–29]. SENSE makes use of spatial information related to the spatially varying sensitivity of the receiver coil. Using an array of multiple receivers, this technique allows a reduction in the sampling density in the k-space. When SENSE and enhanced gradient performance are combined, it may be possible to simultaneously increase spatial and temporal resolution, although the number of phase-encoding steps significantly reduces the signal-to-noise ratio compared with that of nonparallel imaging techniques. Therefore, when combined with SENSE technique and a sharp bolus injection protocol of contrast medium, time-resolved contrast-enhanced MR angiography can simultaneously depict pulmonary vasculature and circulation with high spatial and temporal resolution [29].

We found that interobserver agreement of time-resolved MR angiography was substantial (0.61 ≤ {kappa} < 0.81) and showed no significant difference from that of contrast-enhanced MDCT or contrast-enhanced MDCT with time-resolved MR angiography. In addition, our interobserver agreement for contrast-enhanced MDCT was compatible with that of an earlier study [17]. Therefore, from the point of view of observer performance, time-resolved MR angiography can be used to evaluate pulmonary embolism with a per–vascular zone accuracy comparable to that obtained with contrast-enhanced MDCT.

As for per–vascular zone diagnostic capability, the sensitivities of time-resolved contrast-enhanced MR angiography for pulmonary embolism in the overall (83%) and peripheral (68%) vascular zones were significantly higher than those (75% and 54%, respectively) of contrast-enhanced MDCT, although no significant difference in diagnostic capability between the two techniques was found in the assessment of the central vascular zone. Moreover, when MDCT was combined with time-resolved MR angiography, sensitivities in the overall and peripheral vascular zones of contrast-enhanced MDCT were significantly improved. In our study, time-resolved MR angiography depicted all thrombi in the central vascular zone and 28 pulmonary perfusion abnormalities in the peripheral vascular zone. The same technique revealed 18 (44%) of 41 thrombi with pulmonary perfusion abnormalities. The remaining 10 (24%) pulmonary embolisms were identified as perfusion defects in the pulmonary parenchyma. contrast-enhanced MDCT revealed all thrombi in the central vascular zone and 22 (54%) of 41 thrombi in the peripheral zone.

These findings were compatible with previously reported results in experimental studies [24, 25] and were mainly the result of the thinner sections used in contrast-enhanced MDCT and the higher spatial and temporal resolution of time-resolved MR angiography with SENSE technique. However, the sensitivities and positive predictive values of time-resolved MR angiography, contrast-enhanced MDCT, and contrast-enhanced MDCT with time-resolved MR angiography were less than 70% for the peripheral vascular zones. These results suggest that the spatial resolutions of time-resolved contrast-enhanced MR angiography and contrast-enhanced MDCT are not sufficient for the diagnosis of pulmonary embolisms in the peripheral vascular zone, even if the spatial resolution of contrast-enhanced MR angiography and CT is improved by the SENSE technique and multidetector arrangement.

False-positive diagnoses were made for 35 vascular zones (three central and 32 peripheral). The susceptibility artifacts resulting from the intravascular contrast agent and air in the alveoli, the flow artifacts from contrast medium in the pulmonary vasculature, superimposition of pulmonary veins on pulmonary arteries in the coronal plane, and image degradation of motion artifacts were the main causes of false-positive diagnosis. Moreover, 13 thrombi shown to be incomplete obstructions of pulmonary arteries were not diagnosed as pulmonary perfusion abnormalities on time-resolved MR angiography. The clinical and imaging features of the pulmonary embolism in the peripheral vascular zone have been controversial, and the true- and false-positive perfusion defects seen on time-resolved contrast-enhanced MR angiography may also be controversial if pulmonary angiography does not show parenchymal perfusion defects. Therefore, per–vascular zone visualization of thrombi and perfusion abnormalities was restricted, so that a parametric evaluation of pulmonary circulation similar to the evaluation of cerebral infarction was necessary [37].

As for the per-patient diagnostic capability, time-resolved MR angiography and contrast-enhanced MDCT showed no significant differences in revealing pulmonary embolism. On the other hand, specificity and accuracy of ventilation–perfusion scintigraphy were significantly lower than those of time-resolved MR angiography. The results of our per-patient ROC analysis showed that high and intermediate probabilities of pulmonary embolism determined with ventilation–perfusion scintigraphy were ultimately diagnosed as pulmonary embolisms. The perpatient diagnostic capability of ventilation–perfusion scintigraphy in our study was compatible with that reported in the literature [4, 30, 31]. For evaluation of pulmonary circulation abnormalities in lung parenchyma, no significant difference in diagnostic capability was seen between contrast-enhanced MDCT with ventilation–perfusion scintigraphy and contrast-enhanced MDCT combined with time-resolved MR angiography with SENSE.

These per-patient results were considered to be due to the improvement in spatial resolution of pulmonary angiograms and perfusion images by time-resolved MR angiography with SENSE and to the clearly visualized pulmonary circulation with a sharp bolus profile. In addition, because we obtained thin-section images in this study, thrombi were easily detected even on MR angiography and CT angiography, eliminating a significant obstacle for diagnosis of pulmonary embolism on cross-sectional imaging. Therefore, both time-resolved MR angiography and contrast-enhanced MDCT are both superior to and probably should be used in lieu of ventilation–perfusion scintigraphy when available.

A few limitations apply to our study. First, in our study population, the overall prevalence of pulmonary embolism was 25%, which coincides with that of two previous studies [17, 38] but was slightly lower than that of the PIOPED study [4]. Second, we interpreted contrast-enhanced 4-MDCT system for every 1.25-mm section thickness as previously reported by Schoepf et al. [39]. However, 8- or 16-MDCT systems have been used in clinical situations, and if thinner detector and reconstruction collimations can be adopted, the diagnostic capability of contrast-enhanced MDCT in the peripheral vascular zone may be improved. Therefore, a large-scale prospective study is warranted to determine the real value of MRI for diagnosis of pulmonary embolism in comparison with contrast-enhanced MDCT. Third, the population of our study was small. A large-scale prospective study of MR ventilation imaging such as oxygen-enhanced MRI and hyperpolarized noble gas MRI may be needed to assess the real value of the substitution of ventilation–perfusion scintigraphy.

In conclusion, time-resolved contrast-enhanced 3D pulmonary MR angiography combined with SENSE was found to be useful for the diagnosis of pulmonary embolism, and this technique may offer an alternative to ventilation–perfusion scintigraphy for imaging patients with suspected pulmonary embolism.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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