Visualization of morphological parenchymal changes in emphysema: Comparison of different MRI sequences to 3D-HRCT
Introduction
Chronic obstructive pulmonary disease (COPD) is one of the leading causes of morbidity and mortality worldwide. At present it is the 4th most common cause of death among adults [1]. Pulmonary emphysema is a pathological term and is defined by the American Thoracic Society as an abnormal permanent enlargement of the air spaces distal to the terminal bronchiole, accompanied by the destruction of their walls, most often caused by COPD [2], [3].
Spirometry as well as performance test like 6 min walking distance is the clinical reference for COPD measurement. Since compensation mechanisms of the lungs are pronounced, mild pathological affection and minor changes in disease cannot be detected with global analysis. In contrast to spirometry, imaging might allow for regional assessment of the compartments involved. This might be crucial for early detection of COPD and monitoring of its treatment. High-resolution computed tomography (HRCT) is the imaging modality of choice and a long-standing player for non-invasive imaging and quantification of lung parenchyma and airways [4], [5], [6], [7], [8], [9], [10].
Magnetic resonance imaging (MRI) of the chest is more and more recognized as a versatile diagnostic procedure also for imaging the chest with special focus on functional information [11], [12], [13]. As emphysema is associated with extensive functional impairment, which can be examined by proton-MRI (like perfusion), or hyperpolarized gas MRI (e.g. ventilation or measurement of apparent diffusion coefficient), the capability of MR to visualize parenchymal alterations within the same investigation would be highly appreciated [14].
Morphological imaging of the lung parenchyma using proton-MRI has up to now been successfully applied in diseases with an increase of tissue and therefore signal, such as pulmonary nodules, infiltrates and fibrosis [15], [16], [17], [18]. MR signal intensity of the lung parenchyma depends on the proton density of the lung parenchyma per voxel [19] and inhomogeneous B0-field due to susceptibility artefacts resulting from air-tissue surface. Emphysema, however, is characterized by a loss of parenchyma and reduced blood volume. This decrease of tissue within a voxel results in a low signal [20].
As no study evaluated the potential of morphological MRI in emphysema, the aim of our study was to compare different proton-MRI sequences with modern multislice thin-section-CT of the lung.
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Patients
22 patients (7 females and 15 males) with the mean age of 62 ± 6 years (range 51–73) were included in this study. All patients were suffering clinically from COPD and radiological evaluation revealed emphysema. All patients had a smoking history of 51 ± 20 pack years (range 20–110 pack years). The mean FEV1% predicted was 35 ± 12% (range 17–66%) and the mean weight was 72 ± 14 kg (range 48–95 kg). The study was approved by the Local Ethics Committee. All subjects were informed prior to the investigation.
Results
Due to severe breathing artefacts in one patient a single VIBE and in another patient a single ce-VIBE sequence could not be evaluated. Therefore, 132 lobes were analyzed with CT and HASTE and 126 lobes with all 4 datasets (CT, HASTE, VIBE, and ce-VIBE). In two patients an atelectasis of the middle lobe was present, which was detected correctly in all sequences. These lobes were therefore not eligible for scoring. Results are given for each lobe in Table 1, Table 2, Table 3.
For assessment of
Discussion
Using MRI the severity of emphysema was correctly scored in 41–63%. For the leading type of emphysema the agreement with CT was 55–68%. By MRI, bronchial wall thickening was only seen in one third of the lobes. Nodules <1 cm were mostly missed by MRI. The fibrotic changes were seen in 39–60% using MRI.
CT is well established as the imaging modality of choice for high-resolution imaging (HRCT) of pulmonary structure. CT routinely depicts airways down to the 8th generation [5]. Lumen diameter and
Conclusion
The results of our study showed a potential of MRI in emphysema depiction and characterization. Though, up to now MRI is far away from challenging the 3D-HRCT for imaging the lung parenchyma. But at least in patients who receive functional imaging by MRI, MR angiography or follow-up disease monitoring, morphological MRI could provide a reliable estimate of parenchymal changes without radiation exposure. All sequences showed a similar diagnostic performance, however a combination of HASTE (two
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