We report our initial experience of lung motion analysis on a 3-T MRI scanner equipped with a 128-channel receiver coil. We achieved a temporal resolution for dynamic 3D imaging of 0.62 to 0.76 frames/s, which allowed us to observe free-breathing lung motion in three dimensions. At the same time, the image quality was sufficient to see the boundary of the lung area as well as to apply an automatic segmentation algorithm to extract the lung volume. This then allowed us to determine the time-dependent lung volume.
The applicability of automatic postprocessing is critical for motion analysis using dynamic MRI, in which tens of 3D image frames consisting of tens of slices must be processed. The lung area was segmented automatically with the constant multiplier and threshold parameters throughout the series of frames except several frames near maximum inhalation of the deep-breathing protocol. Examination of shows that the time spent at maximal inhalation for the deep-breathing protocol was very short, with subsequent large changes in lung volume per unit time. This resulted in motion artifacts around the lung boundary. The blurred boundary impeded the growth of the region during the segmentation by the confidence-connectedness algorithm. The multiplier and threshold parameters were adjusted so that the region grew toward the true boundaries even if the region had larger statistical variation of the pixel intensities.
Surface rendering of the segmented image enables the detailed qualitative assessment of free-breathing 3D lung motion, which could never have been achieved with either free-breathing dynamic two-dimensional images or breath-hold 3D images acquired at multiple respiratory phases. shows the nonuniformity of motion of the diaphragm during deep respiration. In the initial stage of inhalation, when the diaphragm starts contracting to enlarge the thoracic cavity in an inferior direction, major displacement of the diaphragm along the superior-inferior direction is observed around the lumbar spine, and afterward, the displacement propagates to the costal area of the diaphragm. During the earlier exhalation state, the costal area of the diaphragm was lower (more on the inferior side) than the other area. These actions of the diaphragm can be explained by a contraction of the muscle connected to the lumbar spine and the friction between the pleurae under the ribcage. Intracycle hysteresis of lung motion has been discussed in previous publications (13
The segmented lung image depicts a variation of the lung volume during deep breathing that is consistent with known tidal volumes of healthy adults. The merit of a lung volume measurement based on MRI is that it provides absolute volume data, including RV, which cannot be measured by spirometry. It would be interesting to perform further validation by comparing tidal volumes measured by spirometry. Although several studies comparing spirometry-based measurement with four-dimensional computed tomography (23
) and dynamic MRI (15
) have been published, to the best of our knowledge, no MRI studies have compared MRI-based volume measurement with spirometry in an MRI scanner, partly because MRI-compatible spirometers are not widely available.
Although the proposed imaging protocol provides temporal resolution to capture lung motion, it was not sufficient to measure the volume variation in shallow breathing, which is the most natural way for a subject to breathe. In addition, the limited temporal resolution also meant that there was substantial heart motion during the acquisition of each slide, thereby causing motion artifacts around the heart. It is clear that one can trade spatial resolution for higher temporal resolution. And for the task demonstrated here (ie, to measure the lung volume as a function of time), lower spatial resolution would suffice. For example, the number of phase encoding steps could be reduced by a factor of two and the temporal resolution increased by the same factor. For future diagnostic use of the 3D free-breathing protocols, however, high spatial resolution will be required to observe the anatomic structures of the lung. This study, with a voxel size of 3.125 × 3.125 × 5 mm, achieved spatial resolution close to that obtained under normal breath-hold MRI pulmonary examinations. Thus, one of the purposes of this study was to demonstrate automatic segmentation in dynamic 3D free breathing with this type of spatial resolution. Temporal resolution is also improved by increasing the acceleration factor for the parallel imaging, which was limited to 4 in this study. Schmitt et al (19
) demonstrated an acceleration factor as high as 8 with this 128-channel receiver coil, with a maximum geometry factor of 2.3 for coronal imaging with a one-dimensional parallel acquisition technique. Moreover, the design of the coil allows the use of a two-dimensional parallel acquisition technique with an acceleration factor of 20, which can dramatically improve temporal resolution.
In conclusion, the initial experience of the 3-T whole-body scanner with a 128-channel coil showed that the scanner and imaging protocol provided dynamic 3D images with spatial and temporal resolution sufficient to delineate the diaphragmatic domes and chest wall during active breathing and intensity homogeneities and a signal-to-noise ratio to perform automatic segmentation. There is potential to acquire dynamic 3D images with higher temporal resolution, and its application to lung motion analysis and diagnosis is promising.