Applications

While magnetic resonance imaging has made great strides in other areas of body imaging, it is not the modality of choice where the lung is concerned. Work has been done in regards to the use of MR to assess central pulmonary emboli, and researchers have taken advantage to the altered MR imaging characteristics at air water interfaces to investigate whether or not MR may in the future serve as a sensitive, quantitative measure of the extent of regional air-water interface throughout the lung. However, this is only in the most preliminary stages of research investigation.

X-ray CT remains the modality of choice for imaging the lung. Early work using Mayo's Dynamic Spatial Reconstructor demonstrated that X-ray CT could be used to accurately assess lung volume to within 3% of known volumes. Regional lung air content was found to be accurate to within 6% of known regional values while relative differences in lung air content was found to be accurate to within 3%. With the advent of the use of high resolution CT reconstruction algorithms (thin slices 1-2mm thick; and high spatial frequence reconstruction with inplane pixel size as small as .2mm) the detailed anatomy has been stunning. Helical CT is beginning to provide the speeds of scanning needed to acquire volumetric image data sets spanning significant portions of the apex to base extent of the lung in a single breath hold. Imatron's Electon Beam CT scanner, in addition to allowing for volumetric data acquisitions in a single breath hold, allows for single slice image acquisitions in as little as 100msec so that it becomes practical to not only image during a single breath hold, but it is also feasible to gather slices gated to the ECG. When pixel dimensions are being reconstructed down to .2mm and slice thicknesses are narrowed to as little as 1-2mm, much of this spatial resolution is lost if scanning times approach the length of the cardiac cycle. Cardiogenic motion of the lung is appreciable, and in lung regions near to the heart, cardiogenic based changes in regional lung volume during apnea can be as great as lung volume changes occuring in response to a tidal respiratory effort!

It is our contention that, if the improvements in X-ray CT are used to their best advantage, CT should offer not only exquisite detail in regards to the anatomic status of the lung, but it should also offer all of the quantitative physiologic detail gathered through spirometry and plethysmography and more.

If one uses known regions of 100% air content (region at the lumen of the trachea and main-stem bronchi) and 100% blood content (sampled in the region of the descending aorta, ascending aorta, or cardiac chambers), then the Hounsfield Units of each voxel can be converted to a % aircontent value of 0-100. With careful measures of transpulmonary pressures simultaneous to assessment of regional lung air content and changes occuring with stepwise inflations to higher or lower lung volumes, one can make accurate assessments of regional ventilation (change in lung air content), lung function on a lobe by lobe basis, and couple this information with the anatomic assessment available from the same high resolution CT images. Regional airway areas are available without the need for indirect measures of airway resistance. By measuring lung function, (air content and air content changes) on a more regional basis, one can begin to quantitatively assess which lobe(s) should be removed in lung reduction therapy, for instance. If scanning is done at a known lung volume such as 50% vital capacity, achieved by fitting the patient with a pneumotachometer during scanning, then grey scale changes over time (lung air content changes) on a regional basis can be used to assess alterations in lung physiology. Lung air volume coupled with anatomic information from the CT sections can allow the physician to begin to develop important quantitative measures regarding objective assessment of disease severity and response to therapy.

Since any changes in the volume of the non-air component of the lung in a single scanning period (ie at two lung volumes) must be due to changes in lung blood volume, it is possible to also begin to understand the role of lung inflation on regional lung blood volume, and once normal ranges are set, it should be possible to develop indices reflecting regional parenchymal / vascular interactions such as expansion of extra-alveolar vessels and compression of intra-alveolar vessels during lung inflation. While clinical utility of such indices are not currently know, it is through the teaching of how one can obtain such indices, that we hope to open the door for new creative uses of X-ray CT for the evaluation of the lung and to aid in the evolution of CT such that it can serve as a cost effective one-stop-shop for the evaluation of the lungs.

This tutorial will focus on how to measure global and regional lung air content.

Last modified: Fri Jun 4 12:59:38 CDT