Info

and (Na X NP)2 is the geometric mean count for the lung, cpm Ainjected = amount of 99mTc-MAA injected, mCi

Factors for the right and left lung and other regions of interest (ROIs) within the lung can be calculated separately by apportioning the amount injected to the area of interest, although this step is not without assumptions, possibly introducing error into the calculations. The factors can then be applied to the emission data for that particular area.

For planar imaging, transmission scans are performed with external pancake sources or 99mTcO4 [109]. To obtain an image, the source is held against the subject's chest and back for a fixed period of time. Both anterior and posterior images are acquired for the calculation of the attenuation factor. In addition, it is necessary to know precisely the dose of radioactivity in the source imaged by the gamma camera and the sensitivity of the gamma camera/collimator system for counting the particular isotope being used.

TAF from the transmission scan (TR):

where

NO = geometric mean of flood source count rate with regions defined from the transmission scan

Nt = geometric mean of count rate from transmission scan for the

E = gamma camera sensitivity, cpm/mCi

This procedure can also be applied to the oropharyngeal region using lateral scans of the head and outlining the oropharynx in the images.

Defining Regions of Interest in the Lung

Defining regions of the lung for determining deposition of an inhaled therapy to the small peripheral (P) vs. large central (C) airways needs to accurately reflect lung geometry. This is especially true with 2D imaging. Because of the overlapping of airway structures, it is not possible to differentiate radioactivity emanating from small versus large airways [36,115], particularly when imaging immediately following inhalation of the tracer aerosol. Repeat imaging at 24 hours allows MCC to remove aerosol deposited on airway surfaces, leaving the remaining activity "representing" aerosol retained on peripheral airways. Aside from the need to align the subject in the same position in front of the scanner, this measurement is not always convenient to obtain. With 2D imaging, the peripheral region should be a narrow region, defined from the outer edge of the lung. Otherwise, the data are confounded by the detection of radioactivity from larger airways. As illustrated in Fig. 10, a number of methods have been applied to planar images to define central and peripheral regions within the lung [114]; a comparison between planar imaging and SPECT, using similar definitions for the lung regions demonstrated greater discrimination between aerosol deposited in central vs peripheral regions for SPECT [116]. As described above, the outer boundary of the lungs must be defined in the gamma camera t

Figure 10 Various methods for defining regions of interest within the lung for images obtained with planar (2D) scanning. (From Ref. 114.)

images—this is the first step in mapping regions within the lung. The whole lung (right and left lungs) region is then further divided into regions of interest (ROIs), which correspond to the large (central), medium, and small (peripheral) airways.

Two methods that have been used extensively for defining three ROIs in planar lung images are the 5 X 8 grid [117] and "onionskin" (OS) contours [113,118] (Fig. 11). The former divides the lung into rectangular areas of variable size around the hilus, while for the latter, concentric rings are drawn from the lung edge to the hilus of approximately 1/4, 1/4, and 1/2 the lung width.

Figure 11 Illustration of two methods for defining regions of interest in the lung to calculate the distribution to the central and peripheral areas. For the 5 x 8 grid technique, rectangular grids, five sections wide by eight sections high, are placed over the right and left gas-filled lungs so that the perimeter of the grids enclose the lungs. The ROIs, containing different numbers of sections, are then drawn as nested rectangles, centered about the hilus, of varying height and width: outer border of C = 2 x 3, I = 3 x 5, and P = 5 x 8 sections. For regions defined by the onionskin contour method, right and left lung areas are drawn by following the outer contours of the gas-filled lungs. The C, I, and P regions are drawn as three concentric regions from the outer edge to the hilus of the lung, with P and I regions each representing one-fourth the width of the lung and C representing one-half the width of each lung. Significant differences were seen between the central and peripheral regions for the two methods affecting the calculation of the P/C ratio. (From Ref. 113.)

Figure 11 Illustration of two methods for defining regions of interest in the lung to calculate the distribution to the central and peripheral areas. For the 5 x 8 grid technique, rectangular grids, five sections wide by eight sections high, are placed over the right and left gas-filled lungs so that the perimeter of the grids enclose the lungs. The ROIs, containing different numbers of sections, are then drawn as nested rectangles, centered about the hilus, of varying height and width: outer border of C = 2 x 3, I = 3 x 5, and P = 5 x 8 sections. For regions defined by the onionskin contour method, right and left lung areas are drawn by following the outer contours of the gas-filled lungs. The C, I, and P regions are drawn as three concentric regions from the outer edge to the hilus of the lung, with P and I regions each representing one-fourth the width of the lung and C representing one-half the width of each lung. Significant differences were seen between the central and peripheral regions for the two methods affecting the calculation of the P/C ratio. (From Ref. 113.)

A comparison of these two methods [113] for defining central (C), mid-, and peripheral (P) lung showed that the cross-sectional areas for the total lung and the peripheral ROIs were significantly greater for the 5 X 8 grid than for the OS contours, while the central lung ROI was significantly smaller, as defined by the grid. When the respective areas were applied to a deposition data set, the values for peripheral lung deposition were significantly greater and the central area deposition significantly less applying the grid as compared to the OS ROIs. This resulted in a P/C deposition ratio significantly greater for the 5 X 8 template, with the conclusion that there was greater drug deposited in the peripheral lung. The comparison points out the need for consensus among investigators imaging the lung for the purposes of assessing drug deposition as to the most appropriate geometry on which to base deposition calculations.

For the other two imaging modalities, SPECT and PET, multiple (up to 10) concentric regions of interest or shells are used to define the geometry for the purpose of calculating regional drug distribution (Fig. 12). The methods used for SPECT images have been developed by Fleming and colleagues and are described extensively in the literature [40,41,43,107,119,120]. For PET images, the shells are generated from the transmission scans for each transverse slice and applied to the specific emission slice (Fig. 13). Because of the way PET data are acquired, absolute quantitation of drug dose is possible for discrete areas within the lung.

Figure 13 Applying the PET shell analysis program to the lung images. One transmission slice from each of the coronal, transaxial, and sagittal planes is illustrated. The number of shells will vary with the geometry (cross-sectional area) of the slice. The shell configurations, obtained for the transmission scan slices, are then superimposed on the emission scans, slice by slice, providing volume and activity information related to the distribution of drug within the lung.

Figure 13 Applying the PET shell analysis program to the lung images. One transmission slice from each of the coronal, transaxial, and sagittal planes is illustrated. The number of shells will vary with the geometry (cross-sectional area) of the slice. The shell configurations, obtained for the transmission scan slices, are then superimposed on the emission scans, slice by slice, providing volume and activity information related to the distribution of drug within the lung.

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