Evaluation of various attenuation corrections in lung SPECT in healthy subjects

Abstract

The effect of increasingly more sophisticated attenuation correction methods on image homogeneity has been studied in seven healthy subjects. The subjects underwent computed tomography (CT), single photon emission computed tomography (SPECT) and transmission computed tomography (TCT) of the thorax region in the supine position. Density maps were obtained from the CT and TCT studies. Attenuation corrections were performed using five different methods: (1) uniform correction using only the body contour; (2) TCT based corrections using the average lung density; (3) TCT based corrections using the pixel density; (4) CT based corrections using average lung density; and (5) CT based corrections using the pixel density. The isolated attenuation effects were assessed on quotient images generated by the division of images obtained using various attenuation correction methods divided by the non-uniform attenuation correction based on CT pixel density (reference method). The homogeneity was calculated as the coefficient of variation of the quotient images (CV(att)), showing the isolated attenuation effects. Values of CV(att) were on average 12.8% without attenuation correction, 10.7% with the uniform correction, 8.1% using TCT map using the average lung density value and 4.8% using CT and average lung density corrections. There are considerable inhomogeneities in lung SPECT slices due to the attenuation effect. After attenuation correction the remaining inhomogeneity is considerable and cannot be explained by statistical noise and camera non-uniformity alone.

Clinical significance of CT density-based, non-uniform photon attenuation correction of deep-inspiratory breath-hold perfusion SPECT

“…Lung perfusion alterations after AC in normal lungs showing systemic increases and enhancement of gravitational ventral-dorsal perfusion gradient are consistent with the previously reported results with other methods of TCT-or CT pixel density-based AC [2][3][4][5][10][11][12]. The present AC method with the use of DIBrH SPECT-CT fusion images appears to provide better assessment of perfusion impairment in diseased lungs compared with uncorrected SPECT, The present DIBrH perfusion SPECT provides reliable fusion images with routinely obtained DIBrH CT [13][14][15][16][17][18], and these fusion images offer detailed outline of the patient's body and local attenuation coefficient of complex body tissues in the thorax, which are prerequisite for reliable AC algorithm for SPECT [3,5,10,11].…”

“…Since photon interaction is significant and complicated in the thorax with variable photon attenuation structures such as chest wall, vertebrae and lungs [1][2][3][4][5][6][7][8][9][10][11][12], non-uniform photon attenuation correction (AC) is indispensable for better assessment of lung perfusion on perfusion SPECT. The use of thoracic transmission computed tomography (TCT) obtained by radioactive external sources is one of the options for non-uniform AC of perfusion SPECT, but it has not been widely used for clinical practice, because of complicated procedure and unavailability of radioactive external sources in some SPECT systems [1,2,4,9,10].…”

“…The use of thoracic transmission computed tomography (TCT) obtained by radioactive external sources is one of the options for non-uniform AC of perfusion SPECT, but it has not been widely used for clinical practice, because of complicated procedure and unavailability of radioactive external sources in some SPECT systems [1,2,4,9,10]. AC with the use of co-registered perfusion SPECT-computed tomography (CT) fusion images and CT pixel density can be a widely available option for non-uniform AC, although the clinical significance of this technique has not yet been well evaluated in lung diseases [1-3, 5, 7, 8, 11].…”

Assessment of Clinical Impact in the Application of Chang Attenuation Correction to Lung Ventilation/Perfusion SPECT

“…As expected by Núñez et al, limited benefit was achieved in cases with large or multiple ventilation/ perfusion mismatch defects interpreted as a high probability of PE. On the other hand, the ability to identify small perfusion defects is positively affected by attenuation correction, improving the sensitivity of SPECT studies of PE and the homogeneity of images (1,14,15), as supported by our study, too. In our opinion, the advantages of attenuation correction are not limited to quantitative studies; qualitative interpretation of lung scintigraphy can also be affected.…”

Generation of planar images from lung ventilation/perfusion SPECT

“…It may be possible to produce an improved version of the attenuation map by an iterative process which would utilize the initial synthetic attenuation map to correct the SPECT emission reconstructions and re-use this corrected emission photopeak data set to yield a better estimate of the lung boundaries, as has been described earlier [23]. We have also used a lower value for the threshold of the lung boundary (25%) than that has been previously reported for relatively uniform distributions (40% being common [28]) as the distribution of radiopharmaceutical in the lungs, especially on the ventilation scan, can often be heterogeneous, and even include focal areas of highly concentrated radioactivity ("hot spots") caused by airways' disease and inertial impaction of the radioactive tracer on the branching airway. This leads to an excessively high maximum count, and therefore the lower threshold we use provides some protection against this effect.…”