Esophageal varices.

Barium swallow is generally used to demonstrate esophageal varices, which appear as serpiginous filling defects in the distal esophagus and cardia of the stomach. Angiography is used for pre- and postoperative shunt evaluation.

Esophageal source measurement of Tc-99m attenuation coefficients for use in left ventricular volume determinations.

A linear attenuation coefficient for water (mu = .15 cm-1) at 140 keV has been used in the determination of left ventricular volumes (LVV) by attenuation-corrected equilibrium methods. This theoretical value ignores the effect of Compton scatter and thus may be too high for human LVV determinations. The effective attenuation coefficient, mu', of the human chest was determined in ten normal volunteers using a Tc-99m esophageal source imaged with a gamma camera. Values for mu' at 30 degrees LAO in end-expiration, quiet breathing, and end-inspiration were .125 +/- .006 cm-1, .125 +/- .005 cm-1, and .113 +/- .007 cm-1, respectively (95% confidence interval). Values of mu' at 45 degrees LAO were .122 +/- .006 cm-1, .119 +/- .007 cm-1, and .099 +/- .009 cm-1, respectively, for the same conditions. The measured value of mu' for the source in a water phantom was .127 +/- .001 cm-1. This suggests that a value of mu' of .125 cm-1 may be appropriate for use in determining LVV in patients.

Factors affecting ventricular volumes determined by a count-based equilibrium method.

Using a 99mTc-filled source ("ventricle") in an elliptical torso phantom, we analyzed the effect of source depth, region of interest (ROI) size, background concentration and source shape on volumes determined by an attenuation-corrected count-based equilibrium method. The calculated volume of a 96 cc sphere decreased linearly from 103 to 82 cc with increasing depth from 4 to 18 cm [vol = -1.48 X depth (cm) + 109, r = 0.99]. The calculated volume of the same sphere imaged at a depth of 9 cm increased from 98 to 117 cc with ROI sizes increasing from 161 to 1,369 pixels (1 pixel = 0.17 cm2). With increasing background concentration from 0-2 microCi/ml calculated volumes decreased from 95 to 85 cc (vol = -5.3 X background concentration (microCi/ml) + 95, r = 0.97). However, with correction for over-subtraction of background, increasing background activity caused no decrease in calculated volume (mean = 95 cc, s.d. = 1). Calculated volumes for the sphere and various cylinders were accurate, while those for cones were up to 37% lower for actual volumes ranging from 56-608 cc. This study demonstrates that multiple factors produce variability in count-based determination of phantom volumes. A careful consideration of the interaction of these factors with the edge-detection and computational algorithms is required.

Tc-99m attenuation coefficients in water-filled phantoms determined with gamma cameras.

Quantitative imaging with gamma cameras requires compensation for attenuation of source photons. Some methods of compensation make use of a constant or average estimated attenuation coefficient mu. A value for mu of 0.15 cm-1 for 140.5-keV photons in water or tissue is commonly used. This value, however, neglects scattered photons which are detected within the energy window in gamma camera imaging. Values for mu of 0.12 cm-1 used in attenuation compensation of Tc-99m single-photon emission computed tomography scans of uniform cylindrical sources have been shown to give improved results compared with use of mu = 0.15 cm-1. In this study, gamma cameras and a multichannel pulse-height analyzer were used to determine effective values of mu for photons in water as a function of energy window. Two cylindrical water-filled phantoms, circular and elliptical, were used with a point source of Tc-99m at depths up to 18 cm. Energy data were integrated over the top half of the photopeak, and over 10%, 20%, and 30% windows centered on the photopeak. Attenuation curves were exponential for all photopeak windows with values of mu of 0.12 +/- 0.014 cm-1 for all windows up to 20% and 0.1 cm-1 for a 30% window. This study suggests that a value of mu of 0.11-0.12 cm-1 is, in fact, appropriate for use in attenuation compensations where an average is required.