DETERMINATION OF SIZE OP HEART BY X-RAYS 425 



routine for an adult of average size six per cent reduction of the 

 silhouette area or three per cent reduction of a given diameter 

 will give the actual size of the heart contour with sufficient ac- 

 curacy to obviate the necessity of measuring the antero-posterior 

 diameter of the chest and making a special calculation. But for 

 very large or very small individuals and for children the simple 

 formula given above should be followed.^ 



1 When a shorter distance from the target to the plate than two meters is 

 used, allowance must be made not only for this variation in distance but also for 

 a variation in distance of the heart contour from the plate which enters in as 

 the target is brought nearer to the heart. This factor also enters in at the two 

 meter distance in ventro-dorsal pictures, the heart contour being further from 

 the front of the chest in the dorso-ventral than in the ventro-dorsal position. 

 For determining the average distance of the heart contour from the plate the 

 stereoscopic method gives the best results. This is based on the distance of cor- 

 responding points in two silhouettes from a fixed line on the plate perpendicular 

 to the line of shift of the tube. The average distance of a series of such points 

 on the contour gives the average distance of the contour. 



Knowing the distance of the target from the plate, the length of the shift of the 

 tube and the distance of the shift of a given point in the two silhouettes it is easy 

 to calculate the distance from the plate of the point on the heart which casts this 



A.C ^ ^ 



point of shadow. The formula is x = -. — ; — - where ' 



A + B 



X = distance of point on heart contour from the plate 

 A = distance of shift of given point in the two silhouettes 

 B = distance of shift of tube 

 C = distance from the target to the plate. 

 The formula for determining the relation of the size of the area of the sil- 

 houette to that of the area enclosed by the heart contour is as follows: 



100 B2 



X = area enclosed by the heart contour 

 100 = area of silhouette 



A^ = square of the distance from the target to the plate 

 B^ — square of the distance from the target to the heart contour 

 The formula for determining the relation of a given diameter of the heart 

 silhouette to a given diameter of the heart is as follows: 



100 S 



X = length of diameter of heart contour 

 100 = length of diameter of silhouette 

 A = distance from the target to the plate 

 B = distance from the target to the heart contour 



