A chest phantom was implanted with different sizes and types of nodular lesions and was imaged with a new X-ray scanner. Let us assume that there are 156 radiographs of the chest phantom screened for detection of nodular lesions. The radiographs showed 44 lesions, out of which four lesions were verified to be false. The radiographs also missed three lesions that could not be seen by an observer. Compute accuracy, sensitivity, and specificity of the X-ray scanner in imaging nodular lesions in the chest.

Explanation:

In this example, total number of radiographs are 156. Hence, n=156 (A+B+C+D).

True positive results= 40

False Positive result= 4

False negative result= 3

Hence the contingency table can be made as follows:

                              Lesion presentLesion AbsentTotal

Test positive               40 (A)              4 (B)                  A+B= 44

Test negative                 3 (C)             109 (D)          C+D= 112

                                   A+C= 43              B+D= 113          n= 156

1) Sensitivity= A / A+C= 40/43 =93.02%

2) Specificity= D/ B +D= 109/113 = 96.46%

3) Accuracy= A+D/ A+B+C+D = 95.51%

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