11 Jun

The role of radiological imaging in the diagnosis of acute appendicitis: NUCLEAR IMAGING Part 1

Nuclear imaging techniques detect accumulations of white blood cells (or of immunoglobulins) in areas of inflammation. Other causes of RLQ inflammation are also detected, not all of which require surgery. Disease in atypical locations can be detected but may be misdiagnosed. Because the liver, spleen, bone marrow and large blood vessels take up radionuclide, the scans are limited in their anatomic scope. The available techniques differ in the length of time required for a positive scan and in the need for preliminary in vitro labelling of the patient’s leukocytes.
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Due to the availability and relatively low cost of the isotope, 99mtechnetium-labelled hexamethylpropylene amine oxime (HMPAO) scans are most commonly performed. A 2 h preparation time is required, during which the patient’s blood is labelled with the radionuclide before the scan can be commenced. Scanning is then undertaken for up to 3 h, although most positive results occur before then. The performance characteristics of this technique have been calculated in several studies (Table 6), but these impressive results may not be easily achieved in nonspecialized centres, because of difficulty in interpreting the images. Although the studies summarized in Table 6 found that the accuracy of this technique was similar in all demographic groups, another team reported poor results in children, as well as poor interobserver agreement in the interpretation of 99mtechnetium-labelled HMPAO scans.

TABLE 6
Prospective studies of the use of mtechnetium hexamethylpropylene amine oxime scans for the diagnosis of appendicitis

Author, year (reference) n Acute appendicitis (%) Sensitivity

(%)

Specificity

(%)

PPV

(%)

NPV

(%)

Accuracy

(%)

Foley et al, 1992 (216) 30 63 81 100 100 73 89
Evetts et al, 1994 (217) 37 73 85 93 96 69 89
Kao et al, 1996 (218) 50 60 93 90 92 93 90
Lin et al, 1997 (219) 49 51 92 92 92 92 92
Rypins and Kipper, 1997 (220) 100 37 97 94 90 98 95
Kipper, 1999 (221) 124 41 98 82 82 98 90

NPV Negative predictive value; PPV Positive predictive value

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