An enlarged spleen is a frequent and important clinical sign. It is thus essential to have a reliable picture of the presence and extent of splenomegaly. The dimensions of the normal sized spleen are given on p. 358. In the adult, an enlarged spleen is usually palpable when its length exceeds 14 cm. However, the measurement of spleen size by means of a physical examination of the abdomen is unreliable, as minor enlargement is often undetected by palpation and even a grossly enlarged spleen may be missed in an obese person.
Conversely, a lax phrenic-colic ligament or loss of tone of abdominal wall may give rise to a wandering spleen, which will be palpable, as will one that is pushed downwards by a flattened diaphragm in obstructive airways disease.
Traditional radiographic imaging may also be misleading and difficult to interpret. However, in modern practice, reliable information is obtained by ultrasonic imaging, nuclear magnetic resonance imaging (MRI) and computerized tomography (CT) scan, all of which give an accurate representation of the anatomy of the spleen and its position in relation to adjacent organs (Figure 21.3). The major advantage of CT scanning is the excellent structural detail that it provides, but it gives no insight into splenic function, whereas the advantage of MRI is that it demonstrates comparatively fine detail of structure and is also capable of identifying function-related parameters that are sensitive to pathological changes.
Another method of scanning the spleen is by means of a scintillation camera following injection of radio isotope-labelled red cells after they have been manipulated by a procedure which ensures that they are removed from the circulation by the spleen. The most effective method is by exposing the red cells, which have been labelled with 51Cr, 111In or 99mTc, to a temperature of 49.5°C for precisely 20 min (Figure 21.3b). As this procedure is laborious and the images obtained are relatively crude, it has been largely overtaken by MRI and CT scanning; however, it has a major advantage in that it provides information on the functional size of the spleen. Functional asplenia or atrophy is
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Figure 21.3 Imaging of the spleen by various methods. (a) Scan of spleen after administration of "'In-labelled autologous red cells showing red cell pool in the spleen; the scan also shows blood flow in the heart. (b) Scan of the same patient as in (a) after administration of labelled heat-damaged red cells which have been taken up exclusively by the spleen. (c) Computerized tomography: transverse section showing liver (left) and spleen (right). (d) MRI: Coronal (longitudinal) section showing liver, spleen and kidneys. (e) Ultrasound scan of enlarged spleen.
well demonstrated by this procedure. It is also useful in diagnosing space-occupying lesions such as splenic cysts and tumour deposits, in determining whether an upper abdominal mass is of splenic origin, and in identifying abnormally positioned and accessory splenic tissue.
From the radionuclide image, the area (A) of the spleen can be obtained from the linear measurements. Several formulae have been proposed for estimating the volume of the spleen from these measurements. The following appears to be fairly reliable:
A method has been developed which combines tomographic imaging and conventional radionuclide scanning by means of single photon emission tomography (PET scan); this has potential value for detecting deep-seated functional lesions. It also provides an accurate three-dimensional measurement of spleen volume.
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