HEPATIC SCINTIGRAPHY
Introduction: Radioisotope scanning, otherwise known as scintigraphy, is a technique by which one can obtain an image of an organ. This technique employs radioactive isotopes and instruments like rectilinear scanner or scintillation camera.
Scanning technique in clinical medicine is defined as mapping out the distribution of a radioactive compound which has been specifically localised in an organ of interest. For example, if we administer radioactive colloids intravenously to a patient, it would specifically localise in the reticuloendothelial system which comprises primarily, the liver, spleen and bone marrow.
The liver takes away most of the administered radioactive colloids. By using a rectilinear scanner we can visualise the liver point by point and line by line so that the whole liver could be covered. At each point, we get information regarding the amount of radioactivity. Thus, the mapping of the distribution of this radiopharmaceutical in the entire liver is carried out. If the distribution happens to be uniform throughout the organ and if the organ appears to have a normal shape and size, the scan is said to be "normal". If there is an abscess in the liver, it would not trap the administered radiopharmaceutical. Hence, corresponding to the site and size of the abscess, the distribution of the radioisotope would be uneven, thus signifying pathology in the liver. Most of the times, an abscess is seen as a void or a cold area, devoid of any concentration of radioactivity. The main limitation of this technique is that the abscess has to be of a fairly appreciable size, i.e. more than 2 cms.
There are four aspects of scintigraphy:

1. Instrumentation-This helps to map the distribution of the radiopharmaceutical in the entire organ. This can be done by a moving detector system called rectilinear scanner or a stationary detector system called scintillation camera.
2. Display-This provides a visual image of the distribution so as to interpret its significance.
3. Radiopharmaceutical-This has to localise itself in the organ under investigation.
4. Observer-The clinician who looks at this display in correlation with the clinical data available for correct interpretation.

All the four aspects have their own limitations so as to make this technique more complicated than what it appears to be.

Instrumentation. The rectilinear scanner introduced by Cassen has undergone a series of developments in its quality so as to give the best possible image in as short a time as possible.
Another progress in this field of instrumentation is a stationary image device, a gamma camera. This has revolutionised the scintigraphy technique to such an extent that an image of an organ could be obtained in a matter of few seconds.
The scanner consists of a scintillation detector which detects the gamma radiation emerging from radiopharmaceutical located in the organ. The detector consists of a sodium iodide crystal and a photo multiplier. When a gamma radiation strikes the sodium iodide crystal, it is converted into a flash of light inside the crystal. This quantity of light is "seen" by the photo multiplier which converts the light energy into a small electrical impulse known as "pulse". The number of pulses in a unit time is a measure of the amount of radioactivity at that point. Thus, if the detector is made to look at the organ point by point and we track down the number of pulses at each point, we get an idea of the amount of radioactivity located at each point. In order to achieve this we have to make the detector look at only one point at a time. This is done by using a focussing collimator.
As far as the liver is concerned, the collimator is usually the one which has a focal distance of about 10 cms. The holes are fairly large in size so as to give a resolution in the order of 1 cm. or slightly less.
The detector system, sodium iodide crystal, coupled to a photomultiplier at one surface and a collimator at the other is driven by two motors, one in X-direction and the other in Y-direction, perpendicular to the former. The speed at which the detector can be moved is variable. The speed of motor of the detector primarily depends upon the amount of radioactivity located at the organ of interest. The scanners in the old days used to operate at a speed of only 33 cms. per minute. The present day scanners can function with a maximum speed of 1,000 cms. per minute. With the quantity of radioactivity that one could administer to a patient, it would still take about 10 minutes to obtain one view of the liver even with this type of fast scanners. It is essential to obtain more than one view of the organ, e.g. anterior, right lateral and also posterior in order to locate the exact site of the abscess in the organ. The availability of two detectors at 180⁰ to each other enables one to obtain two views simultaneously. This reduces the time considerably and causes less inconvenience to the patient. While one detector looks at the anterior surface of the liver and obtains the information about the distribution of the radioisotope, the other detector looks at the posterior surface at the same time and obtains information from the posterior view. This type of rectilinear scanner is known as a dual head scanner.
The scanners have to look at various points in the organ one after the other in sequence. In the scintillation camera all the points are looked at simultaneously and with an ingenious electronic system, it is possible to obtain the information about the distribution from all points at one and the same time. This is feasible because of the large size of the detector. This detector system does not move at all. It looks at the organ, point by point, but observes all points simultaneously. The information about the distribution is obtained on an oscilloscope as an image which can be photographed. Like a camera it looks at the organ and gets a picture of the same. Hence it is known as gamma camera or a scintillation camera. As compared to a scanner, camera hardly takes few seconds to get an image of the liver while scanner takes about 10 minutes. Thus, it is very fast and multiple views can be obtained in a very short time. Because of this virtue, it is also possible to inject the patient underneath the camera and obtain information about the blood flow to the liver and its space occupying lesions. This type of dynamic studies, aided by the computers, have enhanced the utility of the scanning technique in arriving at the proper diagnosis about the nature of the pathology in the organ. For example, as compared to hepatoma, which is a very vascular space occupying lesion, amoebic liver abscess (or a cyst) is avascular.

Display system. The rectilinear scanner obtains the image in 4 different ways employing different types of display systems. In dot scan the electrically driven solenoid makes impressions on paper through carbon paper to produce dots corresponding to the amount of radioactivity. The number of dots in an area is a clear indication of number of radiations coming from that area in a given time. This type of display is the easiest to obtain. Corresponding to the lesion in the organ the dot density would be considerably decreased in liver scanning, thereby delineating the site and size of the lesion precisely (Fig. 35).
Instead of making impressions through carbon paper to produce black and white dots it is possible to get impressions through multi-coloured ribbons to obtain a colour dot scan. In this system of display, different colours depict different range of radioactive concentrations. Usually, corresponding to the maximum concentration, the red colour is displayed and the other colours are displayed in the descending order of the visible spectrum. The change from one colour to the other is due to a definite fixed percent alteration in the radioisotopic concentration. This type of colour display is a pleasant display of an image making it easier to interpret the change in the distribution than black dot scan (Fig. 36).
However, in the photo scan display, the intensity of a light source is varied as the radioactive concentration varies. A photographic film, or usually an X-ray film is exposed to this source of light. Corresponding to the maximum concentration of radioactivity the light intensity will be maximum and the film will be exposed maximally. As the concentration varies the intensity of the light varies proportionately exposing the X-ray film also accordingly. This is the best method of obtaining an image of the organ. As one views the film in an illuminator, as normally X-ray films are viewed, it is possible to detect subtle change in the shades Qf grey in the exposure. This obviously means that the detection becomes even much better than the colour dot scan type of display (Figs. 37 a, b, c).
The display system in gamma camera is either on a polaroid film or in a transparent 35 mm. or 70 mm. film. It can also be obtained on a X-ray film. Images obtained on X-ray films are most convenient for interpretation. X-ray films are considerably slower films as compared to 35 mm. or 70 mm. films. The contrasts in the images are also nicely depicted in X-ray films as compared to the smaller roll films (Fig. 38).

Radiopharmaceuticals. A radiopharmaceutical is one in which a chemical compound is tagged with a suitable radioactive isotope so that when administered to the patient, it localises in the organ c interest by virtue of its biological behaviour.
The liver radiopharmaceutical agents are divided into two major groups namely parenchymal agents and RES agents.

Parenchymal agents. These are ¹³¹l labelled Rose Bengal, ¹³¹l labelled BSP, ^99m Tc labelled HIDA, etc
Rose Bengal is a dye, which when injected intravenously into a patient is specifically cleared from the circulation by the parenchymal cells of the liver. It remains in the liver for a period of an hour or so and clears through the biliary system via the gall bladder to the intestine. This type of hepatobiliary agent permits us to visualise the liver during its stay in the liver. As time passes it is also possible to visualize the gall bladder as well as the patency of the entire biliary system. Thus, if one obtains the picture of the liver at different time intervals, one can assess not only the nature of the liver and its pathological lesions but also the patency of biliary system as a whole.
^99mTechnetium is the most commonly used radioisotope for organ imaging at present. There are quite a few hepato-biliary agents labelled with ^99mTc which have been developed in recent years. Because of the short life, it is possible to administer a large quantity of this radioisotope, yielding better quality images.
RES agents are colloids of few millimicrons in size, which when tagged with ^99mTc serve as good liver scanning agents as they are selectively taken up by the reticuloendothelial system. As the liver forms a major portion of the RES, one visualises this organ and to a less extent the spleen, with these types of radiopharmaceuticals. ^99mTc sulphur colloid is the most commonly used radiopharmaceutical in this type. Phytate labelled with ^99mTc is also a useful agent. ^99mTc phytate is aqueous to start with. When injected intravenously it combines with the blood calcium to form colloid in vivo and thus enables th RES to trap it in the liver and spleen.
Instead of ^99mTc one can use another short lif radioisotope ^113ml in as colloid for RES visualisation. High energy of ^113ml n is not ideal for gamma camera while the ^99mTc is most suited for gamma camera. ^113mln scans are useful in studying the vascuIarity of the cold area seen on liver scan. This is because ^113ml n chloride binds to transferrin and remains in circulation.

Appearances of normal liver scan. The scanner usually gives a scan of actual size of the liver in the patient. However, the camera gives a miniature image of the liver as the picture is usually obtained from an oscilloscope. The scanner has a limitation in detecting lesions less than 2 cms. in the liver. The camera has a slightly better resolution.

Liver scan/scintigraphy using ^99mTc or ^113ml colloids. About 85% of the administered colloid is trapped in the liver, around 10% in the spleen and the remaining in the bone marrow or rest of the RES. If there is any generalised impairment of liver function, the distribution pattern is altered in such a way that liver takes up less and colloids are shunted to the spleen and bone marrow. Better visualisation of spleen and marrow is definitely suggestive of impairment of liver function. This is off and on seen in some patients having amoebic liver abscess. Portal hypertension would also give rise to this type o liver image with spleen and bone marrow showing better concentration. An amoebic liver abscess might cause portal hypertension at times, thus giving rise to the above picture.
The right lobe is the thickest part of the liver. As it constitutes a major portion, most of the radio pharmaceutical is also concentrated there. The left lobe in normal individuals is faintly visualised in the anterior view and probably not visualised at all in the posterior or right lateral view. Any pathology in the left lobe has to be interpreted with this aspect in mind.
The scan gives us the size of the liver in exactness. If the costal markings are available on the scan, it is possible to ascertain or guess the exact dimension of the liver which extends below the costal margin. With the help of other landmarks such as the position of the right nipple and umbilicus, it is feasible to locate the liver exactly.

Dynamic studies. These are very useful to assess the blood flow to the liver and its space occupying lesions. Malignant lesions are vascular in nature while abscesses are usually avascular.
Patient is positioned under camera and the radiopharmaceutical is injected into the antecubital vein. Sequential pictures are obtained at 2 sec. intervals for the next forty seconds or so. If one sees the lesion, which was "cold" on the liver scan, to be avascular, it is likely to be an abscess. However, by this technique it is not possible to differentiate between a cyst and an abscess.
Similar injections can also be given either in the femoral or in the malleolar vein to visualise and trace the entire inferior vena cava in addition to the vascularity of the liver and of the lesion. Such studies when routinely done by DaCosta et al¹ in cases of amoebic liver abscess showed that the abscess, until tapped or reduced in size after appropriate treatment frequently obstructs the l.V.C. (Figs. 39 a,b,c,d). Out of 15 cases of amoebic liver abscess studied by them, 11 had an abnormal l.V.C.

Vascular studies. Instead of taking sequential pictures as described above, it is often recommended to repeat the liver scans with a different radiopharmaceutical which tends to remain in the vascular compartment without localising in any particular organ.^113m In choride is one such agent. This binds itself to transferrin in the circulating blood and remains as a vascular scanning agent. If a liver scan is carried out with this agent, avascular lesions like abscesses or cysts would remain cold while malignant lesions would no more be seen as cold, since they are vascular. A peripheral vascular zone of congestion would be seen around the cold area in case the latter is due to amoebic liver abscess (Fig. 40) and not a cyst (Ref Section II).
This type of study could be carried out with a mere scanner and the need for the camera does not arise. Recently^99mTc has been used to label red blood cells of the patients in vivo or in vitro and these labelled red cells serve as a good vascular agent. This type of study helps in delineating the lower rim of the abscess, showing the true extent of the lesion, especially when the abscess is along the free edge of the liver.

Lung liver scans. For all superior surface lesions in the liver, a lung scan can be carried out simultaneously along with the liver scan by injecting two radiopharmaceutical agents one after the other, one localising in the lung and the other in the liver. If one sees a space between the lung and the superior surface of the liver, it is very suggestive of an abnormality in the superior surface of the liver. One must interpret this type of scan along with the conventional chest X-ray and the clinical features, because a subdiaphragmatic abscess, would also mimic such a lesion.

Cold area produced by amoebic liver abscess on liver scan. According to Ibrahim et al,² in the case of neoplasms and cysts, the contrast is sharp and well marked between the normal liver tissue and the lesion, while the contour of the liver abscess is not well defined owing to the presence of inflammatory reaction in its wall.²
In 1965, Cuaron et al³ studied liver scans of 106 patients with amoebic liver abscess using ¹³¹l Rose Bengal. They observed that anteroposterior scan proved very efficient in the detection of abscesses located in the left lobe, while the right lateral scan was not so. In locating abscesses of the right lobe, the anteroposterior scan proved to be 82.8% efficient, while the right lateral scan and the combination of both demonstrated efficiencies of 92.0% and 98.9%. The accuracy of the anteroposterior scan in the detection of lesions of the right lobe decreased in relation to the thickness of the liver tissue interposed between lesion and probe. Thus, abscesses situated in the anterior third were localized with an accuracy of 100%, as compared to 93.3% and 75.8% for detection of abscesses located respectively in the intermediate and posterior third of the right lobe. Right lateral scan was more efficient in the detection of lesions located in the posterior (98.2%) and anterior thirds (93.3%) of the right lobe than in those of its intermediate third (73.3%). Thus, in this study the combination of anteroposterior and right lateral scans was found to be the best method for the localization of liver abscesses.
The distribution of 106 abscesses in their series suggested that the most affected regions were the posterior (54.7%) and external (51.9%) thirds of the right hepatic lobe.
In 1970 Cuaron and Gordon⁴ studied liver scans of 2,500 cases of amoebic liver abscess. To analyse the distribution of the amoebic abscess in the liver, they empirically divided the right lobe into eight different regions-upper external posterior, upper external anterior, upper internal posterior, upper internal anterior, lower external posterior, lower external anterior, lower internal posterior and lower internal anterior.
Following was the distribution of 4,286 amoebic abscesses of the liver found in 3,379 hepatic scans from 2,500 patients.

83% patients had a single abscess while 17% presented with more than one lesion.
The highest overall efficiency was found in the posterior projection where 94.6% of the lesions were visualised. The anterior projection showed 92.8% of the abscesses while the right lateral scan demonstrated only 72.2%.
The utility of both the anterior and posterior scans were similarly high in the localization of abscesses of the left hepatic lobe where right lateral image was totally inadequate.
The anterior projection was most efficient in locating lesions of the left hepatic lobe (98.4%).
The right lateral view was useful in studyiyng the lesions located in the external regions of the right hepatic lobe.
The usual combination of anterior and right lateral scans showed an overall efficiency of 95.5%. Slightly better results were obtained with the combination of the posterior and right lateral scans (96.4%), while the highest percent of positive result was obtained with the combination of the anterior and posterior views (97.1%). Logically the combination of the three projections was the most effective (98.6%).
Analysis of the distribution of these lesions showed that the most affected regions of liver were the posterior, external and upper thirds of the right lobe.
Amongst Indian series, Habibullah et al^S reported that in a series of 110 cases of amoebic liver abscess, right lobe involvement was present in 82 .7% of cases and left lobe involvement in 17.3% of cases. Similar high incidence of right lobe abscesses were reported by Poulose et al (92%)⁶ and Sharma et al (81%).⁷
In 1974, Geslien et al⁸ did Gallium scanning in amoebic liver abscess. This was because 67Ga Citrate was reputed to localise in inflammatory lesions. In two cases of "acute" hepatic amoebic abscess, 67Ga Citrate was seen to accumulate in the periphery of the abscess cavity. The area of Gallium localization appeared to be in the region of hyperaemia seen histologically and arteriographically. Liver scanning with 67Ga Citrate may be more accurate than radiocolloid liver scanning for assessing the size and resolution of an "acute" hepatic abscess and not chronic abscess.
Radiocolloid scans reflect the distribution of reticulo-endothelial activity in the liver. Cuaron et al^9,10 have shown that normal phagocytic activity of Kupffer cells is interfered within the area immediately surrounding acute amoebic abscesses. This is due to an intense inflammatory response in the boundary area which also corresponds to a zone of increased vascularity demonstrable angiographically. In series of patients studied with both a radiocolloid (^113m In) and a blood pool scan agent (^113mIn transferrin), Cuaron found a marked discrepancy in the size of the individual lesions as seen with the two radiopharmaceuticals. Defects in the blood pool scans represented only the central necrotic abscess cavities themselves. Defects in the radiocolloid studies represented both the necrotic centre as well as the surrounding inflammatory zone of inhibited reticulo-endothelial cells. On an average the abscess itself accounted for only 24% of the total area of the defect seen on the radiocolloid scan.
The rim of increased activity on the ⁶⁷ Ga liver scan is essentially contained within the area of the ^99mTc sulphur colloid scan defects. This combination of findings is explained by an increased hyperaemic zone around the actual abscess and a decreased accumulation of ^99mTc sulphur colloid in both the abscess and in the zone of hyperaemia.
The above facts may account for the discrepancy in the literature regarding the healing rate of amoebic abscess demonstrated by serial radiocolloid scans.^11,12 The apparent rapid resolution time seen on radiocolloid studies is largely due to return of phagocytic function of the R. E. cells secondary to the regression of the inflammed hyperaemic region about the rim of the abscess.
The prolonged liver scan defects reported by some investigators¹³ may present scar tissue replacing liver parenchyma in the central area of necrosis.

Resolution. Poulose et al have reported mea resolution time of the 'cold' area to be 6 weeks.⁶ In another study, liver scans done 3 and 4 weeks after treatment showed normal resolution in less than half the number of patients.¹⁴ However, on most of the occasions, with adequate therapy hepatic scans reveal complete resolution in majority of the abscesses over a period of one to four months.^7,12,15,16 Occasionally it takes as long as a year.¹² There are no reasons ascribed for this slow progress. Poor correlation has been observed between the size of an amoebic abscess and it resolution time. Smaller abscess cavities at time require more time to heal than larger ones. However Johnston et al¹⁷ did not observe any correlation between the size of the abscess and the period o healing. Yet another study showed that multiple and large sized abscesses took a longer time to disappear than single small sized abscesses as see on the scan. Cohen¹⁸ reported persistence of cold areas in a case of multiple amoebic liver abscess upto 2 1/2 years after treatment. In one study using air contrast it was observed that the period of healing ranged from 28 to 115 days.¹⁹ There was some correlation between the size, location and number of abscesses and the time taken for healing.
Needle aspiration or surgical drainage does not hasten the resolution time.²⁰

Amoebic liver abscess, hepatic scintigraphy and the clinician. Until a few years ago, amoebic liver abscess was a clinical diagnosis. Short of diagnostic aspiration, there was no method of confirming the diagnosis except by indirect information revealed by radiology.
Liver scanning with isotope is a simple noninvasive technique of viewing the liver. Its only drawback is that the facility is not available in most of the centres in our country and in many other parts of the world.
I must therefore stress, that clinicians should remember that in an average case of amoebic liver abscess, liver scanning is not absolutely necessary. Many physicians await the liver scan report before starting anti-amoebicidal drugs or at other times, tapping the abscess. It must be understood that a liver scan does not give a diagnosis of an amoebic liver abscess. It only shows a cold area which could also be a hepatoma, a secondary deposit or a cyst of the liver. Although as explained earlier, repeat liver scan with vascular agents or dynamic studies if available would be useful in differentiating them, false positives are known to occur. Thus, only if reports of serological tests like l.H.A. are also available, a diagnosis of an amoebic liver abscess can be confirmed beyond doubt. Unfortunately even the facilities for serological tests are not available in most centres in our country.
Therefore, in an average case of fever, pain and tenderness in the hepatic area, the clinical diagnosis of an amoebic liver abscess is usually good enough, especially in superior surface abscess of the right lobe where the right dome of the diaphragm also shows changes.

What then is the value of liver scan in amoebic liver abscess? It cannot be denied that what was not known in one hundred years about the amoebic liver abscess has been learnt in the last ten to fifteen years, thanks to the liver scanning procedures. It was always taught in the past that most often amoebic liver abscess occurs in the supero-anterior surface of the right lobe and that it is often single. We are now aware that very often the amoebic liver abscess is elsewhere, e.g. in the posterior surface or inferior surface. Nearly half the abscesses occur in the lower half of the liver. In the days gone by, these patients had been labelled as suffering from amoebic hepatitis because the clinicians did not find a raised immobile right dome of the diaphragm in these cases. We have also realised that multiple amoebic liver abscesses are quite common and also that many liver abscesses confirmed on liver scan resolve with drug therapy alone without any aspiration.
In countries, where nuclear medicine centres are sparse and the patient has to be referred to a far off place for a liver scan, when should a physician ask for a liver scan in a case suspected of amoebic liver abscess?

Following are the indications:

1. When the tenderness is generalised, there is no lump, no local intercostal or point tenderness and right dome of the diaphragm is normal, such cases then often have intrahepatic or inferior surface abscesses. It is also prudent to think of other conditions as soon as liver scanning report is seen to be normal.
2. "Where to tap" is usually not the question to which answer can be obtained from the liver scan. On clinical examination, the "point tenderness" as a rule, gives-the answer. However, occasionally very high fever with diffuse hepatic tenderness might create a difficulty, which is usually solved by the liver scan.
3. Sometimes after tapping an abscess, the patient's symptoms and signs persist and he remains ill. Liver scanning is then indicated to exclude a second abscess, which may then be tapped.
4. Any patient who is suffering from fever and right lower chest pain and whose X-ray chest shows an unusual shadow in the right lower zone must be referred for liver scan. Amoebic liver abscess is one of the common causes responsible for such a presentation. These patients may not show the usual signs of an amoebic liver abscess. In fact any obscure shadow in the right hemithorax even in middle and upper zones with a clear lower zone demands ruling out an amoebic liver abscess (in our country) by resorting to a liver scan. Obscure shadows in the left lower zone also necessitate a liver scan. This subject has been discussed in detail elsewhere.
5. In case of any upper abdominal lump, liver scanning must be done to exclude an inferior surface amoebic liver abscess.
6. In any massive hepatomegaly, a 'silent' amoebic liver abscess should be excluded by liver scanning. In such cases, however, only diagnostic tapping of pus or a positive l.H.A. test would positively rule out other conditions like a malignancy.
7. In any patient of acute amoebic dysentery, who is running fever, has leucocystosis and is not doing well, liver scan is an essential investigation to exclude single or multiple amoebic liver abscesses.
8. If any patient diagnosed as 'Viral-hepatitis' has significant hepatic pain and tenderness, or marked upward (raised right dome) or downward liver enlargement or unequal enlargement of the lobes of the liver, scanning must be done to eliminate an amoebic liver abscess.
9. In all patients of P.U.O. Iiver scan must be asked for to exclude amoebic liver abscess. Though large cold areas in such cases are very often due to amoebic liver abscess, small cold areas could be due to other causes like tuberculous infection, etc.
10. In all cases of pericarditis with effusion, it is better to ask for a liver scan to rule out an amoebic liver abscess of the superior surface of the left lobe which can cause amoebic pericarditis.
11. False localisation is often an indication for liver scanning in a case diagnosed as amoebic liver abscess. For example, if in an enlarged tender left lobe tapping is negative, it is possible that the abscess is in the right lobe.
12. Finally in any patient of acute abdomen, if possible, liver scan must be done to exclude rupture of an inferior surface amoebic liver abscess which is a common cause of acute abdomen in our country. Often, however, there is no time for this investigation. Also these patients are quite ill and only camera pictures can be done, since they are not fit enough for slow rectilinear scanning.

Which liver scan to ask for? In an average case of amoebic liver abscess ^99mTc sulphur colloid or ^99m Tc phytate liver scan should be asked for.
In patients who are suspected of having an inferior surface amoebic liver abscess of the right lobe, ¹³¹ I Rose Bengal or^99mTc labelled HIDA scan should be preferred and a follow up scan of the gall bladder should be requested.
Patients in whom there is strong likelihood of a hepatoma (which is a very close differential diagnosis of a chronic amoebic liver abscess), scan may be repeated by using^99m Tc citrate (solcocitran) or ⁶⁷Ga citrate-isotopes which are picked up by malignant liver cells, though this is not so unequivocal and universally accepted fact.
Also ^113m Indium chloride blood pool scan can be asked to be repeated in order to study the vascularity of the cold area. A rim of vascularity around the cold area will be in favour of amoebic liver abscess. In centres where gamma camera is available, dynamic studies should be asked for to study vascularity of the lesion. In patients with suspected amoebic liver abscess having ascites, oedema feet or prominent veins on the abdominal wall, radio-isotopic venography should be requested to study l.V.C.
III patients like the ones with acute abdomen or those in severe agony cannot be scanned by a rectilinear scanner because of the long time taken for scintigraphy. Camera pictures then become a necessity. Children suspected of having amoebic liver abscess would not co-operate and therefore would also need the help of Gamma camera.
A word about nuclear medicine studies in pregnant females, who are suspected to have amoebic liver abscess. This combination fortunately is very rare. Though theoretically scanning is not safe for the foetus, for all practical purposes the danger is minimal, if at all. Also with^99m Tc foetal dose is minimal.

Finally "What not to expect from a liver scan" in a patient suspected of having an amoebic liver abscess?

1. Liver scan is not useful in deciding whether the patient should be tapped or not.
2. Repeat liver scans are not always helpful in planning the-treatment of amoebic liver abscess. This is because it takes about two to three months for the cold area of amoebic liver abscess to disappear. However, a lot will depend on the load of work in a nuclear medicine department.
3. The common^99m Tc sulphur colloid or phytate scan cannot differentiate amoebic liver abscess from other causes of cold areas.
4. Patients suspected of having an inferior surface amoebic liver abscess, espsecially with a "lump" formation should preferably be sent for peritoneoscopy, which is a much superior investigation, in this situation. Liver scan can often be misguiding in these patients and a patient having an extrahepatic lump may be diagnosed as having a cold area on the inferior surface of the liver. However, as mentioned in an earlier section, in our experience, a combination of investigations like liver scan and peritonescopy is ideal for such cases.

ULTRASOUND IMAGING
Ultrasound imaging has the capability of portraying fine structure in considerable detail by a totally non-invasive method.
Diagnostic ultrasonic procedures are classified into four different modes of operation. The "amplitude" mode or "A" mode is the simplest and used commonly in neurology for echoencephalography.
'B' mode is used for liver imaging. In this mode, the ultrasonic probe is attached to a specially designed mechanical arm whose joints are fitted with position and angle sensing devices. By means of this arm, the probe is moved back and forth along a transverse body contour. Each time an echo signal is received it is displayed on storage type oscilloscope as a bright dot at a location corresponding to the point of origin of the echo. In this manner, a complete cross sectional view of the region scanned is obtained displaying an internal organ such as liver.
In 'B' scanning, the probe has to be maintained at all times in good acoustic contact with the patient's skin. This is important because sonic beams are almost totally reflected at air pockets because of the large difference in acoustic impedence between air and tissue. It may also be necessary to move the probe back and forth along the same contour path several times in order to build up a diagnostically useful image on the scope. For these reasons, it has been difficult to achieve automation in 'B' scanning and the standard practice is to apply a suitable substance such as mineral oil on the patient's skin and move the probe back and forth manually. Manual scanning has two major drawbacks. First of all reproducible geometric orientation is almost impossible to achieve which puts a serious limitation on the quality of the image. Angular changes of as small as one degree can produce substantial differences in the echo intensities. Secondly, manual scanning takes a long time and this makes it impossible to observe rapid dynamic changes.
A significant advance in grey scale imaging, however, took place with the advent of video processors referred to as scan conversion storage tubes. With these units, it has become possible to visualize very subtle differences between and within the tissues.
Grey Scale Imaging of the liver in various planes has become possible in last 5 years with the use of ultrasonic frequencies ranging from 2.25 MHz to 3.5 MHz. The recorded image of the liver is in sections as if one was viewing the cut surface of the liver in transverse or longitudinal planes. This is a noninvasive technique and permits examination of a patient repeatedly any number of times without any discomfort to the patient. This has enabled the study of evolution of amoebic abscess from the stage of inflammation of the liver tissue to the formation of abscess. The serial study of the same patient has revealed a change in echo pattern due to liver necrosis which subsequently undergoes liquefaction and then appears as a cystic mass. Though the Grey Scale Imaging does not specifically indicate the nature of change in the tissue, by using a computer it has been recently possible to specify the nature of change in the liver tissue and this procedure is called "Tissue Characterization". By using the Grey Scale Imaging for serial studies it has been possible to observe the. physical changes in the aspirated material. It has also been possible to record the time of resolution of an abscess with replacement of liver tissue. The smallest size of the abscess that can be detected is determined by the frequency used. Blood vessels and bile ducts of the size of 5 mm. are normally recorded during Grey Scale Imaging.
Fig. 41 shows an ultrasonic scan of a patient having a liver abscess. The dotted line is a marker enabling one to assess the depth of the abscess from the skin of the abdominal wall and also permits measurements of the dimensions of the cavity. Thus, as mentioned elsewhere, aspiration of the liver abscess can be done accurately and confidently under ultrasonic guidance. Multiple abscesses situated at different sites can be tackled with ease, whereas in the past one had to resort to laparotomy for these type of patients.
Ultrasound has the disadvantage of (1) inaccessible right lobe under the ribs, (2) difficulty in visualisation due to marked obesity or gas in the bowel, (3) complete or almost complete replacement of the liver by neoplasm.

Computerised Axial Tomography (CAT Scan)
Computerised Axial Tomography is a new imaging device where an X-ray beam is passed through an organ degree by degree in a 360ø traverse of the X-ray tube across a transverse section of the body. Quantitative differences in the transmission of the beam during its passage through the body are recorded by a scintillation detector. The final image is reconstructed with the aid of a high-powered computer.
Computerised Axial Tomography has brought a new dimension to imaging of organs. Its cross-sectional format permits us to find the exact position of a focal lesion in the liver in all its three dimensions. The resolution of the images is excellent and fine structural details can be visualised in each crosssection of the organ, so much so that each image looks vividly like a page from an anatomy atlas.
As far as visualisation of amoebic abscess in the liver is concerned, this device is likely to have several limitations. However, no genuine experience of this kind is yet available, because in developing countries where amoebic abscesses are common the CAT scanners are largely unavailable because of the high cost of this instrument.
CAT image (Fig. 42) is basically a map of the spatial distribution of the absorption co-efficients of the various regions in an organ. Thus, it shows density differences between normal and abnormal areas in the liver. If an amoebic abscess with thick inspissated pus inside it does not differ significantly in density from the surrounding normal liver tissue, the abscess visualisation will be poor. Moreover, the breathing movements affecting the liver do not keep a small abscess constantly in focus in the same cross section, thereby making it difficult to detect small lesions in a thin slice of the liver. CAT devices also expose the patient to radiation. Faster the image, higher is the radiation exposure.
All imaging modalities are complementary to each other. If radionuclide and ultrasound imaging do not confirm the clinical suspicion of amoebic abscess in the liver, it is conceivable that in an occasional case, additional information may be obtained by CAT scanning.
Above all, the most non-invasive of all diagnostic procedures is a good clinical examination of the patient. In amoebic abscess everything else Supplements it, nothing supplants it.

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Addendum

SONOGRAPHY

Following are the advantages of "sonography" as compared to isotope liver scan in the diagnosis of amoebic liver abscess-

1. Sonography can demonstrate that the cold area is sonoluscent-which means it contains fluid. Often a thick irregular wall and some necrotic debris seen in the cavity can distinguish it from a cyst.
2. It can pick up an abscess which is less than 2 cms and is not visualised by isotope liver scan.
3. When an X-ray chest shows a shadow at the base on the right side and the dome of the diaphragm cannot be differentiated, with sonography the diaphragm can be clearly visualised and even the movements can be measured accurately.
4. An amoebic liver abscess can be differentiated from hydronephrosis, biliary calculi or empyema of the gall bladder, acute alcoholic fatty liver and right sided pleural effusion at the same sitting.
5. There are conditions where the isotope liver scan may not help in the diagnosis of amoebic liver abscess.

These are:-

1. multiple small abscesses
2. superficial abscess
3. some of the inferior surface liver abscesses
4. patients having an amoebic liver abscess with poor liver function.

In all these patients sonography is a very useful investigation.

6. Sonography can pick up the complications like rupture of left lobe abscess into the pericardium or a right lobe abscess into the pleura.
7. IVC obstruction, obstruction of the hepatic vein or presence of free fluid in the peritoneal cavity can be picked up by sonography.
8. Academic aspects of amoebic liver abscess like a solid state abscess, the process of healing etc. can be studied with the help of sonography.
9. The most important advantage of sonography is "therapeutic tapping". Tapping of the amoebic liver abscess can be done most scientifically and confidently with the help of sonography. In recent times, the prognosis has improved in cases having multiple abscesses because of the ease with which tapping can be done. Usually such patients have one or two big abscesses and the rest of them are small abscesses. One can also measure the sonoluscent area and calculate the exact amount of pus which will be expected from the abscess.
10. This investigation has no radiation hazards.
11. It is very safe in children and in pregnant women.
12. It is a very useful investigation in cases of P.U.O. when, besides an amoebic liver abscess, one can pick up many other causes of P.U.O.
13. In big institutions, it is important to realise that this investigation is not dependent on the usual isotope supply.
14. Sonography will possibly have more advantages in future, when the real time system units will be freely available.
15. This investigation will be very useful especially in the rural areas when portable sonography units will be available.

Sonography is unreliable for excluding a "small superior surface" amoebic liver abscess. This is because when the transducer is placed on the right lower chest the ribs are a hindrance to the sound waves. Also rarely false positive sonoluscent areas may be reported.

January, 1982.