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Computerised tomography and the liver
Clin. Radiol. (1977) 28, 571-581
COMPUTERISED TOMOGRAPHY AND THE LIVER LOUIS KREEL From the Division o f Radiology, Clinical Research Centre and Northwick Park Hospital, Harrow
Computerised tomography produces an excellent image of the liver. The author discusses the relevant technical factors such as the use of window levels and window widths, tissue attenuation values and filtering. The importance of the applied anatomy and the use of contrast agents are discussed and then the findings on computed tomography in clinical practice are presented. The lesions that can be visualised include cysts, abscesses, primary tumours, metastases, fatty liver, subphrenic abscess, dilated bile ducts and cholelithiasis as well as surrounding ascites. These appearances are described and illustrated. INTRODUCTION There is little doubt that of the diagnostic modalities available, a high resolution system such as the EMI General Purpose Scanner is, at present, the best anatomical display of liver parenchyma (Fig. 1). This must be compared with isotope scanning which may, on occasion, show functional information such as the presence of a malignancy with a gallium scan or an abscess with a polymorphonuclear leucocyte-labelled indium scan otherwise unobtainable by other noninvasive techniques; and compared with ultrasonography which can usually distinguish solid from cystic lesions, show normal portal and hepatic veins and dilated bile ducts. For vascular pathology and for small vascularised tumours angiography remains the method of choice, bearing in mind that this is, of course, an invasive procedure. Computerised tomography produces such an excellent anatomical display because it is a highly efficient method of using X-rays with a high proportion of the transmitted photons being registered and a high sensitivity of small differences in tissue attentuation or tissue density. The anatomical display itself is of axial sections approximately 1 cm in thickness. There are, however, a number of technical factors which must be understood and taken into account if the best results are to be obtained. The equipment, how it operates and the way it is used in clinical practice, has now been discussed in a number of articles (Hounsfield, 1973; Ambrose, 1973; Hill, 1976; Kreel, 1976) and these aspects will be referred to only as they are directly concerned with the interpretation of liver and gallbladder pathology. To this end the meaning and use of window width and window level will be considered as well as the use of 'filtered' or 'smoothed' pictures. However, the anatomy of the liver, as seen on computerised tomography, is very important in the diagnosis of liver disease and will be examined in some detail.
It is also germane to the present discussion that computerised tomography is concerned with spaceoccupying lesions rather than the diffuse pathology, apart from fatty infiltration of the liver which is a notable exception. Computerised tomography at present has no diagnostic role or possibly only a minor role in the various types of liver cirrhosis which depend upon clinical, biochemical and histological methods, although in the future this may change.
WINDOW WIDTH AND WINDOW LEVEL Tissue attenuation values or tissue density have arbitrarily been graded into 1000 EMI units, ranging from the lowest density of - 5 0 0 through water at 0 to the highest possible reading of +500. The computer printout, in fact, gives the data in just this form (Fig. 2). However, its conversion to an anatomical picture immediately limits the display of these tissue density values to that which can be represented on any particular photographic system, which at best is about 20 shades of grey. It is, however, not always necessary nor desirable to show the data on such a long grey scale and indeed, on occasions, a bistable display in black and white can be a great advantage. Whether the data is shown in a picture with a long grey scale or a short grey scale (Fig. 3) is determined by the window width. Thus, window width settings of 400 give the maximum grey scale (Fig. 3a) and a setting of 001 gives a bistable display or black and white picture with no grey scale (Fig. 3d). The window width of 001 is also known as 'measure' because it is used for determining tissue densities (Fig. 4). Window level on the other hand moves the picture up or down the scale from - 5 0 0 to + 500. Thus, if the blood vessels between the air at the lung bases behind the liver are to be seen, the window height must be set at - 4 5 0 , if adipose tissue is to be shown - 1 5 , for abdominal soft tissue + 15, and for bone, a
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setting of between + 5 0 and + 150 is needed. The window level is therefore related to tissue density. To measure tissue density or tissue attenuation values, the window width must be set at the bistable black and white display of 001. When there are an equal number of black and white dots in a particular area then its tissue density corresponds to the value as shown by the window level (Fig. 5). While this is by and large true, it is most important to be aware of the factors that may influence tissue density values.
FACTORS INFLUENCING TISSUE ATTENUATION VALUES As with conventional radiology the most important factor is the atomic numbers of the constituents of the organ. It is obvious then that contrast agents and bone have high atomic numbers and hence high tissue attenuation coefficients, while lungs have low atomic numbers and tissue attenuation. However, there is another important factor in computerised tomography which also determines tissue density and that is the concentration of molecules per unit volume. If molecules are dispersed in water the tissue density is lowered but if concentrated by dehydration or water absorption as occurs with protein solutions, especially blood, then the tissue attenuation coefficient is increased. At first as blood clots, it contracts expelling water and increasing the 'tissue density' as seen on computerised tomography. Later as it disperses by absorbing water
Fig. 1 - C T view of liver using a 'filtered' programme showing porta hepatis and portal veins.
the tissue density is lowered. Similarly, ice has a lower density than water even though it is solid. The third factor influencing tissue density is the kV setting at which the section is done. However, the variation of tissue densities with kV alter in a linear fashion so that their relative tissue densities are unaltered. This does, however, mean that for absolute values of tissue density the printout values must be corrected for the particular kV setting.
Fig. 2 - Computer printout of the axial section shown in Fig. 1.
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Fig. 3a - Section at a window width of 400 with maximum grey scale but minimum contrast. Bone detail becomes visible at wide window settings.
Fig. 3 b - N a r r o w i n g the window to 100 increases t h e contrast but diminishes the grey scale so that bone appears entirely white and no detail is visible within the bone.
Fig. 3 c - At a window of 50 the grey scale is greatly diminished with marked contrast enhancement. This setting is used to detect lesions within large relatively homogeneous soft tissue areas such as the liver and brain.
Fig. 3d - The m i n i m u m window setting (001) gives a bistable black and white display.
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PARTIAL VOLUME E F F E C T There is a very important artefact that must be taken into account when considering tissue attenuation, particularly in the liver. Tissue attenuation coefficients on computerised tomography are only valid if the whole of the lesion is on the particular axial section, otherwise the values will also reflect those of adjacent tissue. Thus, if a low density lesion is only partly within a section of higher density the readings will appear too high and conversely if it is partly within a section of lower density the readings will be too low. Thus the values of a cyst with a
diameter less than the slice thickness and lying deep in liver tissue, can have values as high as + 8 or + 10 but a cyst on the diaphragmatic surface of the liver, with part of the section taken through lung tissue will have values as low as - 10 or - 12 (Fig. 6). This is known as the partial volume effect. Another artefact which at present influences not only tissue attenuation values but also picture quality are the dark bands running across the liver from the intercostal spaces when sections are taken at incorrect kV settings. These dark bands may pass right across the liver making interpretation of space-occupying lesions difficult. The worst artefacts of all, however, are caused b~, movement, especially due to respiration or peristalsis of bowel-containing gas. It is therefore important to have an exposure time for each section less than 20 s so that patients can hold their breath. Bowel movement is controlled by giving an anticholinergic such as propanthaline in a dose of 3 0 45 mg intramuscularly some 1 0 - 1 5 min before the scan starts or Glucagon 0 . 1 - 0 . 2 5 mg i.v. Movement artefacts causing streaking may also occur on the upper margin of the liver possibly due to adjacent cardiac pulsation. This can often be overcome by examining the superior part of the liver in deep inspiration to bring the top of the liver away from the heart.
Fig. 5a F i g . 4 - A t the minimum window setting (W0001) the window level can be raised to 'isolate' the liver shown to have a density value of 35 (+ L 0035).
A filling defect is shown in the liver (arrow).
Fig. 5b - To measure the 'tissue density' of the lesion the window width is reduced to 'measure' (WW 001) and the window level set so that there are equal black and white spots within it, giving a reading of 22 Hounsfield units (WH + 0.22) indicative of a solid lesion.
COMPUTERISED TOMOGRAPHY AND THE LIVER CT ANATOMY OF THE LIVER The most cranial part of the liver, enclosed by the right hemidiaphragm, usually has part of the lung base on the CT sections. In the vast majority of cases a crescent of lung lies posteriorly but just occasionally there is also lung anteriorly. The left and
Fig. 6 a - A round low density lesion in the liver lying posteriorly in the right lobe shown to be a cyst.
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right ventricles, pericardial fat and pericardium lies anteriorly and adjacent to these structures, the cupola of the left hemidiaphragm encloses the gastric fundus and spleen. Because of the variable position of the diaphragm a section showing these features may be as high as D9 or as low as D12 and because of the adjacent lung the density values of the liver
Fig. 6b - Because of the adjacent lung, the partial volume effect reduces the value of the window level readings to 10 Hounsfield units (WH - 010). -
Fig. 7 - Hepatic veins (arrow) draining into inferior vena cava enclosed by liver parenchyma.
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and spleen will be artificially lowered due to the partial volume effect. Two to four centimetres lower the sections no longer include lung and the true tissue density of the liver becomes apparent, particularly if compared to the upper pole of the right kidney. Liver tissue density is significantly higher (25-35 EMI units) than kidneys, muscle, aorta or inferior vena cava (20-25 EMI units) but similar to that of spleen (25-35 EMI units). The right lobe of the liver fills almost the whole of the right half of the section and the left lobe of the liver extends anteriorly, the falciform ligament forming a cleft between. The inferior vena cava is enclosed by the liver and frequently the major branches of the hepatic veins are demonstrated draining into the inferior vena cava (Fig. 7). The caudate lobe of the liver lies just anterior to the inferior vena cava with the hilum of the liver lying in front of the caudate lobe. The hilum of the liver containing the portal vein and bile ducts is of lower density than liver substance. The left hepatic duct, containing contrast medium, may be seen to form the most anterior aspect of the hilum when calcium ipodate has been given. The portal vein within the liver has a similar density to the inferior vena cava and as it proceeds to the periphery forms round or oval transradiancies seen within the liver substance (Fig. 1). The normal liver parenchyma does not therefore appear entirely homogeneous having small circular,
oval or tubular lower-density areas formed by the portal and hepatic veins. The peripheral margin of the liver is closely applied to the peritoneum of the lateral abdominal wall. Even small peritoneal effusions thus become visible showing as a crescent of lower density between the liver and the peritoneum. The gallbladder, lying on the anterior margin of the liver or surrounded by liver substance, is usually visible either because of its natural lower density or because of contrast medium within it. In the more caudal cuts only the right lobe of the liver is visible and Riedel's lobe may, of course, come down to the livel of the iliac crest, particularly in elderly females. CONTRAST AGENTS IN LIVER CT
Oral cholecystographic agents will show the gallbladder with less than half the dose used in conventional examinations and the hepatic ducts and common bile duct in about half the cases. There is, unfortunately, no obvious increase in liver density with these substances. In some cases small amounts of the contrast can be seen in the renal pelves. The density of the liver parenchyma increases significantly with urographic contrast media. This has, however, not proved particularly useful in the diagnosis of space-occupying lesions because the contrast is also taken up by tumours and the relative tissue densities between tumour and normal parenchyma are often diminished rather than enhanced. When this happens the lesions become more difficult to see. Another disadvantage of the urographic Fig. 8a, b - Hepatoma of left lobe of liver. The contrast between normal tissue and turnout is enhanced by using the contrast agents is that peristaltic activity of the renal Ttltering' or 'smoothing' programme. (a). Normal pIo- pelves and ureters containing dense contrast produces
gramme, (b) 'Filtered' programme.
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streak artefacts across the liver and other organs adjacent to the kidneys. The ideal contrast agent for the liver is yet to be produced. It must, o f course, be non-toxic and must selectively enhance the normal liver parenchymal density by only some 1 0 - 1 5 EMI units.
difference between normal parenchyma and tumours. Without the 'smoothing' programme these lesions may not be appreciated and if such a smoothing programme is not available the liver must be viewed at narrower window settings to increase inherent contrast.
FILTERING AND SMOOTHING PROGRAMME
CT OF THE LIVER IN CLINICAL PRACTICE
The new 'filtering' programme is particularly useful in the liver (Fig. 8) and brain. It produces mean tissue values in all adjacent small areas of the section which effectively enhances the natural contrast between adjacent areas. In the liver it often 'brings out' lesions, especially large metastases which are otherwise difficult to visualise. Although by no means proven, it would appear that residual normal parenchyma, in many cases of metastatic disease, loses tissue density, thus diminishing the contrast
The information derived from computerised tomography is ideal for the detection of single or discrete lesions down to 2 cm in diameter (Fig. 9) and can often show such abnormalities when only 0.5 cm in diameter. As has been stressed previously, this will in a large measure depend on relative tissue densities so that cysts will be shown much more easily than solid tumours, unless the relative tissue density of the normal parenchyma can in some way be raised. Small tumours must also be distinguished from portal and hepatic veins, which can usually be done by examining adjacent sections. Venous transradiancies will show continuity through adjacent sections, particularly if such sections are taken only 1-1.5 cm apart. Solid L e s i o n s . - B y and large, liver tumours whether benign or malignant, primary or secondary, have similar tissue density values, some 1 0 - 1 5 EMI units less than normal liver parenchyma.
Fig. 9 - Single metastasis in the liver from a carcinoma of the prostrate. Fig. 10. Multiple well-defined metastases from a leiomyosarcoma. Fig. ll Small, calcified metastasis from an osteogenic sarcoma.
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Provided the tumours are reasonably well demarcated and the normal liver parenchyma retains its usual density, these lesions will be well shown (Fig. 10). Both single and discrete multiple lesions have been clearly demonstrated in lymphoma, testicular teratoma, adenocarcinoma, sarcoma, hamartoma and fibroma. No characteristic features of these lesions have, as yet, been recognised and for a pathological diagnosis histoligical sections are still required. The main value then in showing these lesions is to site them accurately and to show how many are present. Just occasionally metastases are totally calcified, as with osteogenic sarcoma (Fig. 11) or partially calcified as with hepatomas or colloid carcinoma metastases. These lesions are then readily recognised. Metastases from gastrointestinal endocrine tumours such as insulinomas and gastrinomas may take up urographic contrast agents, thus showing up more deafly after an intravenous contrast injection. But, as indicated previously, other metastases are often shown more poorly after such a contrast injection. Multiple diffuse tumours in the liver may be difficult to recognise for a number of reasons. Their margins are poorly demarcated, the normal intervening parenchyma appears to lose its tissue density and as the malignant disease progresses the body fat planes are lost, making it difficult to show the margin of the enlarged liver. Computerised tomography is thus more accurate in the detection of
smaller or single metastases than in late metastatic disease in the liver. Liver Cysts and Abscesses.- The detection of abscesses or cysts (Figs. 6, 12) is usually unequivocal whether single or multiple and down to 0.5-1 cm in size. However, abscesses cannot be distinguished from cysts except within a particular clinical context. These space-occupying lesions are well demarcated and because of their low tissue density, well shown. By the use of the window level and window width settings they can be shown to have a tissue density in the region of +1 EMI unit (Fig. 12) thus clearly distinguishing cystic and solid lesions. Hydatid cysts often, of course, have a surrounding line of calcification and the internal structure may indicate the presence of daughter cysts making almost a pathognomonic picture. Subphrenic Abscess. - In cases of subphrenic abscess the margin of the liver is pushed aside forming a relatively straight edge with an area of low density ( 1 - 2 EMI units) between the liver and diaphragm (Figs. 13, 14). It thus becomes possible not only to show the position of the abscess for aspiration but also to calculate the volume of the contained pus. Ascites and Colloid Carcinoma Infiltration of the Peritoneum. - The homogeneous crescentic inter-
position of ascitic fluid between the liver and diaphragm is clearly demonstrated oll the upper abdominal sections. The spleen is similarly displaced
Fig. 12a - Abscess in liver foUowing choledochojejenostomy Fig.12b - To show density of lesion (WH + 000). to relieve obstructuvejaundice.
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with the ascities being equally well recognised (Fig. 15) and even the pancreas may be more clearly shown because of the ascitic fluid. The diffuse infiltration of the peritoneal cavity by colloid carcinoma of the colon or ovary produces a somewhat similar appearance with displacement of the liver and spleen away from the diaphragm. However, in these cases the tissue attenuation values of
the displacing substance is higher than ascitic fluid which has a density of about +2 E units, whereas colloid carcinoma infiltrate is 1 0 - 1 5 E units (Fig. 16). Aetinomycosis of the Liver. - In a case of actinomycosis the actual extent of the lesion in the liver could not be demarcated but its extension through the abdominal wall and into the subcutaneous tissue
Fig. 13 - Needle in position in a subphrenic abscess.
Fig. 1 4 - Density of subphrenic abscess is +1 H unit (WH + 001).
Fig 15 - Ascitic fluid comes between liver and diaphragm and surrounds the spleen,
Fig. 16 - Infiltrating peritoneal colloid carcinoma resembles ascitic fluid but was shown to have a higher density.
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could be, A smaU, associated round lesion was shown to be at the right lung base and not actually in the liver, The Fatty Liver. - Diffuse fatty infiltration of the liver, as occurs with alcoholism, produces a pathognomonic appearance, The tissue density of the liver becomes extremely low with the portal and hepatic
veins showing as vascular structures of higher density. At present this is thought to be so characteristic that a liver biopsy becomes redundant (Fig, 17). Obstructive J a u n d i c e . - I n t r a h e p a t i c dilated bile ducts produce multiple rounded areas of lower density in the outer part of the liver and a large branching lower density configuration at the hilum.
Fig. 17 - Fatty liver has a characteristic appearance with the portal veins and IVC visible against the background of low density liver. The pancreas and kidney have higher densities than liver parenchyma.
Fig. 1 8 - Carcinoma head of pancreas (arrows) producing obstructive jaundice. Enlarged gallbladder and bile ducts visible.
Fig. 19 - Hypernephroma (arrows) impressing posterior Fig, 2 0 - Calcification in a thrombosed inferior vena cava aspect of the liver which produced an area of 'non-uptake' as (arrows) and collateral veins behind the diaphragm (arrow shown on the isotope scan. heads).
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Furthermore, the cause of the extrahepatic biliary liver tissue but may be suspected when there is obstruction may be shown as a mass in the head of extrahepatic jaundice and the gallbladder cannot be the pancreas (Fig. 18) or the head o f the pancreas demonstrated. may clearly be shown to be normal in size. It should, however, be borne in mind that if the mass has a SUMMARY AND CONCLUSIONS smooth margin, extends more cranially towards the hilum o f the liver and other sections show enlarged Computerised tomography produces an excellent lymph nodes, then the underlying pathology could image of the liver which can be enhanced with the well be a lymphoma. new 'filtered' programme. Normal liver density is Masses in Adjacent O r g a n s . - Mass lesions in the greater than other soft tissue densities, apart from right kidney or suprarenal m a y remain undetected by that of the spleen. An understanding of the use of the other investigations or produce the appearances of an window level and window width settings helps greatly intrahepatic space-occupying lesion. An isotope scan in distinguishing solid lesions from cysts or abscesses. may, for instance, show an area of non-uptake in the Lesions which can be demonstrated include posterior upper part of the right lobe of the liver due tumours, cysts, subphrenic abscess and fatty liver. to a mass lesion on the anterior aspect of the upper CT is particularly useful in showing the size, shape pole of the kidney (Fig. 19). Similarly, a turnout in and position o f space-occupying lesions. However, the right suprarenal m a y be demarcated by a line o f with solid space-occupying lesions histology is still lower density indicating that it lies outside the liver required for a definitive tissue diagnosis. and if due to a metastasis that it carl be excised as there is a line o f demarcation between it and the liver. Vascular L e s i o n s . - Calcification in the inferior ADDENDUM vena cava following thrombosis and the resulting collateral circulation can be clearly demonstrated 2 Hounsfield units = 1 EMI unit. Unfortunately, (Fig. 20). The enlarged azygos and hemiazygos veins EMI units are used on the CT 5005, and most other produce rounded shadows in the fat space between equipment uses Hounsfield units, including the EMI the diaphragm and the vertebral b o d y particularly at brain machine. the level of D l l / 1 2 . Collateral vessels around the kidney and in the peritoneal fat may also be noted. Acknowledgement. - Grateful acknowledgement is made Chronic congestive cardiac failure produces marked enlargement o f the inferior vena cava and to the Department of Health and Social Security, whose farsighted and generous help has assisted greatly in the dehepatic veins which on isotope scanning produce velopment of this project. The assistance and considerable areas o f diminished uptake and can be mistaken for help from Mr John Twydle, who has been managing the metastases. These dilated and enlarged vessels can be machine, is also gratefully acknowledged. clearly demonstrated on CT scanning. The Gallbladder. The visualisation of the normal REFERENCES gallbladder with and without contrast medium is Ambrose, J. (1973). Computerised transverse axial scanning almost invariable with computerised tomography. (tomography). (2) Clinical application. British Journal of Gallstones can be seen but this m e t h o d is far too Radiology, 46, 1023 1047. expensive and time-consuming to be used in this way. Hill, K. R. (1976). EMI total body scanner: technical aspects. However, in certain circumstances the size of the ,, British Journal o f Clinical Equipment, 1,207-214. gallbladder can be o f considerable importance Hounsfield, G. N. (1973). Computerised transverse axial scanning (tomography). (1) Description of system. especially in obstructive jaundice but may also be British Journal o f Radiology, 46, 1016-1022. helpful in suspected empyema of the gallbladder. Kreel, L. (1976). The EMI Whole Body Scannei: an interim Carcinoma of the gallbladder may be difficult to clinical evaluation of the prototype. British Journal of Clinical Equipment, 4,220-227. detect as the density o f the mass is similar to that of
COMPUTERISED TOMOGRAPHY AND THE LIVER LOUIS KREEL From the Division o f Radiology, Clinical Research Centre and Northwick Park Hospital, Harrow
Computerised tomography produces an excellent image of the liver. The author discusses the relevant technical factors such as the use of window levels and window widths, tissue attenuation values and filtering. The importance of the applied anatomy and the use of contrast agents are discussed and then the findings on computed tomography in clinical practice are presented. The lesions that can be visualised include cysts, abscesses, primary tumours, metastases, fatty liver, subphrenic abscess, dilated bile ducts and cholelithiasis as well as surrounding ascites. These appearances are described and illustrated. INTRODUCTION There is little doubt that of the diagnostic modalities available, a high resolution system such as the EMI General Purpose Scanner is, at present, the best anatomical display of liver parenchyma (Fig. 1). This must be compared with isotope scanning which may, on occasion, show functional information such as the presence of a malignancy with a gallium scan or an abscess with a polymorphonuclear leucocyte-labelled indium scan otherwise unobtainable by other noninvasive techniques; and compared with ultrasonography which can usually distinguish solid from cystic lesions, show normal portal and hepatic veins and dilated bile ducts. For vascular pathology and for small vascularised tumours angiography remains the method of choice, bearing in mind that this is, of course, an invasive procedure. Computerised tomography produces such an excellent anatomical display because it is a highly efficient method of using X-rays with a high proportion of the transmitted photons being registered and a high sensitivity of small differences in tissue attentuation or tissue density. The anatomical display itself is of axial sections approximately 1 cm in thickness. There are, however, a number of technical factors which must be understood and taken into account if the best results are to be obtained. The equipment, how it operates and the way it is used in clinical practice, has now been discussed in a number of articles (Hounsfield, 1973; Ambrose, 1973; Hill, 1976; Kreel, 1976) and these aspects will be referred to only as they are directly concerned with the interpretation of liver and gallbladder pathology. To this end the meaning and use of window width and window level will be considered as well as the use of 'filtered' or 'smoothed' pictures. However, the anatomy of the liver, as seen on computerised tomography, is very important in the diagnosis of liver disease and will be examined in some detail.
It is also germane to the present discussion that computerised tomography is concerned with spaceoccupying lesions rather than the diffuse pathology, apart from fatty infiltration of the liver which is a notable exception. Computerised tomography at present has no diagnostic role or possibly only a minor role in the various types of liver cirrhosis which depend upon clinical, biochemical and histological methods, although in the future this may change.
WINDOW WIDTH AND WINDOW LEVEL Tissue attenuation values or tissue density have arbitrarily been graded into 1000 EMI units, ranging from the lowest density of - 5 0 0 through water at 0 to the highest possible reading of +500. The computer printout, in fact, gives the data in just this form (Fig. 2). However, its conversion to an anatomical picture immediately limits the display of these tissue density values to that which can be represented on any particular photographic system, which at best is about 20 shades of grey. It is, however, not always necessary nor desirable to show the data on such a long grey scale and indeed, on occasions, a bistable display in black and white can be a great advantage. Whether the data is shown in a picture with a long grey scale or a short grey scale (Fig. 3) is determined by the window width. Thus, window width settings of 400 give the maximum grey scale (Fig. 3a) and a setting of 001 gives a bistable display or black and white picture with no grey scale (Fig. 3d). The window width of 001 is also known as 'measure' because it is used for determining tissue densities (Fig. 4). Window level on the other hand moves the picture up or down the scale from - 5 0 0 to + 500. Thus, if the blood vessels between the air at the lung bases behind the liver are to be seen, the window height must be set at - 4 5 0 , if adipose tissue is to be shown - 1 5 , for abdominal soft tissue + 15, and for bone, a
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setting of between + 5 0 and + 150 is needed. The window level is therefore related to tissue density. To measure tissue density or tissue attenuation values, the window width must be set at the bistable black and white display of 001. When there are an equal number of black and white dots in a particular area then its tissue density corresponds to the value as shown by the window level (Fig. 5). While this is by and large true, it is most important to be aware of the factors that may influence tissue density values.
FACTORS INFLUENCING TISSUE ATTENUATION VALUES As with conventional radiology the most important factor is the atomic numbers of the constituents of the organ. It is obvious then that contrast agents and bone have high atomic numbers and hence high tissue attenuation coefficients, while lungs have low atomic numbers and tissue attenuation. However, there is another important factor in computerised tomography which also determines tissue density and that is the concentration of molecules per unit volume. If molecules are dispersed in water the tissue density is lowered but if concentrated by dehydration or water absorption as occurs with protein solutions, especially blood, then the tissue attenuation coefficient is increased. At first as blood clots, it contracts expelling water and increasing the 'tissue density' as seen on computerised tomography. Later as it disperses by absorbing water
Fig. 1 - C T view of liver using a 'filtered' programme showing porta hepatis and portal veins.
the tissue density is lowered. Similarly, ice has a lower density than water even though it is solid. The third factor influencing tissue density is the kV setting at which the section is done. However, the variation of tissue densities with kV alter in a linear fashion so that their relative tissue densities are unaltered. This does, however, mean that for absolute values of tissue density the printout values must be corrected for the particular kV setting.
Fig. 2 - Computer printout of the axial section shown in Fig. 1.
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Fig. 3a - Section at a window width of 400 with maximum grey scale but minimum contrast. Bone detail becomes visible at wide window settings.
Fig. 3 b - N a r r o w i n g the window to 100 increases t h e contrast but diminishes the grey scale so that bone appears entirely white and no detail is visible within the bone.
Fig. 3 c - At a window of 50 the grey scale is greatly diminished with marked contrast enhancement. This setting is used to detect lesions within large relatively homogeneous soft tissue areas such as the liver and brain.
Fig. 3d - The m i n i m u m window setting (001) gives a bistable black and white display.
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PARTIAL VOLUME E F F E C T There is a very important artefact that must be taken into account when considering tissue attenuation, particularly in the liver. Tissue attenuation coefficients on computerised tomography are only valid if the whole of the lesion is on the particular axial section, otherwise the values will also reflect those of adjacent tissue. Thus, if a low density lesion is only partly within a section of higher density the readings will appear too high and conversely if it is partly within a section of lower density the readings will be too low. Thus the values of a cyst with a
diameter less than the slice thickness and lying deep in liver tissue, can have values as high as + 8 or + 10 but a cyst on the diaphragmatic surface of the liver, with part of the section taken through lung tissue will have values as low as - 10 or - 12 (Fig. 6). This is known as the partial volume effect. Another artefact which at present influences not only tissue attenuation values but also picture quality are the dark bands running across the liver from the intercostal spaces when sections are taken at incorrect kV settings. These dark bands may pass right across the liver making interpretation of space-occupying lesions difficult. The worst artefacts of all, however, are caused b~, movement, especially due to respiration or peristalsis of bowel-containing gas. It is therefore important to have an exposure time for each section less than 20 s so that patients can hold their breath. Bowel movement is controlled by giving an anticholinergic such as propanthaline in a dose of 3 0 45 mg intramuscularly some 1 0 - 1 5 min before the scan starts or Glucagon 0 . 1 - 0 . 2 5 mg i.v. Movement artefacts causing streaking may also occur on the upper margin of the liver possibly due to adjacent cardiac pulsation. This can often be overcome by examining the superior part of the liver in deep inspiration to bring the top of the liver away from the heart.
Fig. 5a F i g . 4 - A t the minimum window setting (W0001) the window level can be raised to 'isolate' the liver shown to have a density value of 35 (+ L 0035).
A filling defect is shown in the liver (arrow).
Fig. 5b - To measure the 'tissue density' of the lesion the window width is reduced to 'measure' (WW 001) and the window level set so that there are equal black and white spots within it, giving a reading of 22 Hounsfield units (WH + 0.22) indicative of a solid lesion.
COMPUTERISED TOMOGRAPHY AND THE LIVER CT ANATOMY OF THE LIVER The most cranial part of the liver, enclosed by the right hemidiaphragm, usually has part of the lung base on the CT sections. In the vast majority of cases a crescent of lung lies posteriorly but just occasionally there is also lung anteriorly. The left and
Fig. 6 a - A round low density lesion in the liver lying posteriorly in the right lobe shown to be a cyst.
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right ventricles, pericardial fat and pericardium lies anteriorly and adjacent to these structures, the cupola of the left hemidiaphragm encloses the gastric fundus and spleen. Because of the variable position of the diaphragm a section showing these features may be as high as D9 or as low as D12 and because of the adjacent lung the density values of the liver
Fig. 6b - Because of the adjacent lung, the partial volume effect reduces the value of the window level readings to 10 Hounsfield units (WH - 010). -
Fig. 7 - Hepatic veins (arrow) draining into inferior vena cava enclosed by liver parenchyma.
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and spleen will be artificially lowered due to the partial volume effect. Two to four centimetres lower the sections no longer include lung and the true tissue density of the liver becomes apparent, particularly if compared to the upper pole of the right kidney. Liver tissue density is significantly higher (25-35 EMI units) than kidneys, muscle, aorta or inferior vena cava (20-25 EMI units) but similar to that of spleen (25-35 EMI units). The right lobe of the liver fills almost the whole of the right half of the section and the left lobe of the liver extends anteriorly, the falciform ligament forming a cleft between. The inferior vena cava is enclosed by the liver and frequently the major branches of the hepatic veins are demonstrated draining into the inferior vena cava (Fig. 7). The caudate lobe of the liver lies just anterior to the inferior vena cava with the hilum of the liver lying in front of the caudate lobe. The hilum of the liver containing the portal vein and bile ducts is of lower density than liver substance. The left hepatic duct, containing contrast medium, may be seen to form the most anterior aspect of the hilum when calcium ipodate has been given. The portal vein within the liver has a similar density to the inferior vena cava and as it proceeds to the periphery forms round or oval transradiancies seen within the liver substance (Fig. 1). The normal liver parenchyma does not therefore appear entirely homogeneous having small circular,
oval or tubular lower-density areas formed by the portal and hepatic veins. The peripheral margin of the liver is closely applied to the peritoneum of the lateral abdominal wall. Even small peritoneal effusions thus become visible showing as a crescent of lower density between the liver and the peritoneum. The gallbladder, lying on the anterior margin of the liver or surrounded by liver substance, is usually visible either because of its natural lower density or because of contrast medium within it. In the more caudal cuts only the right lobe of the liver is visible and Riedel's lobe may, of course, come down to the livel of the iliac crest, particularly in elderly females. CONTRAST AGENTS IN LIVER CT
Oral cholecystographic agents will show the gallbladder with less than half the dose used in conventional examinations and the hepatic ducts and common bile duct in about half the cases. There is, unfortunately, no obvious increase in liver density with these substances. In some cases small amounts of the contrast can be seen in the renal pelves. The density of the liver parenchyma increases significantly with urographic contrast media. This has, however, not proved particularly useful in the diagnosis of space-occupying lesions because the contrast is also taken up by tumours and the relative tissue densities between tumour and normal parenchyma are often diminished rather than enhanced. When this happens the lesions become more difficult to see. Another disadvantage of the urographic Fig. 8a, b - Hepatoma of left lobe of liver. The contrast between normal tissue and turnout is enhanced by using the contrast agents is that peristaltic activity of the renal Ttltering' or 'smoothing' programme. (a). Normal pIo- pelves and ureters containing dense contrast produces
gramme, (b) 'Filtered' programme.
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streak artefacts across the liver and other organs adjacent to the kidneys. The ideal contrast agent for the liver is yet to be produced. It must, o f course, be non-toxic and must selectively enhance the normal liver parenchymal density by only some 1 0 - 1 5 EMI units.
difference between normal parenchyma and tumours. Without the 'smoothing' programme these lesions may not be appreciated and if such a smoothing programme is not available the liver must be viewed at narrower window settings to increase inherent contrast.
FILTERING AND SMOOTHING PROGRAMME
CT OF THE LIVER IN CLINICAL PRACTICE
The new 'filtering' programme is particularly useful in the liver (Fig. 8) and brain. It produces mean tissue values in all adjacent small areas of the section which effectively enhances the natural contrast between adjacent areas. In the liver it often 'brings out' lesions, especially large metastases which are otherwise difficult to visualise. Although by no means proven, it would appear that residual normal parenchyma, in many cases of metastatic disease, loses tissue density, thus diminishing the contrast
The information derived from computerised tomography is ideal for the detection of single or discrete lesions down to 2 cm in diameter (Fig. 9) and can often show such abnormalities when only 0.5 cm in diameter. As has been stressed previously, this will in a large measure depend on relative tissue densities so that cysts will be shown much more easily than solid tumours, unless the relative tissue density of the normal parenchyma can in some way be raised. Small tumours must also be distinguished from portal and hepatic veins, which can usually be done by examining adjacent sections. Venous transradiancies will show continuity through adjacent sections, particularly if such sections are taken only 1-1.5 cm apart. Solid L e s i o n s . - B y and large, liver tumours whether benign or malignant, primary or secondary, have similar tissue density values, some 1 0 - 1 5 EMI units less than normal liver parenchyma.
Fig. 9 - Single metastasis in the liver from a carcinoma of the prostrate. Fig. 10. Multiple well-defined metastases from a leiomyosarcoma. Fig. ll Small, calcified metastasis from an osteogenic sarcoma.
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Provided the tumours are reasonably well demarcated and the normal liver parenchyma retains its usual density, these lesions will be well shown (Fig. 10). Both single and discrete multiple lesions have been clearly demonstrated in lymphoma, testicular teratoma, adenocarcinoma, sarcoma, hamartoma and fibroma. No characteristic features of these lesions have, as yet, been recognised and for a pathological diagnosis histoligical sections are still required. The main value then in showing these lesions is to site them accurately and to show how many are present. Just occasionally metastases are totally calcified, as with osteogenic sarcoma (Fig. 11) or partially calcified as with hepatomas or colloid carcinoma metastases. These lesions are then readily recognised. Metastases from gastrointestinal endocrine tumours such as insulinomas and gastrinomas may take up urographic contrast agents, thus showing up more deafly after an intravenous contrast injection. But, as indicated previously, other metastases are often shown more poorly after such a contrast injection. Multiple diffuse tumours in the liver may be difficult to recognise for a number of reasons. Their margins are poorly demarcated, the normal intervening parenchyma appears to lose its tissue density and as the malignant disease progresses the body fat planes are lost, making it difficult to show the margin of the enlarged liver. Computerised tomography is thus more accurate in the detection of
smaller or single metastases than in late metastatic disease in the liver. Liver Cysts and Abscesses.- The detection of abscesses or cysts (Figs. 6, 12) is usually unequivocal whether single or multiple and down to 0.5-1 cm in size. However, abscesses cannot be distinguished from cysts except within a particular clinical context. These space-occupying lesions are well demarcated and because of their low tissue density, well shown. By the use of the window level and window width settings they can be shown to have a tissue density in the region of +1 EMI unit (Fig. 12) thus clearly distinguishing cystic and solid lesions. Hydatid cysts often, of course, have a surrounding line of calcification and the internal structure may indicate the presence of daughter cysts making almost a pathognomonic picture. Subphrenic Abscess. - In cases of subphrenic abscess the margin of the liver is pushed aside forming a relatively straight edge with an area of low density ( 1 - 2 EMI units) between the liver and diaphragm (Figs. 13, 14). It thus becomes possible not only to show the position of the abscess for aspiration but also to calculate the volume of the contained pus. Ascites and Colloid Carcinoma Infiltration of the Peritoneum. - The homogeneous crescentic inter-
position of ascitic fluid between the liver and diaphragm is clearly demonstrated oll the upper abdominal sections. The spleen is similarly displaced
Fig. 12a - Abscess in liver foUowing choledochojejenostomy Fig.12b - To show density of lesion (WH + 000). to relieve obstructuvejaundice.
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with the ascities being equally well recognised (Fig. 15) and even the pancreas may be more clearly shown because of the ascitic fluid. The diffuse infiltration of the peritoneal cavity by colloid carcinoma of the colon or ovary produces a somewhat similar appearance with displacement of the liver and spleen away from the diaphragm. However, in these cases the tissue attenuation values of
the displacing substance is higher than ascitic fluid which has a density of about +2 E units, whereas colloid carcinoma infiltrate is 1 0 - 1 5 E units (Fig. 16). Aetinomycosis of the Liver. - In a case of actinomycosis the actual extent of the lesion in the liver could not be demarcated but its extension through the abdominal wall and into the subcutaneous tissue
Fig. 13 - Needle in position in a subphrenic abscess.
Fig. 1 4 - Density of subphrenic abscess is +1 H unit (WH + 001).
Fig 15 - Ascitic fluid comes between liver and diaphragm and surrounds the spleen,
Fig. 16 - Infiltrating peritoneal colloid carcinoma resembles ascitic fluid but was shown to have a higher density.
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could be, A smaU, associated round lesion was shown to be at the right lung base and not actually in the liver, The Fatty Liver. - Diffuse fatty infiltration of the liver, as occurs with alcoholism, produces a pathognomonic appearance, The tissue density of the liver becomes extremely low with the portal and hepatic
veins showing as vascular structures of higher density. At present this is thought to be so characteristic that a liver biopsy becomes redundant (Fig, 17). Obstructive J a u n d i c e . - I n t r a h e p a t i c dilated bile ducts produce multiple rounded areas of lower density in the outer part of the liver and a large branching lower density configuration at the hilum.
Fig. 17 - Fatty liver has a characteristic appearance with the portal veins and IVC visible against the background of low density liver. The pancreas and kidney have higher densities than liver parenchyma.
Fig. 1 8 - Carcinoma head of pancreas (arrows) producing obstructive jaundice. Enlarged gallbladder and bile ducts visible.
Fig. 19 - Hypernephroma (arrows) impressing posterior Fig, 2 0 - Calcification in a thrombosed inferior vena cava aspect of the liver which produced an area of 'non-uptake' as (arrows) and collateral veins behind the diaphragm (arrow shown on the isotope scan. heads).
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Furthermore, the cause of the extrahepatic biliary liver tissue but may be suspected when there is obstruction may be shown as a mass in the head of extrahepatic jaundice and the gallbladder cannot be the pancreas (Fig. 18) or the head o f the pancreas demonstrated. may clearly be shown to be normal in size. It should, however, be borne in mind that if the mass has a SUMMARY AND CONCLUSIONS smooth margin, extends more cranially towards the hilum o f the liver and other sections show enlarged Computerised tomography produces an excellent lymph nodes, then the underlying pathology could image of the liver which can be enhanced with the well be a lymphoma. new 'filtered' programme. Normal liver density is Masses in Adjacent O r g a n s . - Mass lesions in the greater than other soft tissue densities, apart from right kidney or suprarenal m a y remain undetected by that of the spleen. An understanding of the use of the other investigations or produce the appearances of an window level and window width settings helps greatly intrahepatic space-occupying lesion. An isotope scan in distinguishing solid lesions from cysts or abscesses. may, for instance, show an area of non-uptake in the Lesions which can be demonstrated include posterior upper part of the right lobe of the liver due tumours, cysts, subphrenic abscess and fatty liver. to a mass lesion on the anterior aspect of the upper CT is particularly useful in showing the size, shape pole of the kidney (Fig. 19). Similarly, a turnout in and position o f space-occupying lesions. However, the right suprarenal m a y be demarcated by a line o f with solid space-occupying lesions histology is still lower density indicating that it lies outside the liver required for a definitive tissue diagnosis. and if due to a metastasis that it carl be excised as there is a line o f demarcation between it and the liver. Vascular L e s i o n s . - Calcification in the inferior ADDENDUM vena cava following thrombosis and the resulting collateral circulation can be clearly demonstrated 2 Hounsfield units = 1 EMI unit. Unfortunately, (Fig. 20). The enlarged azygos and hemiazygos veins EMI units are used on the CT 5005, and most other produce rounded shadows in the fat space between equipment uses Hounsfield units, including the EMI the diaphragm and the vertebral b o d y particularly at brain machine. the level of D l l / 1 2 . Collateral vessels around the kidney and in the peritoneal fat may also be noted. Acknowledgement. - Grateful acknowledgement is made Chronic congestive cardiac failure produces marked enlargement o f the inferior vena cava and to the Department of Health and Social Security, whose farsighted and generous help has assisted greatly in the dehepatic veins which on isotope scanning produce velopment of this project. The assistance and considerable areas o f diminished uptake and can be mistaken for help from Mr John Twydle, who has been managing the metastases. These dilated and enlarged vessels can be machine, is also gratefully acknowledged. clearly demonstrated on CT scanning. The Gallbladder. The visualisation of the normal REFERENCES gallbladder with and without contrast medium is Ambrose, J. (1973). Computerised transverse axial scanning almost invariable with computerised tomography. (tomography). (2) Clinical application. British Journal of Gallstones can be seen but this m e t h o d is far too Radiology, 46, 1023 1047. expensive and time-consuming to be used in this way. Hill, K. R. (1976). EMI total body scanner: technical aspects. However, in certain circumstances the size of the ,, British Journal o f Clinical Equipment, 1,207-214. gallbladder can be o f considerable importance Hounsfield, G. N. (1973). Computerised transverse axial scanning (tomography). (1) Description of system. especially in obstructive jaundice but may also be British Journal o f Radiology, 46, 1016-1022. helpful in suspected empyema of the gallbladder. Kreel, L. (1976). The EMI Whole Body Scannei: an interim Carcinoma of the gallbladder may be difficult to clinical evaluation of the prototype. British Journal of Clinical Equipment, 4,220-227. detect as the density o f the mass is similar to that of