110
JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION
MARCH, 1976
Principles of Computerized Tomography N. HASSANI, M.D.,
Radiology Department, Queens Hospital Center, Affiliate of the Long Island Jewish-Hillside Medical Center, Jamaica, New York and R. KHOMEINI, M.D., Queens, New York R. BARD, M.D., New York, New York
THE COMPUTERIZED axial tomographic unit or CAT scanner is the latest diagnostic tool available to the neuroradiologist, neurologist, neurosurgeon, and, to a limited degree, the ophthalmologist. The CAT scanner is a computer transverse axial scanning system using a new technique of exceptional accuracy. The noninvasive nature of this method and the information it provides have opened new horizons in the diagnosis of many lesions of the brain and orbit. This is achieved by producing anatomic cross section images of the head. The availability of the CAT scanner system has reduced the use of other invasive diagnostic techniques. Ventriculography, radionuclide brain imaging, and cerebral angiography are performed less frequently. Pneumoencephalography has been markedly reduced in institutions with CAT scanners. This single examination avoids patient discomfort, eliminates residual morbidity, and does not require the injection of air, radionuclides, or radio-opaque contrast agents. The system generally enables the operator to carry out a complete examination of the brain and orbits in less than 20 minutes. The radiation dosage is similar to that of a routine skull series. The examination may be performed on an outpatient basis, on a comatose patient, or under general anesthesia. COMPARISON WITH CONVENTIONAL RADIOGRAPHIC TECHNIQUE
Research has shown that in routine radiographic methods, a high proportion of the
information content on tissue structure present when a beam of x-ray photons has passed through the body is not appreciated in a useful form on a photographic film. The loss of information is due to two factors. There is difficulty in distinguishing on a medium of comparatively low sensitivity subtle differences between tissues of similar radiographic densities. Additionally, confusion results from superimposition of data existing in a three dimensional form on a two dimensional film display. 1 When conventional radiographic techniques are applied to the brain, the problem is compounded by the high x-ray absorption of the bones of the cranial vault. Differentiation between tissues with small density differences requires the use of extremely sensitive detectors. Loss of data from superimposition of structures necessitates the use of tomographic techniques. By this means, the three dimensional object may be accurately presented as a series of thin, two dimensional sections which collectively represent the original object. The design of the CAT scanner combines tomographic scanning and sensitive detection systems with advanced data processing displays to present the information in useful clinical form.2 BACKGROUND
The CAT scanning technique and equipment are the result of research initiated in 1969 by G. N. Hounsfield and the Central Research Laboratories of EMI, Ltd. of England. The equipment was used at Atkinson-
Vol. 68, No
.
Computerized Tomography
Morley's Hospital for 18 months before initial clinical results were reported by the neuroradiologist James Ambrose in 1972. Since then many centers have obtained CAT scanners. PRINCIPLES OF CAT SCANNING
The basic system includes: a) a scanning unit, b) an x-ray control system, c) a computer and magnetic disc unit, d) a display unit e) a line printer and f) a teletype. Scanning Unit. The scanning system has an adjustable couch, water box, x-ray tube and scan and reference detectors. The patient is positioned on a hydraulic adjustable table with the head placed in a rubber bag within the water box. The water provides a region of known absorption with symmetric geometry, acts as a reference medium for the x-ray absorption measurements, and reduces the range of absorption values which the system must analyze. The x-ray tube has a small focal spot and the width of the beam may be varied. Opposite the tube (instead of x-ray film) are two detectors mounted on a common frame. Two contiguous slices of the head are received by the sodium iodide detectors which convert the light energy they produce into amplified electrical pulses by a photomultiplier tube. During scanning, a narrow x-ray beam passes through the patient's head and one half of the emergent photons are received by each detector. The frame on which the x-ray tube and sensors are located traverses linearly across the patient's head. During this movement each detector records 240 transmission readings of x-ray intensity which are stored in the computer. The frame then rotates one degree and the linear transverse is repeated. This series of traverses is repeated for 180 degrees taking five minutes. Each sensor processes 43,200 (240 x 180) readings fed to the computer. A third detector is used in a reference mode to measure the intensity of the primary x-ray beam. This enables the computer to compensate for any minor variations in the emitted x-ray intensity during the scanning procedure. X-ray control unit. The control unit is lo-
111
cated in the control room remote from the scanning unit. It contains the necessary controls and interlocks for operating the scanning unit and the controls for selection of the x-ray tube parameters. The x-ray tube can be set at 100 kv-40 mA, 120 kv-33 mA, or 140 kv-28 mA. It is usually operated at 120 kv-33mA, Computer cabinet. This comprises a minicomputer, a magnetic disc unit, and their associated power supplies. The 43,200 readings of the X-ray intensity from each detector are fed continuously to the minicomputer during the scanning procedure. From these data, the computer rapidly calculates an absorption value for each point in the scanning plane for a total of 25,600 absorption values. These values are expressed as absorption coefficients relative to the absorption value of water which has been set at zero. Air has been designated as -500 and dense bone as +500 units. In this scale with water as the reference standard of zero, fat is -50, circulating blood is +6, white matter +11-16, gray matter + 18-23, congealed blood +28-38, and calcification and bone ranging from +40 up to +500. These absorption values are used to build up a picture of each slice in the form of a matrix of 25,600 picture points. Each picture point indicates the absorption value for a volume of tissue 1.5 x 1.5 mm. x the slice thickness selected corresponding to the point in the section under examination. The processed information is stored on the system's magnetic disc which can hold the results from 40 slices. The magnetic tape unit provides a long term storage of scan information. This information is stored in digital form so that it may be used to reproduce pictures or print-outs by the computer. Also the data may be used for statistical purposes, research, or input into a central data bank of patient records. All or selected pictures from the system disc in the CAT scanner or from the compact flexible "floppy" discs in the diagnostic display console may be transferred to magnetic tape. Conversely, pictures may be selected from the magnetic tape for transfer to either form of disc store. The diagnostic display console is a mobile
112
JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION
unit with a 12 inch screen television monitor and compact floppy discs which provide a completely self-contained console for viewing, photographing, and storing the results of the patient scan. The viewing unit. This unit displays the results on a cathode ray tube. It includes a polaroid camera and associated exposure controls, together with the picture selection control and window width and window level controls. The window width switch selects the range of absorption values that determine black and white on the display. The window level control enables the center of the range selected by the window width switch to be set at any desired point in the system's scale between -500 and +500. Line printer. This printer produces a permanent record of the results of the scan in numerical form allowing quantitative assessment of the x-ray absorption values in different slices. It may also be used to produce a listing of information stored on discs or tapes. Teletype. The teletype machine records the patient's identification number, hospital name, date of scanning, and scan level on each polaroid picture and print-out paper. Positioning of the patient. The head is normally positioned in the water box with the orbito-metal line parallel to the front face of the scanner. This position may be varied for scans of particular areas. For examination of the posterior fossa, the patient's chin may be tilted downwards. For examination of the orbits, the chin is tilted upwards. The routine scan angle is at 25 degrees to Reid's base line. The level of the cut required is now selected and scanning started. Each contiguous pair of slices is scanned in 4 minutes and 45 seconds plus or minus 15 seconds. Thirty seconds later the two processed pictures are available for viewing. CONTRAST ENHANCEMENT TECHNIQUE IN CAT SCANNING
Intravenous administration of water solu-
MARCH, 1976
ble contrast of iodinated compounds was first performed by James Ambrose in his early experiences to enhance visualization of brain tumors. Since that time several investigators have found this method as a valuable aid in increasing the accuracy of scanning. This procedure is most useful when the pre-contrast scan is compared with the post-contrast scan. The amount of contrast material used
varies.3 Indications for contrast enhancement are: a) previously treated brain tumor, b) abnormal brain scan, c) possible cerebellar, brain stem or fourth ventricle tumors, d) possible metastatic disease, e) question of vascular malformation or aneurysm, and f) patients with focal neurological signs or siezures. The tumor blush following enhancement of primary or metastatic brain tumors has been attributed to passage of iodinated media through the basement membrane of the capillary bed into the abnormal tissue. This may also be due to the circulating blood pool of contrast.4 CONCLUSION
CAT scanning now allows the physician to view the brain cross-sectionally as the pathologist does in slicing the gross brain.
The information displayed shows the shape of the lesion and the internal tissue characteristics. The effect of the lesion on regional structures may be evaluated. LITERATURE CITED
1. HOUSNFIELD, G. N. (3mputerized Transverse Axial Scanning (Tomography): I. Description of system. Brit. J. Radiol., 46:1016-1022, 1973. 2. AMBROSE, J. Computerized Transverse Axial Scanning (Tomography): II. Clinical Application. Brit. J. Radiol., 46:1023-1047, 1973. 3. NEW, P. F. J. and W. R. SCOTT, J. A. SCHNUR, et al. Computerized Axial Tomography with the EMI Scanner. Radiology, 110:109-123, 1974. 4. GADO, M. H. and M. E. PHELPS and R. E. COLEMAN. An Extravascular Component of Contrast Enhancement in Cranial Computed Tomography. Radiology, 117:595-597, 1975.
ATTEND THE 81ST CONVENTION IN NASHVILLE, AUGUST 8-12, 1976
JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION
MARCH, 1976
Principles of Computerized Tomography N. HASSANI, M.D.,
Radiology Department, Queens Hospital Center, Affiliate of the Long Island Jewish-Hillside Medical Center, Jamaica, New York and R. KHOMEINI, M.D., Queens, New York R. BARD, M.D., New York, New York
THE COMPUTERIZED axial tomographic unit or CAT scanner is the latest diagnostic tool available to the neuroradiologist, neurologist, neurosurgeon, and, to a limited degree, the ophthalmologist. The CAT scanner is a computer transverse axial scanning system using a new technique of exceptional accuracy. The noninvasive nature of this method and the information it provides have opened new horizons in the diagnosis of many lesions of the brain and orbit. This is achieved by producing anatomic cross section images of the head. The availability of the CAT scanner system has reduced the use of other invasive diagnostic techniques. Ventriculography, radionuclide brain imaging, and cerebral angiography are performed less frequently. Pneumoencephalography has been markedly reduced in institutions with CAT scanners. This single examination avoids patient discomfort, eliminates residual morbidity, and does not require the injection of air, radionuclides, or radio-opaque contrast agents. The system generally enables the operator to carry out a complete examination of the brain and orbits in less than 20 minutes. The radiation dosage is similar to that of a routine skull series. The examination may be performed on an outpatient basis, on a comatose patient, or under general anesthesia. COMPARISON WITH CONVENTIONAL RADIOGRAPHIC TECHNIQUE
Research has shown that in routine radiographic methods, a high proportion of the
information content on tissue structure present when a beam of x-ray photons has passed through the body is not appreciated in a useful form on a photographic film. The loss of information is due to two factors. There is difficulty in distinguishing on a medium of comparatively low sensitivity subtle differences between tissues of similar radiographic densities. Additionally, confusion results from superimposition of data existing in a three dimensional form on a two dimensional film display. 1 When conventional radiographic techniques are applied to the brain, the problem is compounded by the high x-ray absorption of the bones of the cranial vault. Differentiation between tissues with small density differences requires the use of extremely sensitive detectors. Loss of data from superimposition of structures necessitates the use of tomographic techniques. By this means, the three dimensional object may be accurately presented as a series of thin, two dimensional sections which collectively represent the original object. The design of the CAT scanner combines tomographic scanning and sensitive detection systems with advanced data processing displays to present the information in useful clinical form.2 BACKGROUND
The CAT scanning technique and equipment are the result of research initiated in 1969 by G. N. Hounsfield and the Central Research Laboratories of EMI, Ltd. of England. The equipment was used at Atkinson-
Vol. 68, No
.
Computerized Tomography
Morley's Hospital for 18 months before initial clinical results were reported by the neuroradiologist James Ambrose in 1972. Since then many centers have obtained CAT scanners. PRINCIPLES OF CAT SCANNING
The basic system includes: a) a scanning unit, b) an x-ray control system, c) a computer and magnetic disc unit, d) a display unit e) a line printer and f) a teletype. Scanning Unit. The scanning system has an adjustable couch, water box, x-ray tube and scan and reference detectors. The patient is positioned on a hydraulic adjustable table with the head placed in a rubber bag within the water box. The water provides a region of known absorption with symmetric geometry, acts as a reference medium for the x-ray absorption measurements, and reduces the range of absorption values which the system must analyze. The x-ray tube has a small focal spot and the width of the beam may be varied. Opposite the tube (instead of x-ray film) are two detectors mounted on a common frame. Two contiguous slices of the head are received by the sodium iodide detectors which convert the light energy they produce into amplified electrical pulses by a photomultiplier tube. During scanning, a narrow x-ray beam passes through the patient's head and one half of the emergent photons are received by each detector. The frame on which the x-ray tube and sensors are located traverses linearly across the patient's head. During this movement each detector records 240 transmission readings of x-ray intensity which are stored in the computer. The frame then rotates one degree and the linear transverse is repeated. This series of traverses is repeated for 180 degrees taking five minutes. Each sensor processes 43,200 (240 x 180) readings fed to the computer. A third detector is used in a reference mode to measure the intensity of the primary x-ray beam. This enables the computer to compensate for any minor variations in the emitted x-ray intensity during the scanning procedure. X-ray control unit. The control unit is lo-
111
cated in the control room remote from the scanning unit. It contains the necessary controls and interlocks for operating the scanning unit and the controls for selection of the x-ray tube parameters. The x-ray tube can be set at 100 kv-40 mA, 120 kv-33 mA, or 140 kv-28 mA. It is usually operated at 120 kv-33mA, Computer cabinet. This comprises a minicomputer, a magnetic disc unit, and their associated power supplies. The 43,200 readings of the X-ray intensity from each detector are fed continuously to the minicomputer during the scanning procedure. From these data, the computer rapidly calculates an absorption value for each point in the scanning plane for a total of 25,600 absorption values. These values are expressed as absorption coefficients relative to the absorption value of water which has been set at zero. Air has been designated as -500 and dense bone as +500 units. In this scale with water as the reference standard of zero, fat is -50, circulating blood is +6, white matter +11-16, gray matter + 18-23, congealed blood +28-38, and calcification and bone ranging from +40 up to +500. These absorption values are used to build up a picture of each slice in the form of a matrix of 25,600 picture points. Each picture point indicates the absorption value for a volume of tissue 1.5 x 1.5 mm. x the slice thickness selected corresponding to the point in the section under examination. The processed information is stored on the system's magnetic disc which can hold the results from 40 slices. The magnetic tape unit provides a long term storage of scan information. This information is stored in digital form so that it may be used to reproduce pictures or print-outs by the computer. Also the data may be used for statistical purposes, research, or input into a central data bank of patient records. All or selected pictures from the system disc in the CAT scanner or from the compact flexible "floppy" discs in the diagnostic display console may be transferred to magnetic tape. Conversely, pictures may be selected from the magnetic tape for transfer to either form of disc store. The diagnostic display console is a mobile
112
JOURNAL OF THE NATIONAL MEDICAL ASSOCIATION
unit with a 12 inch screen television monitor and compact floppy discs which provide a completely self-contained console for viewing, photographing, and storing the results of the patient scan. The viewing unit. This unit displays the results on a cathode ray tube. It includes a polaroid camera and associated exposure controls, together with the picture selection control and window width and window level controls. The window width switch selects the range of absorption values that determine black and white on the display. The window level control enables the center of the range selected by the window width switch to be set at any desired point in the system's scale between -500 and +500. Line printer. This printer produces a permanent record of the results of the scan in numerical form allowing quantitative assessment of the x-ray absorption values in different slices. It may also be used to produce a listing of information stored on discs or tapes. Teletype. The teletype machine records the patient's identification number, hospital name, date of scanning, and scan level on each polaroid picture and print-out paper. Positioning of the patient. The head is normally positioned in the water box with the orbito-metal line parallel to the front face of the scanner. This position may be varied for scans of particular areas. For examination of the posterior fossa, the patient's chin may be tilted downwards. For examination of the orbits, the chin is tilted upwards. The routine scan angle is at 25 degrees to Reid's base line. The level of the cut required is now selected and scanning started. Each contiguous pair of slices is scanned in 4 minutes and 45 seconds plus or minus 15 seconds. Thirty seconds later the two processed pictures are available for viewing. CONTRAST ENHANCEMENT TECHNIQUE IN CAT SCANNING
Intravenous administration of water solu-
MARCH, 1976
ble contrast of iodinated compounds was first performed by James Ambrose in his early experiences to enhance visualization of brain tumors. Since that time several investigators have found this method as a valuable aid in increasing the accuracy of scanning. This procedure is most useful when the pre-contrast scan is compared with the post-contrast scan. The amount of contrast material used
varies.3 Indications for contrast enhancement are: a) previously treated brain tumor, b) abnormal brain scan, c) possible cerebellar, brain stem or fourth ventricle tumors, d) possible metastatic disease, e) question of vascular malformation or aneurysm, and f) patients with focal neurological signs or siezures. The tumor blush following enhancement of primary or metastatic brain tumors has been attributed to passage of iodinated media through the basement membrane of the capillary bed into the abnormal tissue. This may also be due to the circulating blood pool of contrast.4 CONCLUSION
CAT scanning now allows the physician to view the brain cross-sectionally as the pathologist does in slicing the gross brain.
The information displayed shows the shape of the lesion and the internal tissue characteristics. The effect of the lesion on regional structures may be evaluated. LITERATURE CITED
1. HOUSNFIELD, G. N. (3mputerized Transverse Axial Scanning (Tomography): I. Description of system. Brit. J. Radiol., 46:1016-1022, 1973. 2. AMBROSE, J. Computerized Transverse Axial Scanning (Tomography): II. Clinical Application. Brit. J. Radiol., 46:1023-1047, 1973. 3. NEW, P. F. J. and W. R. SCOTT, J. A. SCHNUR, et al. Computerized Axial Tomography with the EMI Scanner. Radiology, 110:109-123, 1974. 4. GADO, M. H. and M. E. PHELPS and R. E. COLEMAN. An Extravascular Component of Contrast Enhancement in Cranial Computed Tomography. Radiology, 117:595-597, 1975.
ATTEND THE 81ST CONVENTION IN NASHVILLE, AUGUST 8-12, 1976
