Inr

J Rodmm

Oncology Btol. Pkyr..

1977. Volume 3. pp 45.51.

??Localization,

hrgamon

Press

Inhomogeneities

PHYSICS

Fnnted m the U S.A

and Treatment Planning for Particle Therapy

OF CT SCANNERS:

PRINCIPLES

AND PROBLEMS

H. E. JOHNS, J. BATTISTA, M. J. BRONSKILL, A. FENSTER and M. YAFFE Physics

Division,

Ontario

Cancer

Institute.

500 Sherbourne

R. BROOKS,

Street, Toronto,

Ontario,

Canada

M4S lK5

In the Iast three years, many research laboratories and commercial firms have been very actively interested in computer-ass&e-d tomography (CAT) and the very rapid expansion of the field makes whatever Is wrItten to&y out-of-date hefore it appears In print. In our laboratory we have been investIgatIngbask prIncIpks of whokbody CAT scanning. We are b&Ming a 256 xenon detector array in a fan geometry, havIug beforehand carried out extensive measurements on a lo-channel prototype array, operating at a pressure of 5 atm of xenon and obtalned detailed data on the saturation characteristii of this prototype. For beams from dkgnostic X-ray machines, saturation is dIfIkult to achieve. With about 1000 V across a 1.5mm gap in the chamber, about 95% of the ions can be coIkcted. Chambers with wider gaps are probably usable. IksignIng a chamber which Is linear over a wide range of exposure rates Is dUllcult; by attenuating the X-ray beam with Iayers of ahuninum, we have been abk to test the detector for linear&y. The absorption of radiation in one chamber introduces about 1.5% “cross-taIk” radiation into adjacent ones, which Is probably not a serious problem. Scatter of radmtkn from the whok fan into the detectors can be minimized by radkl coIIImatorsand reduced to the level where it Is not a probkm. The emission of 660ff-focus”X-rays, a more serious probkm, can be removed only in part with tdabk diaphragms. Improved design of X-ray targets may be required. One of our two experimental X-ray sources produces 30 pulses per set, 8.3 ms long. The puking, done In the primary of the generator, is very dIfIicult to stab&e. Sequential pukes change in sixe as dlerent parts of the rotating target are bombarded by the ekctron beam, which suggests that the emission from even a new target k depndent on Its angk. We are cornparIng thk source to a highly stabII&d DC X-ray source. Ekctrouk circuits that measure the currents In xenon ion chambers are needed to present them in usable form to the PDP-llT55 computer. One particular circuit is able to handle a large dynamic range, and measure to a precision of about 0.2%. !Several types of computers are ideally suited to image -etion. use of the convolution algorithm to construct an image from fan geometry causes artifacts that are cowkkrably dilferent from thooe produced in a ‘%ranshte-rotate” geometry. in theory, liquid xenon chambers should be about Bve times more sensitive than gaseous chambers; liquid xenon might be no more complkated to handle than high-pressure gas. CAT scanners may shortly become indkpensibk for radiation therapy pknnbtg, especkIly where high-LET radiation k used and corrections for inhomogeneities become very important. In principk, we now have sufficient information to cakukte precke kodose dktributions in patients. Computer-assisted tomography (CAT) scanners, Radiation scatter. INTRODUCTION

engineering aspects of the subject. The main trend today is to produce faster and faster scanners so that motions of the internal structures of the body do not influence the image. The brain scanner has already been so successful in diagnosing neurological disorders that it is now estimated that one such machine is required for every 500,000 people.* The value of whole body scanners has not yet been determined but extensive tests are now being carried out in many radiology departments. To date almost none of these machines has been

During the last three years there have been many exciting developments in CT scanners; first for the brain and now for all sections of the body. Almost every manufacturer of X-ray equipment is developing a version of the whole body scanner. In some cases, sales and promises have far outstripped the manufacturers’ ability to deliver a machine which will produce good images. In fact. development is so rapid that few of the technical details on the expected performance of a CT scanner are available to the buyer when the sale is made. There are many papers dealing with the clinical use of CT scanners,’ but very few detailed technical papers cover the physics and

placed primarily in radiotherapy centres-but this will certainly take place in the next year or two. Radio-

therapists

Acknowledgements-The authors wish to acknowledge the financial support of the National Cancer Institute of Canada

hope

(and sometimes

assume)

and the Medical Research Council of Canada. 4s

that

CT

46

Radiation

Oncology

0

Biology

0

Physics

scanners will image tumors in relation to healthy tissues, and thus make possible much more precise beam direction than has been possible in the past. It is also hoped that with these devices we will be able to study the change in size of tumors with time or dose, thus adding to our knowledge of the action of radiation or drugs on tumor cells in uiuo. Finally, with the whole body scanner, it will be possible to correct dosimetry for inhomogeneities on a realistic basis. This will be a useful addition to conventional radiotherapy and an absolute necessity when particles such as pions, protons or stripped nuclei are used. With these particles the position of energy deposit is shifted forward or backward when low or high density inhomogeneities occur between the source and the tumor. Without specific knowledge concerning these inhomogeneities gross errors in dosimetry will arise. Our group at the Ontario Cancer Institute is studying the technical problems associated with scanners. We have acquired two types of X-ray generatorsa pulsed source, and a constant-current stabilized source and are studying their relative merits. We are building a fan-type, high-pressure xenon detector system and are studying the problems inherent in such a system. We have acquired a PDP-llT55 computer to construct images of phantoms. By introducing controlled motion in parts of the phantom we will be able to study motion artefacts and see how they depend upon the particular algorithm used in the image construction. We intend to determine the best type of algorithm to use with a fan detector. We are not planning to build a scanner to be used on patients but will acquire a commercial unit when we know the important technical specifications which should be insisted upon to give good images in a realistic clinical condition.

GENERATIONS OF SCANNERS Figure 1 shows the basic principles of the three types of scanners which are available now or soon will be. Generation I illustrates the first version of the EM1 scanner. In it the source and detector. which are coupled together, translate and then rotate through one degree until 180 views have been obtained, requiring some 3-5 min. To increase the speed of such a device the number of detectors can be increased to 5 as illustrated in Generation Ia. After one traverse the system is rotated through 5” rather than lo to give a speed about 5 times as fast. Translate-rotate systems with up to 30 detectors are now available. To achieve still higher speeds, n detectors (up to 300) may be used as illustrated in Generation II. Now, translation is no longer necessary and rotatior ?lone is used. This device must now rotate through at least (7~+ (u) to obtain the necessary

iC

iI

__.

3

31 Ds

translate-then rotote

translate-then

in 1’steps

1~

to 77

Rel speed

!

5” steps -_5

rotate to

77

rotate

continuowy

v *a

or better 27

,L

2

Fig. 1. Diagram to illustrate three types of scanners: Generation I translates, then rotates; Generation Ia uses more detectors (in the diagram 5 are shown), and again the motion is translate, then rotate; Generation II uses very

many detectors, n, in a fan array and rotates only.

data, but usually, for the sake of symmetry, the device is rotated through 27r. This system will have a speed of about n/2 times that of Generation I. In scanning a body in which there is no motion, a Generation II scanner should give just as good an image as a Generation I scanner. Although Generation II scanners have been advertised for sale for over a year, only a few prototype models are now in use in clinical departments. The reasons for their slow development have not been publicized so no unequivocal remarks can be made. From experimental work on our system, we believe there may be several reasons for the apparent lack of performance of the commercial units of Generation II. A major problem is certainly the stabilization of the n detectors and electronic circuits which must maintain their calibration through a complete scan. In contrast, the detectors in a Generation I scanner can be calibrated after each traverse. Another problem is and has been the linearity of the detection system. With improper design of the xenon detector, which is usually used in Generation II scanners, serious problems can arise. The collection of all the ions in a xenon detector is difficult. To achieve full collection efficiency one requires plate separations of only l-2 mm and this, in turn, can introduce serious microphonic and alignment problems. The choice of the best algorithm for image construction from a fan geometry is not clear. At least two major methods are available. The first method to be used involves rearrangement (called rebinning) of the diverging rays into pseudo-parallel sets. This can only be done after a complete 27r scan: thus data must be stored in the computer before computation can begin. A more serious problem may arise from motions. since adjacent structures in the patient are

Physics of CT Scanners: Principles and Problems 0 H. E.

not “looked at” in consecutive time intervals, but at a later interval when rotation of 7~ has taken place. Thus scans of a phantom can give good images but scans of a patient yield images showing many motion artefacts. An alternative method of image construction for a Generation II scanner is a direct one from the fan beams. Such a method is not described in detail in the literature but preliminary findings in our laboratory suggest that it is nearly as fast as the rebinning method and should overcome motion artefacts. Further work is required. DESIGN OF XENON DETECTORS Generation I scanners use sodium iodide or calcium fluoride crystals coupled to photomultipliers as the detection system. When a fan-type scanner is contemplated certain potential problems arise when one attempts to use a conventional scintillation crystal detection system. To achieve adequate resolution the distance between adjacent detectors should be 3 mm or less. Packing crystal/photomultiplier systems in such a small space could be a problem. The use of a large ion chamber filled with xenon at high pressure with radial spaced plane electrodes as illustrated in Fig. 2(a) seems the logical choice. The high voltage electrodes can be made thick enough to prevent radiation scattered in one detector from affecting the next, and collimators in line with these high voltage electrodes can be added to reduce scattered radiation from the patient as illustrated in Fig. 2(b). Before designing such a multidetector system we built a number of prototype detectors to test their operating characteristics. We found, by measurement and by calculation, that with an X-ray source operating at 130 kVp, with 5 mm Al filter, plus absorption by a 25 cm patient, 90% of the incident photons interacted photoelectrically with a 20 cm path of xenon at 5 atm and so are absorbed. About 30% of this absorbed energy is re-emitted as fluorescent radiation and part of this is reabsorbed in the chamber. For a chamber of the dimensions described in Table 1 about 10% of the fluorescence is reabsorbed.

JOHNS et al.

47

The total energy absorption is thus about 65%. If higher pressure xenon is used then a chamber of smaller radial length will give the same photoelectric absorption but we then face the difficult task of collecting all the ions before recombination takes place (see later section). SCATTERED AND OFF-FOCUS RADIATION When a fan beam is used the detector responds to primary radiation as well as scattered radiation, as illustrated in Fig. 2(b). The larger the fan, the larger is the potential problem arising from scattered radiation. We have studied this problem by mounting two radial ion chambers close together and carefully shielding one with a strip of lead so that it “sees” no primary radiation but only scattered radiation while the adjacent chamber sees both primary and scattered radiation as seen in the insert of Fig. 3. The results of such an experiment are shown in Fig. 3. With a collimator 15 cm long, scattered radiation can be reduced to about 0.3% of the primary. In our early investigations of this problem,3 we believed that scattered radiation amounted to about 2%. Now we know that much of this was stray radiation arising from regions of the X-ray tube slightly remote from the focal spot as represented by the shaded region of Fig. 2(b). This can be partially removed by an aperture placed very close to the tube. The aperture should be made as small as possible consistent with not interfering with the beam near the extreme edges of the fan nor with the required beam height. In the X-ray tube which we have tested (Dunlee Durotron 300, 2 mm focal spot) off-focus radiation accounted for 2.5% of the primary even with good collimation. Other tubes may be worse or better. Of course, the radial design of the detectors and collimators tends to discriminate against off-focus Xrays since the chamber will be most efficient in detecting radiation from the focal spot. The off-focus radiation will certainly reduce the spatial resolution and contrast of the system but we have not yet determined to what extent. b)

electrodes colkctlng electrode

/

discrtmlnotes ogolnst scotiered and slbghtly ogoms, off-focus rodiotlon

Fig. 2. (a) Schematic representation of radial detector in a fan geometry. (b) Diagram illustrating scattered and off-focus radiation.

Radiation Oncology ??Biology 0 Physics

Fig. 3. Scattered

radiation

and off-focus radiation

SATURATION OF XENON ION CHAMBER When a high pressure xenon ion chamber is exposed to the X-rays from a diagnostic X-ray tube the ion density at the front of the chamber can be high enough to cause serious recombination problems. Figure 4 shows some of our measurements where

W=O6cm

H;‘sCm Coilectlng

I 0

1 c2

1 04 ‘Iv2

0

200

400

600

800

Fig. 4. Saturation

curves

for xenon

1000

i200

1 high pressure xenon detectors diagnostic X-ray tube. Collecting

of collimator

length.

charge is plotted as a function of collecting voltage. At 2 atm saturation is easily achieved but at 9 atm saturation is far from being attained even at 1800 V. The diagram shows that the curves cross and that at 800 V more charge is collected from an ion chamber filled with xenon at 4atm than in a chamber pres-

641 gap = 0.3 cm

as a function

potertiol

1 0.6

5

1 0.8

I

ikVi’ i400

1600

1800

(volts

exposed

to pulsed

X-rays

from a

Physics of CT Scanners:

X-ray

Principles

and Problems 0 H. E.

JOHNS

49

ef al.

conditions

I30 kVp + 7 300 mA 8.33 msec

mm AI pulse

Xe detector L = 20 cm W= 6mm q=l5cm Gop = 3 mm 5 otm Xe

01 Meosured

Fig. 5. Correction

IO

1.0

to be applied to response

cnorge

per

pulse

of a xenon detector

(nC)

to account

for the loss of charge due to

recombination. surized to 9 atm even though the latter absorbs 15% more of the incident radiation. An ion chamber with 1.5 mm gap at 5 atm is a good compromise. It has been reported at several meetings that radiation detectors should be accurate to 0.1% to avoid reconstruction artefacts. This means that even at 5 atm it is essential to correct the observed reading for loss due to recombination. By’plotting the inverse of the charge collected against the inverse of the collecting voltage squared (see insert of Fig. 4) a good estimate can be made of the saturation value and thus for each exposure rate a corrected charge can be obtained. Figure 5 shows such a correction curve for a typical detector. With 1000 V across a 3 mm gap, the correction amounts to close to 15% for high incident fluxes. By reducing the gap to 1.5 mm this correction

will be much

reduced.

DESIGN DETAILS ON XENON FAN DETECTOR Approximate design specifications for our fan detector are given in Table 1. Items (1)-(12) are self evident. Items (12)-(16) describe how the data can be processed using the rebinning method with this detector. If direct reconstruction were to be used many other alternatives are open. We plan to collect charge from a pulsed machine for 8.3 ms and process this charge for the next 16.6 ms. The total time for a 2~ scan will then be 400 x 25 ms = 10 sec. A slower or faster scan is certainly possible. X-RAY MACHINE There are at least four possible ways an X-ray machine may be used in a CT scanner. The machine may be operated (a) from a stabilized constant current supply; (b) as a pulsed machine using primary switching; (c) as a pulsed machine using tetrode

Table 1. Specification

of detectors

and scanner

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

Number of detectors: 239, arranged radially Spacing of detectors: 4.4 mm (centre to centre) Angle subtended by each detector: 0.18” Detector arc (front surface): 14Ocm Fan angle: 239 x 0.18“ = 43” Width of active area of detector = 3 mm Height of detector 15 mm; depth of detector 17.5 cm Pressure: 5 atm xenon High voltage electrodes made of 0.38 mm tantalum Front wall of detectors: 2-3 mm Al Collecting voltage: 1500 V across 1.5 mm gap Readings taken every 0.9” over 360” to give 400 readings (13) Central ray combined with 2 on each side to give 5

pseudo parallel rays (14) Number of readings per scan: 239 x 400 (15) Opposing pairs averaged to give 239 x 200 data values (16) Number of parallel sets 200; number (17) 130 kVp, 300 mA, 8.3 msec pulse.

in each set 239

switching in the secondary or (d) as a pulsed machine using a grid controlled X-ray tube. We are testing methods (a) and (b). Our preliminary data suggest that constant current machines are usually not stabilized to a level claimed by the manufacturer and to obtain a system with an X-ray flux stabilized to better than 0.1% is dif’hcult. The pulsed machine using primary pulsing also creates some problems. Figure 6 shows the output of a rotating anode tube as a function of time. The source was primary pulsed to give a pulse every 8.3 ms and the pulse pattern is shown in the oscilloscope trace of Fig. 6. Alternate pulses from opposite phases of the power line are not balanced. This effect is no doubt due to improper matching of the silicon controlled rectifiers used for switching and could be overcome. The cyclical variation in Fig. 6 must be due to one half of the rotating anode producing more

50

Radiation Oncology 0 Biology ??Physics

*Ill PUlWd

x-ray

3OUVZO

Fig. 6. X-ray yield as a function of time for a rotating anode tube excited from the 60 cycle single phase power line. Successive pulses are shown in the oscilloscope trace. Fig.

than the other half. Visual inspection of this anode showed no obvious cause for this effect, and the severity of the effect was tube dependent. Clearly the use of such a source would require very careful monitoring. Its importance in image construction is being assessed. X-rays

COMPTON-SCATTER

representation Scanner.

of

Compton-Scatter

density of slice 17 of the Rando phantom measured by removing this slice from the phantom and measuring the amount of Compton scatter as the primary pencil was scanned over the area of the slice. The lung structures and part of the vertebrae are visible. The Rando phantom was then reassembled and a scan obtained using a single detector positioned as in Fig. 7, making no corrections for attenuation and scattered radiation. The result is shown in Fig. 8(b). Note the pattern is badly distorted with gradations in density toward the position of the detector. By making a number of corrections for attenuation and multiple scattefi the image in Fig. 8(c) was obtained. This is a rather good representation of Fig. 8(a). The picture certainly does not show the details of a CT scan but it has the important advantage of giving true electron densities which are required for inhomogeneity corrections in treatment planning when 1-25 MeV radiations are used.

SCANNER

An alternative method for producing images of sections of the body is to determine the amount of radiation scattered by the Compton process from a small volume at P (Fig. 7), irradiated by a pencil of radiation from a @Co source along the line QQ’. The radiation observed by the collimated detector will be a function of the electron density at P, the attenuation of the primary by the path QP and the attenuation of the scattered radiation by the length 1 of patient. Multiply-scattered radiation can also reach the detector. Figure 8 shows a reconstructed image of a section of a Rando phantom through the chest region. Figure 8(a) is a representation of the true electron

(4

7. Schematic

(b)

(cl

Fig. 8. (a) Electron density distribution of slice 17 of the Rando phantom. (b) Electron density of the same slice measured

with an assembled Rando phantom, when no corrections are made for attenuation multiple scatter. (c) Same as (b) after all corrections are made.

and

Physics of CT Scanners:

Principles

SUMMARY At this point, no one can specify how a CT scanner should be built to give best results. There are many possible designs which should be compared with each other and there are many sources of artefacts which should be investigated. Thus far we have investigated xenon detectors and have shown how some of their problems can be overcome. Our work has pointed out to us stimulating problems that

and Problems 0 H. E.

JOHNS et al.

51

remain to be solved in the areas of X-ray sources, electronic circuits and algorithm development for fantype CT scanners. The solution of these problems and the presentation of findings in the open literature will be of great benefit in defining CT scanner specifications and performance and consequently in helping the radiologist choose the most appropriate scanner for his work.

REFERENCES I. Am. J. Roentgenol. July 1976. 2. Battista, J.J., Santon, L.W., Bronskill, M.J.: Compton scatter imaging of transverse sections: corrections for multiple scatter and attenuations. Phys. Med. Biol. in press.

H.E.: New methods of imaging in diagnostic radiology. Br. 1. Radiol. 49: 745-764, 1976. 4. Report of the Task Force on the Placement of Instruments for Computerized Axial Tomography, Ontario Ministry of Health, 26Feb. 1976. 3. Johns.