Authorized licensed use limited to: Illinois Institute of Technology. Downloaded on January 30, 2010 at 11:33 from IEEE Xplore. Restrictions apply.
KAK: COMPUTERIZEDTOMOGRAPHY
1251
P8(k7)=[
p(s, y)dr= an N i nray path AB
NdX-ray source
B\
\
Detector Array Rotating instrument Plate
Fig. 4 . The figure depicts a fan-beam rota
tional type scanner.
p\\
Ninwidth o f one detectorT
A
Fig. 3. A parallel beam o f X-rays propagating through tissue.
Later in this section we will address thefactthat since, in practice, this assumption is never completely satisfied, artifacts are created in X-ray CT images. In this section, we wdiscuss three topics concerningX-ray l i CT: two scan configurations that have been introduced during the last three years, polychromaticity artifacts and the properties of noise in the reconstructed images. We have not discussed X-rayCTusingvideo recorded fluoroscopic data[ 31 -[ 51,[ 791. Ideally, a CTimage should represent a cross-sectional slab of constant thickness. We have not discussed here the variations in this thickness caused by the collimation geometry[191.A . Different Scan Configurations in X-Ray CAT There are two scan configurations that lead t o rapid data collection. These are i) fan-beam rotational type; and ii)fixed detector ring with rotating source type. 1) Fan-Beam Rotational Scanners: Here a fan-beam of Xas shown in rays is used to illuminate a multidetector array Fig. 4. Both the source and the detector array are mounted on ayoke which rotatescontinuouslyaround thepatient. Data collection time for such scanners range from 1 to 20 s. In t i time up to a 1000 projections maybe taken. If the hs projections are taken 'on the fly' there is a rotational smearing present in the data; however it is usually so small that its effects are not noticeable in the final image. Most such scanners use fan-beams with fan angles ranging from 3' to 45'. 0 The detector bank usuallyhas 500 to 700 detectors. Images are reconstructed on 256 X 256 or 5 12 X 5 12 matrices. Most such scannersuse xenon gas ionization detectors. Three such detectors are shown in Fig. 5. Each detector consists of a central collecting electrode with a high-voltage strip on each side. X-ray photons that enter a detector chamber cause ion-
L_ L
Ionization chrnber forxenonone detector
x-ray
photons
A~Thigh wltage surface alumlnm entrance windm a t grovld potential hlgh voltage
'(-vel
electrode collecting
electrode
Fig. 5. Xenon gas detectors for fan-beam rotational type Scam'wrs.
izations with high probability (which depends upon the length I of the detector and the pressure of the gas). The resulting current through the electrodes is a measureof the incident X-ray intensity. In the EM1 6000 scanner the collector plates are made of copper and the highvoltage strips of tantalum. In the same scanner the length I (shown in Fig. 5) is 8 cm, the voltage applied between the electrodes 170 V and the pressure of the gas 10 atm. The overall efficiency of this particular detector is around 60percent. The primary advantages of xenon gas detectors are that they can be packed ciosely and they are inexpensive. The entrance width T in Fig. 5 may be as small as 1 mm. This is in direct contrast t o the scintillator-photomultiplier detector (to be described later) which
suffers from the disadvantage that the smallest commercially available photomultiplier tube has a diameter of 12 mm. Yaffe et al.[ 1251 have discussed in detail the energy absorption efficiency,linearity of response and the sensitivity t o scattered and off-focus radiation for xenon gas detectors. Williams[ 1231 has discussed their u e in commercial CT syss
Authorized licensed use limited to: Illinois Institute of Technology. Downloaded on January 30, 2010 at 11:33 from IEEE Xplore. Restrictions apply.
1252
PROCEEDINGS OF THE IEEE, VOL. 67, NO. 9, SEPTEMBER 1979
(c) Fig. 6. Three examples of reconstructions obtained on a fan-beam rotational scanner. Number of projections: 504. Number of rays in each projection: 504. (Courtesy of EMI.) (a) Noncontrast CTscan at thelevel of thethalamus. Theanterior horns of the lateral ventricles, superior cerebellar cistern and slightly calcified pineal gland are seen. There is differentiation -of gray and white matter in the region of the head of thecaudate nucleus and the internal capsule. (b) High skull CTscandemonstrating corticalatrophy. There is differentiation of gray and white matter in the individual gyri. The falx cerebri i also seen. (c) CT scan of the abdomen revealing a right renal cyst. Vascular s structures demonstrated include the aorta, celiac axis, hepatic and splenic arteries, inferior vena cava and portal vein.
tems. In Fig. 6 we have shown three examples of reconstructions made on a fan-beam rotational scanner. 2 ) Fixed Detectors Rotating Source Scanners: In these scanners a large number of detectors are mounted on a fixed ring as shown in Fig. 7. Inside this ring is an X-ray tube that continuallyrotatesaroundthepatient. During this rotation the output of the detector integrators facing the tube is sampledevery few milliseconds. A l such samples foranyone l detector constitute what is known as a detector fan. Of course, as far as a reconstruction algorithm is concerned there is no difference between a detector fan and the more conventional source fan. Note that the detectors do not have to be packed closely (more on this toward the end of this section). This observation together with the fact that the detectors are spread all around on a ring dictates the use of scintillation detectors as opposed to ionization gas chambers. Most scintillation detectors currently in use are made of sodium iodide, bismuth germanate and cesium iodide crystals. (See[43] for a comparison of sodiumiodideandbismuth germanate.) Thecrystal of which a scintillation detector is madeserves two purposes. First it traps most of the X-ray photons which strike it, with a degree of efficiency which depends upon the photon energy and