ProcIEEE_Kak_computerized_tomography_with_xray_emission_ultr

PROCEEDINGS OF THE IEEE,

VOL. 67, NO. 9, SEPTEMBER 1979

1245

Computerized Tomography with X-Ray, Emission, and UltrasoundSourcesAVINASH c. KAK, MEMBER, IEEE

A b m t - T h i s paper reviews the major developments that have taken images with this resolution the more recent scanners use from phce during the hst three y e m in imaging with computed tomography 500 to 1000 projections, and from 500 to 1000 rays in each (cr)wiug x-my, emi on, md u d l sourws. space limitations projection. The reconstruction time has also gone down conhave resulted m some selection of topics by the authar. There are four major sections dealing with algorithms, X-ray CT, emission CT, and siderably over the years. By exploiting parallelism inherent in ultrasound CT. S e most of the cmreatiy USXI are of the reconstruction process and array processing the per slice i n algxithms wtered*kprojdon tp, we have coacentnted ( 1 tfrese m the reconstruction times have beenbroughtdown to less than ye 1 section on algorithm (with emphasison their implementation a p cs. 30 s. s et) I n X - m y C T m i m p o r C P n t q~ n i s e d d u r i n g t h e k s t f e w y e~ b r s A great deal morecould be said abouttheperformance concernedthepaune6er~byaCTsanner,gnenthetrctthat

~ X - r r y s~ i n C T s a n w r s r r e~ d u o m a t i c m d t h e f r c t t h . t t i characteristics andother capabilities of scanners on the market. si sue attenuation coefficients are energy dependent. A m e to this However, the a m of this paperis not totabulate such informar yn questionarereviewedmthe~onX-my~wherewehaverlso tion (which after all is readily available from the manufacturdiscussed the a cIp&d by the pdychr~maticityd the X-my ers) but to review the recent advances in the basic science and photons.Methods f a the removal of these artifacts have .Is0 been r ve e . InemissionCTtheb&stdevelopmentofthehtthree engineering of computerized tomography. eiwd so is m alth~& sl#~e m - Although the results far have been particularly spectacular~ yerr~ the W t m t in p i t r o n -hy, have dictated an essentially mlmductay treatment rad not rll with X-ray transmission,thetechnique of computerizedtorspeCg of the h g photoa and positron tomognphy have been sur- mography has also been successfully applied in nuclear medik veyed. we have reviewed recent developments in ultrasound cine imaging. The result of X-ray CT is a two dimensional map CT. W have pointed out that becam of the sensitivity of this tech- of the linear attenuation coefficient values (at some effective e nique to refmcticm, it ie currently limited to soft tissue structures, with ultmsonic detection of tumors m the fern& breast a significant photon energy). WithCT in nuclear medicine, on the other application. hand, the image yields information about a location and concentration of aphotonorpositronemittingradioisotope, I. INTRODUCTION which is administered to the patient either by inhalation or OUNSFIELD'S invention of the computed tomography

by injection. For this reason such scanners in nuclear medi(CT)scanner, revealed tothe public in 1972, was a cine arecalled emission CT (ECT) scanners. major breakthrough in biomedical imaging[69],[70]. The clinical usefulness ofX-ray and emissionCT systems To be sure, even before this invention there were researchers has already been established. X-ray derives CT its clinical who were concerned with the problem of constructing cross- significance from the fact that it shows the anatomical morsectional images of an object from its projections. Theearliest phology of any desired cross section of the human body withmathematical work on this subject is by Radon[ 101I. More out superposition, with great sensitivity (around 1 percent)2 recently, some of the fmt investigators to examine this p r o b and with good resolution. The clinical usefulness of emission lemtheoretically and/or experimentally (and often indepenCT lies in the fact that after a gamma-ray emitting isotope is dently) include (in roughly a chronological order): Bracewell administered to a patient, its distributionin the body gives[lll,Oldendorf[961,Cormack[401,[411,KuhlandEdwards physicians information regarding the functional state of[841, DeRosier and Klug[451, Tretiak e t u1.[1161, Rowley various organs. Andby examining the temporal dependence[ 1051, Berry and Gibbs[ 91, Ramachandran and Lakshmina- of thisdistribution,dynamicstudies may be performed on rayanan[102], Bender et al.[8], Hermanand Rowland[641, various organs. and Bates and Peters[ 71. Just on the horizon there is yet another tomographic imagDuring the last few years great improvements have been ing mode currently undergoing clinical t i l . This is the techras madein X-ray tomography, with the result that the patient nique of ultrasonic computed tomography (UCT). The basic scan time has been reduced to less than 10 s from over 4 min. On some of the latest scanners thehigh contrast spatialresolujs recon'Thisdoesnotimplythatlinearattenuationcoefficient with accuracy. Because of hardening beam and tion is beginning to approach10line pairs per centimeter structed 1-percent (discussed here in Section 111-B) the of andthelowcontrast(1-percentcontrast) spatialresolution other effects attenuation coefficient (at the effective numerical valuesthe the computed energyof is now around 3 to 5 line pairs per centimeter, the resolution CT scanner) may be distorted by as much as 5 percent or more[83]. criterion in both cases being visual detectability.' To present Insome cases thiserror might be much greater. In adramaticex-

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Manuscript received December 18, 1978;revised March 15,1979. The author is with the School of Electrical Engineering, Purdue University, West Lafayette, IN 47907. 'The low contrast resolution dependa upon the signal-to-noise ratio in the reconstructed image and, therefore, is a function of radiation dosage.

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