INTRODUCTION TO X-RAY
X-ray 1895년 11월 5일 : Wilhelm Conrad Roentgen(1845-1923) The 1st Nobel Prize in Physics 1901 : "in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him" 1895, Marconi : Hertz의실험 ( 인접한두 coil 사이에 electrical force가건너가는현상 ) 을토대로커다란안테나를사용함으로써코일간의인접거리를수 km까지늘려 radio transmission 의개념을확립 1895 Roentgen : cathode ray에빠져있었음.
Cathode Ray Tube 1860년대말독일의 Plucker와 Hittorf가 rarified gas 에서의전기전도도에대한연구로부터시작되었음. 방전관의 cathode 반대쪽유리벽에서 fluorescent light를관찰, cathode 에서나와직진함을확인. 영국의 Crookes에의해이cathode ray 가자장에의해휘어짐을확인, 이것은 Faraday의법칙에의해 negatively charged particle임을확인. 그는이것이음전하를띤분자 (molecule) 로생각했다. ( 그당시는누구도전자의존재를모르고있었다. 원자가가장작은입자로생각 ) 독일의학자들은빛과같은 EM wave로생각, 영국의학자들은미립자로생각 결과적으로이것을규명하기위해수많은과학자들이실험실에서모든장비를동원하여음극선관을분석하기시작하였다.
Cathode Ray Tube 1890년미국의 Phildelphia의 Goodspeed는음극선관을가지고실험중에사진원판이쓸수없도록뿌옇게된것을발견하고그냥버렸다.( 나중에이것이보물임을깨달음 ) Crookes도수많은사진원판을쓸모없이뿌옇게되어버렸다고제작자에게돌려보냈다. 빛에대한 EM theory를실험적으로입증한 Hertz도음극선관에관심을갖게되어그의조수인 Lenard와함께실험적으로음극선이매우얇은금이나알루미늄 foil을관통하는것을보고이것이particle일수는없다고단정. Hertz는 1894년악성종양으로죽음으로써더많은실험을수행하지못함. Hertz 사후에그의조수Lenard는다음의 2가지사실을확인. (1) ray의 absorption은방전관의희박정도에따른다. (2) 8cm 떨어진형광판은빛을내지못했다.
Wilhelm Conrad Roentgen Roentgen 은 Wurzburg 의새롭게선출된학장이었다. 그는이전에도물리학자로서명성을날리고있었다. 1895 년후반기그는더욱음극선관에많은시간을투자하고있었다. 그는 Hittorf, Crookes, Lenard 의진공관을모아서 Hertz 와 Lenard 의실험을반복하였다. 1895 년 11 월 8 일, 그의음극선관에전류를흘리자음극선만이보일수있는어두운실험실에서한 bench 로부터약한빛이나오는것을발견하였다. 그빛은음극선관으로부터약 1m 정도떨어져있었고, 그는진공관의전류를끊고성냥을켜서어디서빛이나오는지살펴보았다. 그것은작은 barium platinum cyanide screen 이었다. 그는외부에서들어오는빛이없음을다시한번확인한뒤음극선관에전류를흘렸다. 그러자또다시초록빛깔의약한구름같은빛이 screen 에서계속움직이고있었다. 이작은신기루는어찌했든지음극선관으로부터나오는것이었다. 그러나음극선관은멀리떨어져있지않은가?
Wilhelm Conrad Roentgen Roentgen은 screen을점점멀리떨어뜨려보았으나빛은계속나오고있었다. 그는이수수께끼로부터자신을떼어놓을수없었다. 그는실험실과같은빌딩에서살았는데그의부인이저녁준비가되었다고해서저녁을먹고는곧바로실험실로되돌아왔다. 질문은꼬리를물었고며칠후그의친구에게 " 나는매우재미있는것을발견했는데, 내관찰이맞는지틀리는지도무지모르겠다 " 라고말했다. 이 barium platinum cyanide 판을음극선관앞에서들고있으면그사이에천페이지의책이나나무조각, 고무판, 이나유리등을놓아도여전히빛이나왔다. 이모든것이투명했고단지납이들어있는유리의경우투명도가떨어졌다. 그는이사이에손을넣으면선명한뼈의그림자가좀더흐린손바닥그림자내에서볼수있음을확인했다.
X-ray 물, 탄소, 여러가지용액, 구리, 은, 금, 납, 백금등도아주두껍지만않으면빛을통과시켰다. 이실험으로투명도는물질의 density 와관련있음을알게되었다. 그는사진원판이음극선관에민감했던것을알아차리고수학에서미지수를나타내는 X 로그 ray 를이름지을수밖에없었다. 그러면이 X 선은무엇이음극선과어떻게다른가? 이것이빛과같다면굴절시킬수있을것이나프리즘이나딱딱한고무, 그리고알루미늄으로도굴절되지않았다. 그는물질들을가루로만들어빛을여러방향으로반사되도록해보았으나 X-ray 는반사되지않았다. 공기는음극선보다작게 X- ray 를흡수했다. 더욱이 X-ray 는자석에의해휘어지지않았다. 이것으로음극선과는다른것임을확인하였다. 수주간에걸친열띤실험의결과, "X-ray 는음극선과같지않으나, 음극선으로부터생성된다 " 라는결론을얻었다. X-ray 와가시광선은어떤관계가있지만완전히다르게활동한다.
The Beginning with X-ray 그가이전에들어보지못한새롭고중요한것을발견했음을알고 Wurzburg Physical-Medical Society에그의논문 "A New Kind of Rays : A Preliminary Communication" 을제출하였다. 편집자는막그해의마지막 issue를마감하였으나내용의중요성을알아차리고 1895년마지막 Proceedings의마지막 9 page에이논문을실었다. Rontgen의 1895년논문에대한첫영어번역은 1896년 1월 23일 Nature지에실렸는데, 이잡지는그후 "The Rontgen's Rays" 라는특별주제의 section을만들어전세계의과학자들로부터엄청난양의실험결과를모아계속발간하였는데, 과히광란적인과학적인열정으로 Rontgen의첫논문이후 1 년안에 1,000개이상의논문이발간되었다. 이들중가장중요한것은 Rontgen 자신에의해계속쓰여진것으로서그는과학史家들에의해당대의최고실험과학자로인정되었다.
The Beginning with X-ray 의학분야의응용은매우빠르게이루어져진단에있어서 X- ray 의사용은곳 routine 화되었다. 미국에있어서 X-ray 의첫번째임상응용은 1896 년 2 월 3 일이루어졌는데 New Hampshire 의 Hanover 에사는 Eddie McCarthy 라는소년이처음환자였다. 그는 2 주일전에스케이트를타다넘어져손목을다쳤는데, 그의주치의인 Dr. Gilman Frost 가 Dartmouth 에서물리학과교수로있는그의형인 Edwin Frost 에게연락하여환자를 Dartmouth 대학의 Reed Hall 에있는물리실험실로보냈고, Edwin Frost 교수는 battery-powered Crookes' vacuum tube 와감광유리판을사용하여최초의 American Rontgenogram 을만들어냈고콜리스골절 (Colles' fracture) 임을밝혀냈다. 노출시간은 20 분이었다.
The Beginning with X-ray X-ray 사진에의해조직을 characterize할수있음을보여주기위한실험의첫번째보고는rontgen의첫논문발표후불과 3달이내에 Nature지에실렸다. Cormack과 Ingle은두개의동일한사람손가락뼈를이용하여 calcium 농도가 X- ray 영상의 opacity를결정함을보여주었다. 이후일련의실험들이영상의구성에있어서특별한 "shadows(i.e. densities)" 와 "blackening(i.e. lucencles)" 의기저에깔린성질을밝혀내기위해 design되었다. 이러한새로운 ' 방사선사진술 (radiography)' 와 ' 형광투시법 (fluroscopy)' 기술은미국의발명가인 Thomas Alva Edison의관심을끌어 1896년에 Edison 은 Randolph Hearst로부터살아있는사람의뇌에대한방사선영상을만들어내라는도전을받아들였다. 물론그는서로다른뇌조직의 electron density의작은차이를구별해내는데있어서기술적인한계로실패하였다.
FUNDAMENTALS, GENERATION, & DETECTION OF X-RAY
X-ray Generation X-ray generation : electrons with high energy strikes a target such as tungsten or molybdenum White Radiation : high-energy electron interact with nuclei Characteristic radiation : interact with orbital electron, similar to photoelectric effect
X-ray Generators For heat problem : Rotating anode(3,000~ 10,000 rpm) f = Fsinθ space charge effect : escaped electrons form a cloud around the filament Heat generation : 99% energy converted into heat, tungsten s melting point = 3370 C line focus principle : large focal spot(f) for heat capacity, small focal spot(f) for better image
X-ray Tube Ratings Target Material : the higher atomic number, the greater the efficiency of X-ray production Tube Voltage(kVp) : intensity kvp 2, maximum energy kvp Tube Current(mA) : intensity ma, number of X-ray photon produced ma Filament current : over saturation voltage, the current is limited by the filament current 2 I Z( ma)( kvp) F :for fixed i f with rectification factor F
X-ray Filters X-ray Filter : filtering out the undesired portion of the X-ray spectrum for reduction of X-ray dose to the patient filter, absorber : thin sheets of metal placed between the X-ray source and the patient, Al for low-energy X- rays, Cu for high-energy X-rays
Beam Restrictors Restrictor : regulate beam size & shape (aperture diaphragm, cones & cylinder, collimator) for reduction patient exposure & less scatter radiation Collimator : most popular beam restrictor : size adjustable, indicating light beam P= D L I
Interactions between X-ray & Matter Coherent Scattering : 1. Loosing little energy 2. Negligible change in wavelength 3. Occurs in low-energy radiation 4. Does not cause ionization
Interactions between X-ray & Matter Photoelectric Effect : 1. Freed K-shell electron photoelectron 2. Vacancy in the orbit 3. Filled by higher energy level electron radiation or other electron escaped (Auger effect) 4. End products : Positive ion, characteristic radiation or Auger electron, photoelectron 5. Most desirable type in imaging completely absorbed, producing little scattered radiation
Interactions between X-ray & Matter Compton Scattering : 1. Scattered radiation encountered in X-ray imaging 2. Freed electron from outer shell 3. X-ray photon deflected retaining some of the energy 4. Major problems : - Background noise, -Safety hazard
Intensity of an X-ray Beam Roentgen (R) : 1. product of power contained in the beam and time of exposure 2. Total number of ion pairs produced in 1 cc of air under standard condition(at 760mmHg, 0 C) by the radiation or the amount of radiation that produces an electric charge separation of 2.58x10-4 coulomb/kg of air Radiation Absorbed Dos (rad) : 1. Amount of radiation actually absorbed by a medium 2. 1 (rad) = 0.01 joule of energy absorbed by 1 kg of material 3. Different materials have different X-ray absorption characteristics the amount of energy absorbed by different materials may be different for the same amount of radiation
Attenuation di = nσ Idx di / dx = nσ I I = I e β 0 x n : atoms per unit volume of the material I : X-ray intensity at x I 0 : incident X-ray intensity β:linear attenuation coefficient[np/cm or cm -1 ] Half-value layer : the propagation length required to the intensity of the original beam by ½ HVL = 0.693/β mass attenuation coefficient is independent of its physical state (ex, water, ice, water vapor), [cm 2 /g] MAC= β/η, where η is mass density of the material
Attenuation Coefficient attenuation decreases as photon energy increases (Xray as smaller particle) abrupt increase in attenuation at binding energy level of orbital electron (photoelectric effect) β = βs + βa β = β + β + β β β β coh pho com coh pho com ηz E 2 1 ηze 3 3 ηρ E e 1 Z : atomic number η: density ρ e : electron dencity
Radiographic Grids Scattered X-rays : noise degrading image quality, increase patient exposure Grid : series of lead foil strips separated by X-ray transparent spacers, blocks the scattered radiation while letting the primary radiation pass
Intensifying Screens To convert invisible X-rays to visible photons Fluorescent screen A layer of phosphor with 0.05~0.3mm thickness emitting light photons when struck by X-ray photons calcium tungstate (CaWO 4 ): emits blue light(430nm), good for X-ray film exposure, not optimal for human visualization by the eye
Image Intensifiers To brighten the image produced on the image-intensifying screen Vacuum tube with (1) input phosphor and photocathode, (2) focusing plates, (3) an anode, and (4) output phosphor
X-ray Films
Radiation Detectors