Physics of medical imaging – an introduction

Physics of Medical Imaging – An Introduction Dove – Physics of Medical Imaging 9222003 Physics of Medical Imaging – An Introduction Edwin L Dove Biomedical Engineering The University of Iowa Table of Contents Physics of Medical Imaging – An Introduction 1 1 Introduction 3 2 X ray Modality 8 2 1 History 8 2 2 X ray physics 9 2 3 Brief review of the structure of the atom 12 3 Interaction between X rays and matter 15 3 1 Coherent (Rayleigh) scattering 15 3 2 Photo disintegration 15 3 3 Photo elec.

Dove – Physics of Medical Imaging 9/22/2003 Physics of Medical Imaging – An Introduction Edwin L Dove Biomedical Engineering The University of Iowa Table of Contents Physics of Medical Imaging – An Introduction 1 Introduction X-ray Modality 2.1 History 2.2 X-ray physics 2.3 Brief review of the structure of the atom 12 Interaction between X-rays and matter 15 3.1 Coherent (Rayleigh) scattering 15 3.2 Photo-disintegration 15 3.3 Photo-electric effect 15 3.4 Compton scattering 17 3.5 Pair production 19 3.6 Summary 19 Dose and Exposure 21 4.1 Dose equivalent 22 4.2 Maximum permissible levels 22 4.3 Environmental dose 23 4.4 Body parts – whole body dose 24 Propagation model 26 5.1 Simple transmission imaging 26 5.2 Attenuation coefficient 27 5.3 Transmission imaging 29 Brief Summary So Far 31 Generation of X-rays 32 7.1 White radiation 32 7.2 Characteristic radiation 33 Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 7.3 X-ray generators 36 7.4 Grids 38 7.5 Detectors 39 7.6 Miscellaneous X-ray procedures 44 Summary and history 48 References 53 Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Introduction Images of the human body are derived from the interaction of energy with human tissue The energy can be in the form of radiation, magnetic or electric fields, or acoustic energy The energy usually interacts at the molecular or atomic levels, so a clear understanding of the structure of the atom is necessary In addition to understanding the physics of the atom, learning imaging jargon is also necessary For example: • • • • • • • Tomography: a cross-sectional image formed from a set of projection images The Greek word tomo means cut CT: Computed (or Computerized) Tomography MR, or MRI: Magnetic Resonance Imaging This was first called nuclear magnetic resonance (NMR), but the mention of anything nuclear scared patients, so the “N” was dropped PET: Positron Emission Tomography Understanding this phenomenon requires acceptance of the theory that there is antimatter in the universe, and when antimatter meets matter, then both kinds of matter are annihilated, and pure energy is formed SPECT: Single Photon Emission Tomography Ultrasound: Sonar in the body OCT: Optical Coherent Tomography – the use of infrared light to image (particularity) the walls of an artery A modality is a method for acquiring an image MR, CT, etc are all imaging modalities Modalities are sometimes categorized based on the amount of energy applied to the body For example, the X-ray modality produces energy that is sufficient to ionize atoms (i.e., eject an electron from an orbit of an atom, thereby creating a positively charged ion that damages human tissue) The modalities that cause ionizing radiation are X-rays, CT, SPECT, and PET Non-ionizing modalities include MR and ultrasound Other classification schemes are used in modern medical imaging For example, many radiologists consider there to be four methods for obtaining images: X-ray transmission, radionuclide emission, magnetic resonance, and ultrasound Other methods are “under research.” Each of the four methods depicts different categories of information reflective of anatomical or physiological processes Table 1.1compares the four medical imaging techniques Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Table 1.1 Comparison of medical imaging techniques Method Transmission computed tomography Parameter measured Density and average atomic number Emission computed tomography (positron and single photon) Magnetic resonance Concentration of radionuclides Ultrasound Concentrations, relaxation parameters T1, and T2 and frequency shifts due to chemical form Acoustic impedance mismatches, sound velocity, attenuation, frequency shifts due to motion Medical applications Anatomy, mineral content, flow and permeability from movement of contrast material Metabolism, receptor site concentration, flow Anatomy, edema, flow, and chemical composition Anatomy, tissue structure characteristics, flow Figure 1-1 depicts the major modes of medical imaging Some modern imaging modalities (PET, CT and MR) require that the patient enter a ring of detectors For some, this is challenging due to disease state, claustrophobia, or other Ultrasound requires a simple probe be placed on the skin of the subject Figure 1-1 Schematic representation of major imaging modalities in medical imaging Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 An example of a PET image is shown in Figure 1-2 Figure 1-2 PET scans of a brain tumor (Taken from the Harvard Medical School Nuclear Division web site) An example of a SPECT scan is shown in Figure 1-3 Figure 1-3 Bottom row - SPECT scans of a brain tumor (Taken from the Harvard Medical School Nuclear Division web site) Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 A schematic diagram of an MR machine is shown in Figure 1-4 A typical MRI image is shown in Figure 1-5 Figure 1-4 MRI block diagram Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 1-5 Knee study from MRI image In this class we will only study primarily X-rays and ultrasound The other modalities (and more advanced image processing algorithms) are coved in subsequent elective imaging courses (51:185, 186, 188, and 189) Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 X-ray Modality 2.1 History On Friday evening, November 1895, Wilhelm Conrad Röntgen (also sometimes spelled Roentgen) discovered a “new kind of ray” that penetrated matter Röntgen, a 50-year old professor of physics at Julius Maximilian University of Wurzburg, named the new kind of ray X-strahlen “X-rays” (“X” for unknown) Röntgen was looking for the “invisible high-frequency rays” that Hermann Ludwig Ferdinand von Helmholtz had predicted from the Maxwell theory of electromagnetic radiation Röntgen’s discovery was submitted for publication on 28 December 1895 and was published on January 1896 A portable X-ray unit was available from the Sears catalog in late 1896 The cost was $15 Röntgen developed the first X-ray pictures on photographic plates, and one of the first materials tested was human tissue The most famous picture was an image of his wife’s hand with a ring on her finger (Figure 2-1) Figure 2-1 The first reported image of human tissue Mrs Röntgen’s hand with a ring, taken in 1895 In 1901 Röntgen received the Nobel Prize for Physics, which was the first Nobel Prize in physics ever awarded Unfortunately, Röntgen, his wife, and his laboratory workers all died prematurely of cancer Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 The first medical use of the X-ray was on 13 January 1896 by Drs Ratcliffe and HallEdwards, in which they showed the location of a small needle in a woman’s hand As a consequence, Dr J.H Clayton performed the first X-ray guided surgery nine days after the publication of the existence of X-rays Also in 1896 Randolph Hearst (of the famous Hearst publishing dynasty) offered a challenge to scientists to capture an image of the brain Many tried, and all failed, even though some novel imaging enhancement techniques were invented For example, air was injected into the fluid-carrying compartments of the brain (pneumoencephalography) The test subjects reported no major physical discomfort (the brain has no pain receptors), though they developed unusual behavior, mentation, cognition, and motion patterns Allan Macleod Cormack (Tufts University) and Godfrey Newbold Hounsfield (research labs of EMI, Ltd.) developed the necessary mathematics (1962) and the first hardware implementation of the CT scanner (1972) that was able to image the brain This scanner was able to compute one CT image in about 24 hours Cormack and Hounsfield shared the Nobel Prize in Physiology and Medicine in 1979 Note: Hounsfield never claimed that he invented CT The original concept was published in 1917 by Radon Oldendorf (1961) rotated a head phantom on a gramophone turntable and provided simultaneous translation by having an HO-gauge railway track on the turntable This contraption was pulled slowly through a beam of X-rays falling on a detector Oldendorf showed the internal structure of the phantom Even earlier, there were reports (1957, 1958) from Russia (actually CCCP at that time) that a working CT machine was built 2.2 X-ray physics An X-ray is electromagnetic (EM) radiation similar to light, radio waves, TV waves, etc Table 2.1 shows some of the components of the EM spectrum, their frequency, wavelength, energy, and use Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Table 2.1 Electromagnetic Wave Spectrum (from [Enderle et al.]) Figure 2-2 graphically shows the electromagnetic spectrum and the corresponding energy of each component Figure 2-2 The Electromagnetic Spectrum The photon energies are given in electron volts (eV) Obviously there is a relationship between frequency and energy The relationship between energy and frequency for EM waves is E = hf (2.1) where E is energy in kilo electron volts (keV), h is Planck’s constant (4.13x10-18 keV s, or 6.63x10-34 J s), and f is the frequency (Hz) (1 keV=1.6x10-19 joules) At least in a Page 10 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-7 Scattered X-ray photons can be removed from the image by positioning a grid between the patient and the detector (or film) Grid strips are usually made of lead, which is an effective X-ray absorbing material If the grid strips are thin enough, then their image on the detector may be negligible However, if the image quality requirements necessitate thick lead strips, then the grid may be moved during exposure to blur out the image of the grid lines The grid shown in Figure 7-7 is called a linear grid Other forms of grid have been used For example, when the grid strips are focused towards the X-ray source, then grid is called a focused grid 7.5 Detectors The human eye cannot see X-ray photons Our rods and cones of our eyes respond to lower frequency EM waves, not the high-frequency X-ray EM waves As a consequence, the distribution of transmitted X-ray energy must be converted into a form that we can “see.” This conversion is usually done by exposing a photographic film: the X-ray energy excites the silver halide crystals, which are washed off leaving a viewable film; estimating the photon density by measuring the ionization in a gas; converting the X-ray photons to visible light, amplify this light with a photomultiplier tube, and view; or building a solid state detector with current flow proportional to incident photon density The resulting images suffer from the non-ideal nature of the X-ray source and detectors These non-deal qualities are due to geometric unsharpness, beam size, and object magnification Page 39 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-8 illustrates the blurring effect of a finite focal point If the X-ray beam has a beam width of f then a point in the patient appears as a smeared or blurred point with width d This type of unsharpness is sometimes called geometric unsharpness, or the penumbra To reduce this effect, increase S by moving the source further from the patient, reduce t by moving the patient as close as possible to the detector, or reduce f by installing a point collimator near the tube Figure 7-8 Finite aperture of X-ray source causes blurring of the image Figure 7-9 illustrates the effect of beam size divergence The beam size increases with increasing distance from the source because the photons not travel in exactly parallel trajectories To reduce this effect, decrease the distance from the source to the patient Of course, this solution exacerbates the penumbra blurring effect Oh well Page 40 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-9 Effect of diverging X-ray beam size Figure 7-10 illustrates the magnification effect The apparent size of an object is affected by its position in the scanner field of view Objects closer to the source appear larger than similarly sized objects located further from the source To minimize this effect, increase the distance form the patient to the source Oh well, again Figure 7-10 X-ray image of object magnified by ratio ( Sf Sf − t ) Since the human eye cannot see the information carried by the X-ray directly, the images must be converted to a “visualizable” form Usually this process is initiated with an Page 41 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 intensifying screen An intensifying screen is basically a layer of phosphor (from 0.05 to 0.3 mm thick) that emits light photons when struck by X-ray photons The most popular phosphors are calcium tungstate (CaWO4) and terbium-activated rare-earth oxysulfide, though newer phosphors are sometimes used The efficiency of CaWO4 is only 5%, whereas the newer phosphors (such as gadolinium (Gd2O2S)) can achieve efficiencies of greater than 15% These new phosphors emit light in narrow bands (usually green or blue) Many electronic imaging systems use image intensifiers (show schematically in Figure 7-11) The incoming X-ray photons that have propagated through the patient will be absorbed by the fluorescent screen (usually 15 to 35 cm in diameter) with emission of light photons The light photons strike the photocathode kept at ground potential, causing it to emit electrons in a number in proportional to the brightness of the screen The photocathode is usually made of antimony or cesium compounds The electron beam will be accelerated and focused onto the output fluorescent screen by the anode, which sets about 25 kV higher than the cathode The output screen is usually only 1.5 to 2.5 cm in diameter Figure 7-11 Physical construction of an image intensifier Two types of radiation detectors are currently used for X-ray detection: scintillation detectors and ionization chamber detectors Figure 7-12 shows a scintillation detector, which consists of a scintillation crystal (usually sodium iodide with traces of thallium) coupled with a photomultiplier tube Page 42 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-12 Physical construction of a scintillation detector with a photomultiplier tube The photocathode emits electrons when struck by light The electrons are accelerated by the dynodes, which is covered by material that emits secondary electrons when struck with an electron In this way the number of electrons is multiplied as the electron beam passes from dynode to dynode The output current is proportional to the number of electrons striking at Vn The efficiency of this type of device is greater than 85% The second type of detector is the ionization chamber shown in Figure 7-13 This detector consists of a chamber filled with a gas (usually xenon) The molecules in the chamber are ionized by the X-ray photons The ions are then attracted to the electrodes by a voltage difference between the electrodes This device is cheap, and has a relatively small form factor (small size) Page 43 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-13 Physical construction of a radiation detector: ionization chamber 7.6 Miscellaneous X-ray procedures There are many X-ray based procedures used in medical diagnosis fluoroscopy, mammography, and Xeroradiography Some are X-rays can be captured on film, or viewed directly on a fluorescent screen Figure 7-14 illustrates a conventional fluoroscope In a typical fluoroscopic procedure for examining the GI tract, a contrast medium (usually barium sulfate) is taken orally or by enema Figure 7-15 shows a colon radiograph where colon containing the contrast medium appears darker than the surrounding tissues Because the patient is being continuously exposed to X-ray radiation, the radiation dose can be very high Figure 7-14 Basic components of fluoroscopy Page 44 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-15 X-ray radiogram of the colon following air-barium double-contrast enema X-ray mammography is usually performed without contrast injection Mammography has a couple of special requirements For example, low energy (typically 20 keV) X-rays are used since the tissues are soft As a result, the anode in the X-ray tube is made of molybdenum Modern mammography units can achieve spatial resolution of better than 0.1 mm with very low radiation dose Page 45 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Xeroradiography is an X-ray technique developed by the Xerox Corporation that uses Xray energies between 35 and 45 keV and an electrostatic technique similar to the Xerox photocopy machine Figure 7-16 illustrates the physical attributes of such a machine Figure 7-17 shows a typical xeroradiographic image of a breast Figure 7-16 Physical construction of Xeroradiographic system Page 46 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 7-17 Xeroradiogram of breast where ribs and breast vasculature are seen Page 47 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Summary and history Before X-rays were discovered by Dr Röntgen, the first role of a physician was to diagnose what was wrong with the patient before considering the prognosis and providing treatment Accurate diagnosis was most dependent on the history obtained and the physician’s skill at questioning the patients But at the same time the physician used the senses, sight, touch, hearing, smell and even taste to identify abnormalities in the patient This is illustrated, in part, as shown below: Figure 8-1 The Grecian- Roman era of medical diagnosis Hippocrates had developed observation of the patient and the progress of their disease as the science of medicine He described the appearance of the patient, felt their temperature, and smelled their vomit Although doctors became particularly skilled at examining the lumps, cuts and breaks of the body (i.e., "external" medicine), they rarely tried to examine the inside of the body There were exceptions such as when Hippocrates shook the patient with pleurisy to detect a splash when there was fluid in the pleural space; thus the Hippocratic Succussion Splash Observations are considered to be more scientific if measurement of them can be made In Alexandria, Herophilus 300 BCE would feel the pulse and count it using a water clock Galen, 129 ACE, relied heavily on touch or palpation for diagnosis and essential skill for assessing wounds and injuries Galen had practiced sports medicine with the gladiators He would describe the general appearance of the patient but also tasted sweat for jaundice and listen to the rumbling abdomen Galen learned much from feeling the pulse, which he did in both wrists using three fingers The pulse was thought to reveal disorders of the Page 48 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 organs of the body There is much in common with what was understood by him from the pulse and Chinese medicine - perhaps a spread of ideas along the Silk Road The Chinese did not measure the pulse but were said to learn many things from its feel Figure 8-2 Methods of feeling the pulse, and interpreting the information gained In 1583 a medical student was bored by a sermon and observed that the swinging of the altar lamp was unvarying and that this could be used to measure time The student was Galileo (1564-1642) who, as we know, gave up medicine for astronomy fame and ill fortune at the hands of the Catholic Church Galileo's pendulum clock was adapted by Sanctorius (1561-1636) to measure the pulse (Figure 8-2) Sanctorius was considered to be a major physiologist because he used a thermometer to measure temperature, and a weighing chair to measure the intake and output of food and fluid But since there was little understanding of the function of the body, these measurements did not advance medicine This was to change with the Italian Renaissance and the lessons of the anatomists who identified the true state of the organs of the body and set the scene for understanding their function The work of the Englishman William Harvey (1575-1657) who had studied in Italy described the circulation of the blood but added nothing to the diagnosis of disease He thought the heart distributed the humours and spirits In 1707 Sir John Floyer (16491743) introduced the pulse watch and thought it of more value than Harvey's work Physicians began to count the pulse regularly and would note it in various ways In 1731 Page 49 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 a new dimension was added to the measurement of the body with the work of Stephen Hales (1677-1771) who studied the pressure in arteries and veins He would insert a cannula into the vessels and measure the height of the column of blood – that is he measured the blood pressure (see Figure 8-3) But these advances were relatively meaningless until the understanding that diseases were often abnormalities of the structure or function of organs Figure 8-3 Schematic of Harvey’s blood pressure measurement system After Dr Röntgen’s discovery, medicine inalterably changed (of course, X-ray discovery was not the only impetus for medical change, others included anesthesia, the notion of sepsis, etc.) It wasn’t long before knowledge of the knee or hip (for example) was provided without the knife as in the following example images Page 50 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 8-4 Anatomic information obtained from a modern X-ray study Page 51 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Figure 8-5 Acetabular fracture on the left posterior lip Page 52 of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 References Berger, S., W Goldsmith, and E Lewis (eds.) Introduction to Bioengineering, Oxford, New York, 1996 This reference is complete, but it is written at a beginning graduate level The section on medical imaging is brief, but useful Brown, B., R Smallwood, D Barber, P Lawford, and D Hose Medical Physics and Biomedical Engineering, Institute of Physics Publishing, Bristol, 1999 This was to be our textbook for the semester Dhawan, A., Medical Image Analysis, IEEE/John Wiley Press, 2003 Shung, K M Smith, B Tsui Principles of Medical Imaging, Academic Press, San Diego, 1992 This is a wonderful brief text from which many of the images are drawn Unfortunately, this text is out of print Webb, S (ed) The Physics of Medical Imaging, Institute of Physics Publishing, Bristol, 1992 This is a complete text covering all aspects of medical imaging Unfortunately, this text is out of print Many electronic images are stored in DICOM format, which is promulgated by the IEEE and NEMA See: http://www.ctmed.ru/DICOM_HL7/ which states: Hаpяду с буpным pостом компьютеpных технологий, в медицине все более остpо встает вопpос о создании единых международных стандартов обмена медицинскими данными.В разных странах этот вопрос решается по-разному и именно поэтому существует множество различных медицинских стандартов: ASTM, ASC X12, IEEE/MEDIX, NCPDP, HL7, DICOM и т.п Как правило, стандарты носят названия групп/комитетов и прочих некоммерческих организаций их разрабатывающих Page 53 of 53 XrayF03.doc ... the skin of the subject Figure 1-1 Schematic representation of major imaging modalities in medical imaging Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 An example of a PET... angle is given by (3) Page 34 of 53 XrayF03.doc Dove – Physics of Medical Imaging where and 9/22/2003 are the initial and final velocities, respectively and the range of wavenumbers In the soft-photon... 53 Page of 53 XrayF03.doc Dove – Physics of Medical Imaging 9/22/2003 Introduction Images of the human body are derived from the interaction of energy with human tissue The energy can be in