TOPIC 24: MEDICAL PHYSICS Topic 24: Medical Physics 24.1 Production and use of ultrasound 24.2 Production and use of X-rays 24.3 PET Scanning Medical imaging Medical Imaging involves the instruments, techniques and principles of ‘seeing’ inside the human body. Some possible non-invasive ways (modalities) are: a. b. c. d. e. Ultrasound imaging X-ray imaging PET scanning Nuclear Magnetic Resonance (MRI) imaging Radionuclide or radio-pharmaceutical imaging Medical Imaging Modalities 24.1 Production and use of ultrasound 1 Understand that a piezo-electric crystal changes shape when a pd is applied across it and that the crystal generates an emf when its shape changes. 2 Understand how ultrasound waves are generated and detected by a piezoelectric transducer. 3 Understand how the reflection of pulses of ultrasound at boundaries between tissues can be used to obtain diagnostic information about internal structures. 4 Define the specific acoustic impedance of a medium as Z = ρc, where c is the speed of sound in the medium. 5 Use IR / I0 = (Z1 – Z2)2 / (Z1 + Z2)2 for the intensity reflection coefficient of a boundary between two media. 6 Recall and use I = I0e-μx for the attenuation of ultrasound in matter. Ultrasound Ultrasonic imaging Used for imaging lower density materials /soft tissue and fluids such as fetus or the brain which can be damaged by ionizing radiation. Technique involves ultrasound and SONAR Ultrasound is high frequency sound not detected by the ear. (f = 1 – 15/20 MHz). Ultrasound waves are longitudinal, travel through solids, gases and body fluids as pressure or elastic waves. Ultrasound waves are reflected and attenuated (due to inverse-square law of reduction and to absorption in a medium; with higher frequencies attenuated more). Enables looking at ‘real time’ images. Transducers A Transducer is a device for converting one form of energy or signal into another. E.g. the eye (optical to electrical) the ear (acoustic to electrical) the thermometer (thermal to mechanical) Other common transducers are the microphone, loudspeaker, motor, LED, LDR, thermistors, etc. Generation and detection of ultrasound waves Piezoelectric crystal: a quartz crystal that changes shape when a potential difference is applied across it Production: apply an AC voltage to generate a vibration at desired frequency Detection: received sound wave causes it to vibrate and generate an AC voltage that can be measured The Ultrasound Probe and Piezoelectric Effect A piezoelectric transducer interchanges electrical and mechanical energy. Used to both generate and detect ultrasound. Ultrasound is produced and detected through high frequency oscillations in a piezoelectric crystal capable of interchanging mechanical and electrical energy. A common piezoelectric material used is the synthetic ceramic, lead zirconate titanate, PZT Ultrasound pulses are created by piezo-electric crystals. A piezo-electric crystal changes shape when a pd. is applied across it and the crystal generates an emf when its shape changes. Thus, a pd is applied across the crystal, causing it to vibrate at high frequency and produce ultrasonic pulses. The same crystal changes shape when the reflected pulses return, generating an emf A piezoelectric transducer on the body’s surface directs the pulses of ultrasound into the body. These reflect off internal boundaries e.g. between tissue and bone. The probe can detect the reflected pulses and the information is converted into an image. Reflection of ultrasound pulses at boundaries Pulses of ultrasound are emitted and reflected on meeting an organ or tissue. The reflected pulses are detected and timed to give the distance to a boundary so that the location and surface of the organ can be mapped or imaged. 2d = c x t d = distance from probe to reflecting boundary c = speed of sound in the medium t = time interval between emission and detection of the ultrasound pulse. The greater the difference between two media, the more reflection occurs. Role of jelly layer Ultrasound reflects at internal boundaries between media because the acoustic impedance of such media differ. A layer of jelly is smeared on the stomach or flesh to eliminate any air layer between the probe head and the body/skin. This produces better transmission of ultrasound pulses into the body (and less reflection from the skin). - Impedance matching. Otherwise, nearly all the transmitted pulse will be reflected at the surface of the skin. (Prevents reflections of ultrasound at skin surface because most reflection occurs at air/skin boundary). Specific acoustic impedance of a medium The acoustic impedance, Z of a substance is the product of the density of the substance and the speed of sound in that substance. Z=ρc units: kg/(m2s); rayl Intensity reflection coefficient of a boundary The greater the difference between two media, the more the reflection of ultrasound occurs at the boundary of the media. Intensity Reflection Coefficient is the relationship between acoustic impedance and intensity. IR / I0 = (Z1 – Z2)2 / (Z1 + Z2)2 at normal incidence It can be used to calculate the reflected intensity of the ultrasound or the specific acoustic impedance, allowing identification of media. Attenuation of ultrasound in matter Attenuation refers to the reduction in the intensity of the ultrasound as it travels or passes through a medium. It is a lessening of radiation intensity. I = I0 e – μx Attenuation occurs as the ultrasound pulse i. spreads out or scatters (simple scattering) ii. is absorbed by matter Production and use of ultrasound Strengths Affords clear examination of soft tissue. Limitations Small depth penetration and so not useful for adults. No known harmful effects. Scan can take a long time. Safe and easy to use equipment. Poor image clarity due to reflection at boundaries. Offers real-time imaging. Cost effective. There is a limit to the size of objects that can be imaged. 24.2 Production and use of x-rays 1 Explain that X-rays are produced by electron bombardment of a metal target and calculate the minimum wavelength of X-rays produced from the accelerating pd. 2 Understand the use of X-rays in imaging internal body structures, including an understanding of the term contrast in X-ray imaging. 3 Recall and use I = I0e-μx for the attenuation of X-rays in matter. 4 Understand that computed tomography (CT) scanning produces a 3D image of an internal structure by first combining multiple X-ray images taken in the same section from different angles to obtain a 2D image of the section, then repeating this process along an axis and combining 2D images of multiple sections. X-Rays Wilhelm Roentgen X-ray photo of Bertha Roentgen’s hand Discovered X-rays in 1895. Being used in medicine by 1896! Interaction of X-rays with matter X-ray quality: the degree to which X-rays penetrate matter. It depends on the spread of wavelengths present in the beam. Also called penetrating power. Intensity: amount of energy per unit area carried by the X-ray beam per second. The effects of X-rays on matter depend on the quality and the intensity of the beam. X-ray production X-rays are electromagnetic radiations and have enough energy to cause ionization. Electrons from a hot filament are accelerated by a high pd towards an anode.(often made of copper, iron, molybdenum or tungsten or a crystalline material). K.E of the electrons is given to the atoms of the anode, producing heat (99%) and X-rays (1%). X-ray wavelength range: 10 nm – 0.01 nm X-ray tubes operate at potentials around 50 kV X-ray intensity is the product of the number of photons emitted for a given wavelength and the energy of each photon. The minimum wavelength, λmin of the X-ray photon is given by hc/λmin = eV where V is the accelerating pd across the tube. So, λmin = hc / eV ( fmax = eV / h ) The X-ray tube The X-ray tube Attenuation Attenuation refers to the reduction in the intensity of the radiation as it travels or passes through a medium. It is a lessening of radiation intensity. Attenuation occurs as the X-ray beam i. spreads out or scatters (simple scattering) ii. is absorbed by matter (e.g. for photoelectric effect) X-ray Attenuation in an Absorber When a beam of mono-energetic X-rays of intensity Io passes through a medium of increasing thickness x, the intensity, I in the medium is given by I = Io e –μx μ = the linear attenuation coefficient, a constant which depends on the X-ray energy and the nature of the absorbing material. (units: m -1 , cm-1, mm-1 ) I = Io e –μx Half-value thickness, X 1/2 Half-value thickness, X 1/2 is the thickness of material that reduces the intensity of a given X-ray beam to half its original value. (i.e. Thickness of material needed to reduce intensity by 50%). Units: m, cm, mm. Can show that : X ½ = ln 2 / μ = 0.6931/ μ Relationship between X1/2 and μ The graph at right shows the transmitted intensity of a parallel beam of X-rays versus the thickness of a certain type of tissue. Determine the: a) half-value thickness of this tissue b) attenuation coefficient of this tissue Example The hvt of a 30 keV X-ray photon in aluminum is 2.4 mm. If the initial intensity of the X-ray beam is 4.0 x 102 kWm -2, a. What is the intensity after passing through 9.6 mm of aluminum? (25 kWm -2 ) b. Calculate the linear attenuation coefficient of aluminum. ( 0.29 mm -1 or 2.9 x 102 m -1) c. What is the intensity of the beam after passing through 1.5 mm of aluminum? (2.59 x 10 2 kWm -2 ) Analyse the intensity graph at the top by straightening it. ln I = - μ x + ln I0 1. A parallel beam of X-rays passes through muscle and reduces to oneeighth of its initial intensity. Determine the fraction of the intensity of this beam when it is transmitted through the same thickness of fatty tissue. The half-value thickness of muscle is 4.0 mm and the half-value thickness of fatty tissue is 6.0 mm. 2. The half-value thickness for soft tissue (muscle) is about 20 cm and for bone is 150 times less than this. Determine the attenuation coefficient for bone and for soft tissue. X – ray Imaging The basic principle is that some body parts (e.g. bones) attenuate the X-ray beam much more than other parts. (e.g. skin, fat, muscles and tissues). i. X-rays pass through a person and fall on a photographic film which darkens when the radiations hit it. ii. Bone absorbs more of the X-rays than soft tissue and so the portion of film for such parts are dark and those for bones are lighter. X-rays cause ionization and are dangerous so intensity used for patients needs to be kept at an absolute minimum. How X-rays image an internal organ X-rays that pass through the body to the film make the film dark (black). X-rays that are totally blocked do not reach the film and render the film light (white). Five basic radiographic densities are air, fat, soft tissue / fluid, mineral and metal Air : low Z, X-rays get through and image is dark (black). Bone or metal: high Z, X-rays blocked and image is light (white). Outline the basis for the formation of X-ray image of a limb. - attenuation coefficient greater for bone than for muscle/tissue - so intensity of X-ray less for section of film where bone is = lighter image for bone - good contrast to darker image for muscle Producing a radiographic image X – ray Imaging X – ray Imaging X – ray Imaging X – ray Imaging X – ray Imaging X-ray Image Contrast • Some body parts are difficult to image against the background of other parts. E.g. the stomach or intestinal tract. • To improve contrast: Introduce a contrast-enhancing medium (e.g. ‘barium sulfate or bismuth) which are solutions of heavy metals into the body before taking the X-ray photograph. The solutions have a large attenuation coefficient and their progress in the body can be monitored. E.g. Use of barium meal for stomach pains and intestinal ulcers; Iodine for cardiovascular system , kidney and the brain. Why/how are barium meals used to assist X-ray imaging of stomach or intestinal tract? - All tissues in the abdominal cavity have approximately the same attenuation coefficient so there is little to no contrast on photographic film. - The attenuation coefficient for barium is greater than for the tissues in the abdominal cavity. - Barium meal lines the stomach. - Outline of stomach becomes clearer (greater contrast). Computed Tomography (CT) Scan Computed Tomography (CT) Scan This modality helps to identify problems within soft tissue such as the brain, lung, breast, etc. Tomography involves focusing on a certain region or ‘slice’ through the body whilst all other regions are blurred out of focus. Achieved by moving the source of X-rays and the film (or scintillation detector) together. Computer technology is used to build up an axial scan of a section of an organ or part of the body. X-ray tube voltages are around 130 kV and the exposure time is greater than that of the standard X-ray imaging. CT Scanning Acquire X-ray projection Translate X-ray beam and detect across the body and record the output Rotate to the next angle and repeat translation Assemble all the projections. Computed Tomography (CT) Scan Computed Tomography (CT) Scan CT scanning Comparison of simple X-ray Imaging and CT Scan X-ray Imaging • Uses ionizing radiation (X-rays) CT Scanning • Uses ionizing radiation (X-rays) • Short duration of exposure • Long duration of exposure • Smaller dose of radiation • Large dose of radiation • 2 – D shadow image • 3 – D images possible Simple X-ray Imaging and CT Scan Strengths • X-rays relatively cheap to obtain • Screening or elaborate preparation not necessary • Good bone resolution and soft tissue contrast • Relatively versatile (used for lung, breast, brain, dental) • 3 – D images are possible Limitations • Risk of cancer from ionizing radiation • Electrical danger from high voltages used • Poor soft tissue resolution unless with CT • Radiation dose is cumulative 24.3 PET Scanning 1 Understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied. 2 Recall that a tracer that decays by β+ decay is used in positron emission tomography. 3 Understand that annihilation occurs when a particle interacts with its antiparticle and that mass-energy and momentum are conserved in the process. 4 Explain that, in PET scanning, positrons emitted by the decay of the tracer annihilate when they interact with electrons in the tissue, producing a pair of gamma-ray photons travelling in opposite directions. 5 Calculate the energy of the gamma-ray photons emitted during the annihilation of an electron-positron pair. 6 Understand that the gamma-ray photons from an annihilation event travel outside the body and can be detected, and an image of the tracer concentration in the tissue can be created by processing the arrival times of the gamma-ray photons. Use of tracers in imaging A tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied. e.g. a glucose based molecule with fluorine-18 attached. Annihilation during particle and anti-particle interaction In annihilation, both particles cease to exist and their mass is converted into energy in the form of photons. When the positron, emitted by β+ decay of the tracer, meets an electron, it annihilates and produces a pair of gamma-ray photons travelling in opposite directions. The gamma-ray photons will travel out of the body due to their high penetrating power. Role of annihilation in PET scanning • PET is a technique similar to CT scan but uses an isotope of carbon, C-11, as a radioactive tracer for imaging. Used to image the functioning of the brain (e.g. to measure changes in blood flow). The patient is given a specific task (such as reading or drawing) and the PET scan identifies the areas of the brain that receive more blood or the metabolism of glucose in the brain as a result of doing the given task. • The process involves the annihilation of an electron and a positron (the anti-particle of the electron) and the detection of the two photons so produced. • • e- + e+ → 2γ + 511MeV • (Positrons are given off during the decay of fluorine-18 and Oxygen-15) • The patient is ejected with a solution of radioactive material containing isotopes that decay by positron emission, or in the case of C-11, it can be introduced into the body by breathing a small sample of carbon monoxide. The radioactive carbon is taken up by the red blood cells and flows around the body. When it decays, it dos so by positive beta decay and a positron is emitted. • When the positron is emitted it collides with an electron in the tissue of the patient and the pair (electron and positron) will annihilate (destroy) each other and two gamma photons are emitted. The two gamma photons are in opposite directions with the same energy (in accordance with the Law of Conservation of Momentum). Detectors around the patient can pick up the line along which the two gamma photons move and determine the position of the source of the gamma rays. PET scans are used mainly in biochemical and metabolism related studies and they produce superior brain images. It employs short half-life radio-nuclides so that the activity is kept to a minimum for patient safety. Because of their short half-lives, the radioisotopes are usually made in a small particle accelerator called a cyclotron located within a hospital complex. PET scans are not only used for the detection of cancers but are a diagnostic tool in investigating blood flow, heart disease and brain injuries, and they are also being used to investigate Alzheimer’s disease and other forms of dementia. Reflection It is about 120 years since X-rays were discovered. Modern medicine has many methods for looking inside the bodies of people who are unwell or have suffered injuries. Use the internet to find as many different methods as you can. Try and draw a timeline to show when these methods were developed. What did you learn about yourself as you worked on this activity? Did you find it a useful way of learning?
