Production of X-rays and X-ray Imaging PDF

Production of X-rays and X-ray Imaging BME 229 Fall 2024 1  X-rays X-rays are electromagnetic radiation with short wavelengths – Wavelengths less than those for ultraviolet – Wavelengths are typically about 0.1 nm –...

Production of X-rays and X-ray Imaging BME 229 Fall 2024 1  X-rays X-rays are electromagnetic radiation with short wavelengths – Wavelengths less than those for ultraviolet – Wavelengths are typically about 0.1 nm – X-rays have the ability to penetrate most materials with relative ease Discovered and named by Roentgen in 1895 The first X-ray image of a human hand taken by Roentgen (believed to be of Roentgen’s wife). 3 Electromagnetic Waves X-rays are electromagnetic waves v  = = v T f 4  X-rays Production X-rays are produced during interactions of energetic electrons with atoms in a medium. An atom is a positively charged nucleus surrounded by a cloud of electrons. An impinging electron can interact with either the nucleus or an electron (two types of interactions). The interactions of electrons with electrons and nuclei of a medium are termed “Coulombic interactions”. 5  Coulomb’s Force 0 is the permittivity of the free space  Production of X-rays X-rays are produced when high-speed electrons are suddenly slowed down – Can be caused by the electron striking a metal target A current in the filament causes electrons to be emitted These freed electrons are accelerated toward a dense metal target under a high electric field The target is held at a higher potential than the filament and rotates to reduce the heat Overview of the X-Ray Tube 9  Production of X-rays: Step by Step Electrons are produced by thermal emission in the cathode (filament). This is heated by a relatively low voltage supply. At a cathode current of 100 mA, for example, 6×1017 electrons travel from the cathode to the anode of the X-ray tube every second. They are accelerated from the cathode to anode across a high voltage (high electric field). As the kinetic energy of the electrons increases, both the intensity (number of X-rays) and the energy (their ability to penetrate matter) of the X-rays produced are increased. X-ray Production 10  Production of X-rays: Step by Step When these electrons bombard the heavy metal atoms of the target (anode), they interact with these atoms and transfer their kinetic energy to the target. These interactions occur within a very small depth of penetration into the target. As they occur, the electrons slow down (brake!) and finally come nearly to rest, at which time they can be conducted through the X-ray anode assembly and out into the associated electronic circuitry. 11  Production of X-rays: Step by Step The interactions result in the conversion of kinetic energy into thermal energy and electromagnetic energy in the form of X-rays. By far, most of the kinetic energy is converted into heat. The electrons interact with the outer-shell electrons of the target atoms, but do not transfer sufficient energy to these outer-shell electrons to ionize them. Rather, the outer-shell electrons are simply raised to an excited, or higher, energy level. 12  Production of X-rays: Step by Step The outer-shell electrons immediately drop back to their normal energy state with the emission of X-ray radiation. The constant excitation and restabilization of outer-shell electrons is responsible for the heat generated in the anodes of X-ray tubes. Generally, more than 99% of the kinetic energy of projectile electrons is converted to thermal energy, leaving less than 1% available for the production of X- ray radiation. In this sense, the X-ray machine is a very inefficient apparatus. The production of heat in the anode increases directly with increasing tube current. Doubling the tube current doubles the quantity of heat produced. 13  Cathode (Filament) Heating – Electric Current and Ohm’s Law Electric current is the rate of flow of charges through some region of space or a circuit The SI unit of current is the ampere (A) – 1A=1C/s The symbol for electric current is I Q I t 14  Ohm’s Law In a conductor, the voltage applied across the ends of the conductor is proportional to the current through the conductor V R= I The constant of proportionality is called the resistance of the conductor SI unit of resistance is ohms (Ω) – 1Ω=1V/A Schematic 18  Dissipated Power in a Resistor Filament is a resistor. Dissipated power is almost entirely converted to heat! P=VxI Here P is power in watts. V is voltage in volts. 21  Target Material Needs to have a high Z (proton number) so that transitions of high enough energy to emit X-ray radiation are possible Needs to have a high melting point because so much heat is produced Tungsten is an ideal target material (Molybdenum is usually used for lower energy X-rays needed for breast X-rays in mammography) 24  Production of X-rays: Physics An electron passes near a target nucleus The electron is deflected (scattered) from its path by its attraction to the nucleus – This produces an acceleration It will emit electromagnetic radiation when it is accelerated, called Bremsstrahlung (stopping radiation) Bremsstrahlung Radiation 25  X-ray Spectrum ⚫ When a metal target is bombarded by high-energy electrons, X-rays are emitted ⚫ The X-ray spectrum typically consists of a broad continuous spectrum (Bremsstrahlung) and a series of sharp lines ⚫ The lines are dependent on the metal and its atomic structure ⚫ The lines are called characteristic X-rays o = Cutoff wavelength hc o = eV Bremsstrahlung 26 Characteristic X-ray vs. Bremsstrahlung Characteristic X-rays is a result of electron interactions with electrons of the target atoms Bremsstrahlung is a result of electron interactions with nuclei of the target atoms 27 X-ray Minimum (cut-off) Wavelength 0 or min hc o = eV Where h is the Planck’s constant; c is the speed of light in vacuum; e is the charge of an electron; and V is the tube voltage. 28  The Photon Energy Formula 1: E [J] = h. f [Hz] Formula 2: E [keV] = 12.4 /  [A] 1 eV = 1.6022 × 10-19 J 1 Angstrom (A) = 10-10 m 29 Example Using both formula, calculate the photon energy for an X-ray radiation with the frequency of 30  1018 Hz? Answer: E = 1.9878  10-14 J = 124,068.78 eV  124.07 keV. 30 X-ray Imaging 31  Medical Imaging Systems Main medical imaging modalities: X-ray radiology Ionizing 1. Projection radiography 2. Computed tomography (CT) Radiation Nuclear medicine 1. Conventional radionuclide imaging (scintigraphy) 2. Single-photon emission computed tomography (SPECT) 3. Positron emission tomography (PET) Magnetic resonance imaging (MRI) Ultrasound Etc. 33  Projection Radiography It creates a 2D projected image of a 3D object (body). The x-ray radiation has a 3D cone beam shape. Image is formed based on variations in x-ray attenuation transmitting through various parts (organs) of body. Various projection radiography systems: ✓ Chest x-ray ✓ Fluoroscopy ✓ Mammography ✓ Digital radiography ✓ Angiography ✓ Neuroradiology ✓ Etc. 34  Taking an X-ray The generation of an X-ray beam The interaction of the beam with bone wrist_hand_x_ray and soft tissues of a patient The formation of an image 35  Taking an X-ray Filter stops all useless Collimator X-rays, shapes the thus lowers X-rays beam the wrist_hand_x_ray patient’s dose Grid lets through only the X-rays that 36 still travel in their original directions  The Interaction of the Beam with Bone and Soft Tissues of a Patient The X-rays (photons) entering the patient’s body will be either – Absorbed (adsorbed) Due to a photoelectric effect – Scattered Due to a Compton effect – Attenuated Absorbed and scattered X-rays do not contribute to the formation of an X-ray image, since they are removed from the beam Adsorption: Absorption at the surface of the material. 37  The Interaction of the Beam with Bone and Soft Tissues of a Patient The attenuation of X-rays in the The attenuation can be body depends on: defined as: – The thickness of material the beam will go through I = I o exp( − x) – The density of material  I = I o exp( −( ) x) – The chemical properties of  Where: material (Z) I (or N) is the number of photons exiting from material I0 (or N0) is the number of photons entering into material  is the linear attenuation coefficient / is the mass attenuation coefficient  is density of material x is the thickness of material 38 Quiz 5.5 40  The Interaction of the Beam with Bone and Soft Tissues of a Patient Bone, being more dense and made of higher Z materials (Ca, P), will attenuate more incoming X-rays than the soft tissue, which is made of a lower Z material (H, O) and has lower density As a result, bone will produce the higher contrast shadow (light gray/white) on the image, while the soft tissue will be recorded as a darker gray shadow 41  Formation of an X-ray Image (Radiographic Film) wrist_x-ray Radiographic film is an emulsion of dry gelatin crystal made of silver and bromide ions (Br−) The X-ray reaching the film will deposit energy in a single grain of silver bromide and “sensitize” it Wrist X-ray During the film development sensitized silver wrist_hand_x_ray bromides will lose bromide ion and transform into a silver flacks Not sensitized silver bromide grains are removed from the film during the fixation phase of the film development. The film will be more Wrist and opaque to light there and this appears darker to hand X-ray the eye 42 Medical X-ray Imaging: Pros and Cons Pros – Fast – Not expensive Cons – Ionizing radiation involved – radiation dose – Image quality is moderate - Image quality = Resolution, Contrast, and SNR – Low blood vessel and soft-tissue contrasts 43  Mammography Mammography is an X-ray based imaging modality adopted for the imaging of breast pathology and cancer, by the addition of a breast compression device and the use of lower energy X-rays. Mammogram must reveal subtle differences between tissues with similar densities (tumor and soft tissue), as well as a presence of calcifications (bone like materials) in the breast. 44  Breast Calcification Mammography_adeno_small Breast calcifications are calcium deposits within breast tissue. They appear as white spots or flecks on a mammogram and are usually so small that one cannot feel them. Although breast calcifications are usually noncancerous (benign), certain patterns of calcifications, such as tight clusters with irregular shapes, may indicate breast cancer. A breast calcification 45  Benign and Malignant Tumors Malignant Tumor (Cancerous) By definition, a malignant tumor has three properties: 1- It grows in an unlimited, uncontrolled and aggressive manner. 2- It invades surrounding tissues. 3- It is metastasized. Benign Tumor (Non-cancerous) A group of abnormal cells that has grown out of control, but lacks the other two malignant properties of a cancer. Some benign tumours may change over time and become malignant! 46  Cancer Terminology Hyperplasia: The process through which body makes extra cells in organs or tissue. This process is usually completely normal, or at least not malignant, i.e. it is benign. Adenoma: A benign (non-malignant) tumor made up of glandular tissue. Carcinoma: Most common form of cancer characterized by the growth of a malignant tumor in surface tissues of an organ or on the skin. 47 Quiz 5.6 48  Mammography - History German surgeon, Dr. Salomon performed first breast radiographs; By the late 1920s mammography was performed in US at a number of locations; 1953 – Dr. Raul Leborgne described a number of innovations related to breast compression and use of very low energy radiation (first time a relation between microcalcification and breast cancer was established); Development of x-ray grids and screen film for mammography Development of modern digital mammography 50  Mammography It is made of – X-ray generator – Brest compressing paddle – Grid – Cassette/film Mammogram image must have: – High spatial resolution – High contrast – Low noise – Low dose 51  Mammography Use of very low energy (20-30 keV) X-rays can enhance the contrast among the radiologically similar tissues of breast; The low energy X-ray are more attenuated, and thus penetrate less in the tissue, but they provide better relative difference in attenuation between different soft tissues (tumor vs. normal breast tissue); By compressing the breast between the pair of parallel paddles, a uniform tissue thickness is achieved for better visualization; 52 Mammogram Content 53 Quiz 5.7 54  Breast Cancer in Canada In 2020, there was an estimated 27,400 Canadian women diagnosed with breast cancer. One in 9 Canadian women is expected to develop breast cancer during her lifetime, and one in 30 Canadian women will die from breast cancer. The second leading cause of cancer deaths in Canadian women, after lung cancer. 90% of breast cancer arises in cells lining the ductal system of the breast - ductal carcinomas; 56  Mammography - Pros Randomized trials demonstrate that mammography screening may lead to a relative risk reduction in breast cancer mortality by 20-30%, depending on age; Mammography finds cancers at an earlier stage, prior to when they would be detectable with a self examination; Smaller cancers are less likely to metastasize, thus prognosis for a small tumor is better; There is no other technology that has been shown to systematically find breast tumors at any earlier stage; Many developed countries have adopted and implemented national screening programs; 57  Mammography - Cons Mammography does not find all breast cancers; Sensitivity is 80-95% depending on age. Mammography has a relatively high false positive rate; Mammography is resource intensive, and the quality varies depending on implementation; Finding cancers early does not guarantee that the tumor will be curable; 58  Mammography - Cons Mammography has reduced mortality rate by up to 30% → Gold standard for screening BUT, problems remain: The false negative problem ~15% false negative rate → missed cancers Clinical Solution: Improving sensitivity The false positive problem ~80% diagnostic false positive rate → high biopsy rate Clinical solution: Improving specificity Treatment monitoring problem Monitoring with imaging is difficult / expensive Clinical solution: Safe, inexpensive whole breast imaging 59 Detection Theory and Classification Model True Positive / True Negative False Positive / False Negative Detection theory and Test Result Table classification model Two-class prediction problem (binary classification) There are four possible outcomes from a binary classifier 60  Sensitivity and Specificity of an Imaging Modality Sensitivity = [ A / (A+C) ] ×100 61 Specificity = [ D / (D+B) ] ×100 Quiz 5.8 62  Digital Mammography Digital mammography is a specialized form of mammography that uses digital receptors and computers instead of X-ray film to help examine breast tissue for breast cancer. Advantages: 1- Make use of lower X-ray dosage to get the same image contrast and resolution 2- High patient throughput 3- Use of advanced digital image processing techniques to further enhance mammograms 65 X-ray Image Resolution and Contrast Image Contrast: The ability of the imaging modality, e.g. x-ray imaging, to differentiate different tissue types. Examples: Muscle-bone contrast, or muscle-fat contrast. Image Resolution: Spatial Resolution: The ability of the imaging modality to display, as two separate images, two point (small) objects that are very close together. Temporal Resolution: The ability of an imaging modality to track and image fast-moving targets. Imaging frame rates of equal to or above 24 fps are considered real time in medical applications. 66 Typical Medical Imaging Spatial Resolutions 67 X-ray Fluoroscopy seated Fluoroscopy 68 X-ray Fluoroscopy – Basic Operation Live image = Real-time image (~24 fps) 69  Modern Fluoroscopy with Image Intensifier 70 Fluoroscopy : Xray intensifier tube Fluoroscopy – System Components 71  Image Intensifier Convert x-ray photons to light photons and intensify it. 72 Quiz 5.9 73 X-ray Computed Tomography (CT) “Tomography” : an image from many slices (Greek: tomos = slice) Digital geometry processing is used to generate a three- dimensional image of the inside of the body from a large series of two-dimensional X-ray images taken as slices around a single axis of rotation. http://www.stlukeshouston.com/OurServices/DITR/images/Slices_Compare.jpg Video 1 Video 2 75 X-ray Computed Tomography (CT) 76  CT Principle of Operation Individual slice data is generated using an X-ray source that rotates around the patient; X-ray sensors are positioned on the opposite side of the circle from the X-ray source. These sensors use scintillation systems based on photo diodes. Many data scans are progressively taken as the body is gradually passed through the gantry. The images are combined together by mathematical procedures known as tomographic reconstruction. Contrast within soft tissues can be increased by injection of iodine-based solutions. 77  Basic CT Scanner Components Gantry X-ray tube Detector Control console 78  CT - History EMI Ltd., UK Tufts University, Boston 80 CT Generations 81 CT Generations 82 CT Generations Helical (Spiral) Scanning (a.k.a. Volume Scanning) 83  CT Detectors More detectors and smaller detectors to give better resolution Enhanced image data can be reconstructed through algorithms 84  CT Image Resolution Specifications First CT Scanner Modern CT Scanner (circa 1970) (2009) Time to acquire one CT image: 5 minutes 0.5 seconds Pixel size: 3 mm x 3 mm 0.5 mm x 0.5 mm Number of pixels in an image: 6,400 256,000 Early scan, ~1975 Scan, ~2003 Scan, ~2009 85  Radiation Dose and Risk of the X-ray Imaging Risk can be defined as a possibility that something bad (harmful) may happen and it takes into account both the probability of occurrence and the extent (severity) of potential harm. Risk of exposure to any diagnostic modality can be defined as the product of radiation dose and the risk of being exposed to the radiation per unit dose, or Risk = (Dose) × (Risk/Dose) 86  Radiation Dose and Risk of the X-ray Imaging Risk = (Dose) × (Risk/Dose) The generally accepted estimation of the average Risk/Dose for lethal cancers is around 5×10-4 per rem. This suggests that if 10,000 people receive a uniform, whole body dose of one rem (or 0.01 Sv), five of them will die prematurely because of a cancer induced by the radiation. [Remember: 1 rem = 0.01 Sv or 1 Sv = 100 rem] 87  Radiation Dose and Risk of the X-ray Imaging Number Radiation Tissue Diagnostic of dose weighting modality: images: (rem) factor: Risk: CT-scan - Upper GI: (Interventional Fluoroscopy) 1 245 0.12 1.5x10-2 CT-scan - Chest: 1 6 0.05 1.5x10-4 Mammography 2 0.15 0.05 7.5x10-6 88  Risks of Diagnostic Radiology More than 350 million medical and about 200 million dental x-ray examinations are performed each year in the United States. About 20 million doses of radiopharmaceuticals are administered each year in the United States. A radiopharmaceutical is a radioactive isotope that contains radioactive material combined with chemical or biological materials. They are used in various nuclear medicine or radiotherapy procedures. The radiation doses involved in diagnostic radiology, except for interventional procedures, do not result in deterministic effects. The risks are mostly stochastic effects, i.e. carcinogenesis and hereditary effects. Some of the largest doses in diagnostic radiology are associated with fluoroscopy and CT scan procedures. 90