Imaging Technologies PDF

IMAGING TECHNOLOGIES Prof. Kawal Rhode Department of Surgical & Interventional Engineering School of Biomedical Engineering & Imaging Sciences King’s College London  Learning Objectives  Describe the fundamental principles and terminology of di...

IMAGING TECHNOLOGIES Prof. Kawal Rhode Department of Surgical & Interventional Engineering School of Biomedical Engineering & Imaging Sciences King’s College London  Learning Objectives  Describe the fundamental principles and terminology of digital imaging  Relatethe brief history and basic physical principles of X-ray imaging and X-ray Computed Tomography (CT)  Understandthe brief history and basic physical principles of: Nuclear Medicine Imaging, Ultrasound Imaging, Magnetic Resonance Imaging (MRI)  Differentiate between images taken with different imaging modalities  What is Medical Imaging?  Wikipedia:  “Medical imaging is the technique and process used to create images of the human body (or parts and function thereof) for clinical purposes (medical procedures seeking to reveal, diagnose, or examine disease) or medical science (including the study of normal anatomy and physiology). Although imaging of removed organs and tissues can be performed for medical reasons, such procedures are not usually referred to as medical imaging, but rather are a part of pathology.”  Medical imaging is used for much more than diagnosis:  Studying normal populations  Treatment – selection, planning, guidance, monitoring  Scientific research in humans and animals  Why is Medical Imaging Important?  Medical imaging is the cornerstone of medical diagnosis and treatment  Medical imaging has transformed the practice of medicine over the last 100 years Invasive, high infection risk Minimally-invasive, low infection risk, go home same day Examples of Medical Images X-ray, abdomen Images of the same anatomy can appear MRI, heart very different depending on how they were acquired US, liver All of these are digital PET, head images … CT, (OK, not strictly medical …)  What is a Digital Image? Digital images are acquired by an imaging device Overlay a grid, assign a number (usually an integer) to each grid position …  Some Terminology … Pixels (picture elements) are the grid 63 85 49 … elements Image resolution / matrix size / pixel count is often used to refer to the number of pixels along each axis Intensity/grey-scale is the number for a pixel Intensity/grey-scale resolution is the range of possible intensity values Spatial resolution is a measure of the smallest discernible detail in an image  Pixels and Voxels Many medical images are 3D, i.e. stacks of 2D images … E.g. some MRI images, CT images, some US images In 3-D images, the pixels are known as voxels (volume elements) Because medical images represent real things, the pixels/voxels have a size Pixel/voxel size normally specified in mm In most cases (but definitely not all) 2D (x, y) dimensions are the same. The third (z) dimension is usually larger, e.g. slice thickness in CT  Image File Types – There are many different image file formats: Compressed/non-compressed Lossy/lossless – E.g.JPG, BMP , GIF, TIFF – But, for medical imaging the most common is DICOM (Digital Imaging and Communications in Medicine) DICOM is a non-compressed file format – Normally lossy compression file formats should not be used to store medical images  Visualisation Techniques – Two main techniques: Slice mode Volume mode (images from 3d-doctor.com)  1895 Projective X-Rays William Crookes Crookes’ Tube 1832 – 1919 Wilhelm Röntgen 1845 – 1923  Covered Crookes’ tube with cardboard but still got fluorescence from a barium platinocyanide Discovered X-ray in screen 1895  Noticed that he could make shadows of his Nobel Prize Physics hand on the screen using the “X-rays” 1901  Replaced the platinocyanide screen with film  Projective X-Rays  Projective X-ray imaging was widely used from 1895 onwards  First reported clinical use was for the removal of a needle from a patient’s hand in 1896  Projective X-ray imaging is still the mostly widely used form of medical imaging Fluoroscopy 1955  Modern X-ray Systems Mobile C-arm X-ray Fixed C-arm Single Plane Fixed X-ray Room Portable X-ray Robotic C-arm Single Plane Fixed C-arm Bi Plane  Foundations for Computed Tomography  Problems with projective X-rays  All structures are superimposed  Cannot know the depth of objects  Radon worked out a method for reconstruction of a 2D function from projections Johann Radon 1887 – 1956 Radon Transform 1917  Computed Tomography First practical realisation of CT Hounsfield & Cormack Practical CT in 1972 Nobel Prize Physiology/Medicine 1979 Oldendorf ? Modern Systems are Generation 3 Helical MDCT = helical multidetector CT  Discovery of Radioactivity  Discovery of radioactivity by Becquerel 1896  Extensive research on radioactivity by the Curies 1896- Nobel Prize Physics 1903 Henri Becquerel 1852 – 1908 Pierre Curie Marie Curie 1859 – 1906 1867 – 1934 Discovered Radioactivity Extensive Research on Radioactivity 1896 1896- Types of Emissions Beta + Decay  Positron Annihilation  F-18 is a commonly used beta+ emitter  It has a half-life of 110 mins  It is given as fluorodeoxyglucose (FDG)  Isomeric Transition  Tc-99m is a pure gamma emitter  It has a half-life of 110 mins  It is widely used for nuclear medicine imaging  Imaging in Nuclear Medicine  First image of thyroid using radioactive iodine 1948  Ansell & Rotblat  Widespread availability of 99Tcm – gamma emitter 1947  Gamma camera concept 1949  Copeland & Benjamin  Practical gamma camera 1952  Anger  3D Imaging in Nuclear Medicine  Positron Emission Tomography (PET) 1953  Use ring of detectors to count co-incidental gamma photons  Brownell & Sweet  Laboratory demonstration of Single Photon Emission Tomography (SPECT)1963  Similar to CT; rotate gamma camera around the patient  Kuhl & Edwards F-18 PET Cardiac SPECT  Modern NM Imaging Systems Siemens Biograph mCT These are combined PET-CT systems PET-MR systems are also Philips available Ingenuity TF PET-CT  Foundations for Magnetic Resonance Imaging (MRI)  Nuclear magnetic resonance (NMR) discovered 1948  Bloch & Purcell (independently)  Nobel Prize Physics 1952  Damadian patents the idea of a NMR scan 1972 Raymond Damadian 1936 – Invented MRI 1972  Magnetic Resonance Imaging Mansfield & Lauterbur Implemented MRI in 1974 Nobel Prize Physiology/Medicine 2003 Brain MRI Cardiac MRI Components of MR Scanner Magnet Gradient coils RF coils  Nuclear Magnetic Resonance  Sample placed in a strong magnetic field  Sample irradiated with radio waves Signal at resonance frequency RF N S  Sample re-emits radio waves  Emitted signal can be analysed to yield information about the sample  A standard method for chemical analysis and non-destructive testing  MRI involves localising the signal using magnetic field gradients to form an image  The Hydrogen Nucleus  The most common isotope of hydrogen (1H) has the simplest nucleus of all – consisting of a single proton.  1H also has the highest NMR sensitivity, has high natural abundance (99.98%) and is present in the body in large quantities, mainly in water (H2O).  Therefore almost all biomedical MRI makes use of 1H nuclei, and specifically those located in water molecules.  Foundations for Ultrasound Imaging  Langevin invents the hydrophone for locating icebergs 1915  Pulse-echo principle  Followed Titanic disaster in 1912  Later used for submarine detection in WWI & II  Patent for metal defect detection 1940  Firestone  First transmission ultrasound image of a human 1942  Dussik Paul Langevin 1872 – 1946 Invented hydrophone 1915  Ultrasound Imaging  2D (or B-mode) imaging 1957  Wild & Reid  Obstetric ultrasound 1958  Donald  3D imaging 1987  von Ramm & Smith  Echo Ranging The time between emitting a pulse and receiving a response tells us how far away the pulse was reflected Assumed speed of sound (average soft tissue) is 1540 m/s or 154000 cm/s 154000  t d Ultrasound transducer 2 e.g. t = 26 μs d = 2 cm d  Imaging with Echoes Ultrasound imaging detects waves reflected from tissue structures The wave energy is gradually used up as it travels through the tissue Tissue properties determine the amount of reflected energy at each point Ultrasound transducer Tissue 1 Tissue 2 Tissue 3  Modern Ultrasound Systems  More compact  On-board image analysis  Faster Imaging 3D or 2D Spatial Soft Tissue Ionising Contrast Agents Artifacts Interventional / Notes Modality Resolution Contrast Radiation Dynamic Imaging Doppler for blood Ultrasound 2D, 3D +++ ++ NA Microbubbles +++ +++ flow Fast acquisition CT fluoroscopy CT 3D +++ +++ +++ Iodine, Barium + ++ can be used for interventional Fast acquisition Different MRI 3D ++ +++ NA Gadolinium ++ + sequences give different contrast, e.g. T1 / T2 Slow to acquire Radiography is Planar X-ray 2D +++ + ++ Iodine, Barium NA +++ static and fluoroscopy is dynamic Fast acquisition Endoscopy / Optical 2D, 3D +++ NA NA Fluorescent dyes NA +++ Laparoscopy Imaging Fast acquisition Mainly used in Nuclear 2D, 3D + NA + Radiotracers + + oncology Medicine Slow to acquire Imaging Relative Properties and Features of Different Imaging Modalities  Reading Material Webb’s Physics of Medical Imaging Edited by MA Flower Second Edition 2012 CRC Press ISBN 9780750305730