Medical Image Modalities Celina Imielinska Ph D Columbia

Medical Image Modalities Celina Imielinska, Ph. D Columbia University ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

The study of medical imaging is concerned with the interaction of all forms of radiation with tissue and the development of appropriate technology to extract clinically useful information (usually displayed in an image format) from observation of this technology ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Classical medical imaging: utilizes images that are direct manifestation of the interaction of radiation with tissue (conventional: X-ray, nuclear medicine, medical ultrasound) Contemporary medical imaging: two-part process: (1) the collection of data concerning the interaction of some form of radiation with tissue (2) The transformation of these data into an image (or a set of images) using specific computational tools (X-ray CT, SPECT, MR, NMR…) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Classical medical imaging: direct and intuitive Contemporary Medical imaging: indirect and often counterintuitive ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Sources of Images • Structural/anatomical information (CT, MRI, US, VH) - within each elemental volume, tissuedifferentiating properties are measured. • Information about function (PET, SPECT, f. MRI). _______________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY Nov 8, 1895 - Wilhelm Konrad Röntgen reported discovery of new “rays” (Nobel Prize in physics in 1901) Jan 23, 1896 - his paper translation appeared in the Nature Journal within 1 year- over 1000 publications on the “Röntgen rays” were published ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY • X-ray production: a stream of electrons is accelerated by applying a high voltage between cathode and anode metal shielding around X-ray tubes absorbes unwanted X-rays and a beam of X-rays emerges ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY X-ray images: • a beam of X-rays is directed through a patient onto a film • the resulting unabsorbed X-rays cause blackening of a photographic film • the developed film provides a shadow image of the patient • an image provide some measure of the attenuation of X-ray in tissue ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY X-ray image is dependent on different absorption level by: • calcified structures • soft tissue • fat • gas ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY X-ray images are not quantitative only qualitative: • the perceived sizes of image structures are distorted by X-ray divergence from a point source (differential magnification factor). Measurement of size and distance is not accurate. 3 D information is reduced to 2 D (image) projection ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY Projection: • Postero-anterior (PA) - beam traverses from the back to the front of patient’s body • Antero-posterior (AP) – the beam passes from the front to the back (patient’s back is against the film ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY Stereoscopic Radiography (1897): • acquire a pair of views from radiograph that can be visualized in a standard stereoscope ( stereoscopic photographs were popular since mid-XIX century) • only qualitative impression is possible (an illusion of feeling of depth) • abandoned with an introduction of 3 D imaging ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

ANGIOGRAPHY • in use since the early X-rays • a contract agent injected into blood vessels to increase their visibility against surrounding tissue • Substraction Angiography (1963): - obscuring (bony) structures could be removed from the image by adding postcontrast image to a negative of the precontrast radiograph (vessels in the brain) • Digital Substraction Angiography (1980) (DSA) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

X-RAY Radiation Hazards: • X-ray cause ionization and alter molecules in tissue • induction of cancer and genetic effects • avoid X-ray during pregnancy • use alternative imaging (MRI, ultrasound) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

NUCLEAR MEDICINE • a radioactive material is injected and its course is followed by a detector • radionuclides can be tagged to certain substances which concentrate in different parts in the body • radionuclides emit gamma radiation which can be detected an image produced by a gamma camera Radionuclide bone scan ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

NUCLEAR MEDICINE • Image has poor spatial resolution but it provides a measure of physiological function from the time of the radioisotope uptake • while X-ray visualizes the structure, nuclear visualizes physiological function or functional metabolism • same technology used in SPECT (single photon emission computed tomography) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

ULTRASOUND • a pulse of ultrasonic energy is propagated into the patient from a transducer placed on the skin and back-scattered echo signal is recorded by the same transducer • the image is constructed from the backscattered echo strength ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

ULTRASOUND Structures as fat, bone, Structures with watery calcification (stone) and fluid (cyst) transmit gas reflect a high propagation ultrasound very well of the ultrasound beam, appearing (acoustic enhancement) echogenic (acoustic shadowing) (No echo from the cyst) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

ULTRASOUND DRAWBACKS: • gas filled and bony structures cannot be imaged (they absorb ultrasound beam) • US can’t capture tissue-gas and tissue-bone boundary • never used in lung or bone pathology ADVANTAGES: • good for cysts/ cystic structures and fetus in its amniotic fluids • good for two structures with a large difference in acoustic impedance (liver metastasis) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Introduction of CT (1970 s) transformed 2 D qualitative imaging into a quantitative 3 D format ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

3 D Imaging • All processing operations that are applied to acquired multidimensional data to facilitate visualization, manipulation and analysis. ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Sources of Images (2 D) • • • Digital Radiography CT - computer tomography MRI - magnetic resonanse imaging PET - positron emission tomography SPECT - single photon emission computed tomography • US - (2 D) ultrasound • f. MRI - functional MRI • VH - Visible Human data _______________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MR image of left ventricle ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Visible Human color cryosection data ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Sources of Images (3 D) • A time sequence of radiographic images or tomographic slice images of a dynamic object • a volume of tomographic slice images of a static object • a volume of Visible Human data set ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Sources of Images (4 D) • A time sequence of tomographic volume images of a dynamic object it is not feasible at present to acquire 4 D images to capture dynamics (approximations are made o capture “stop action”) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT (1972) – Modern Medical Imaging • G. Hounsfield (computer expert) and A. M Cormack (physicist) (Nobel Prize in Medicine in 1979) • CT overcomes limitations of plain radiography • CT doesn’t superimpose structures (like X-ray) • CT is an imaging based on a mathematical formalism that states that if an object is viewed from a number of different angles than a crosssectional image of it can be computed (reconstructed) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

Stages of construction of a voxel dataset from CT data (a) CT data capture works by taking many one dimensional projections through a slice (scanning) (b) CT reconstruction pipeline ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT CT is the beginning of quantitative imaging • X-rays are focused into a thin beam that only passes “through” a slice of tissue • the beam strikes very sensitive detectors which can quantify subtle differences in tissue density • relative (radiation) transmission value is a function of the attenuation of the X-ray by a tissue ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT – DATA ACQUISITION Slice-by-slice acquisition • X-ray tube is rotating around patient to acquire a slice • patient is moved to acquire the next slice Volume acquisition • X-ray tube is moving continuously along a spiral (helical) path and the data is acquired continuously ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT – DATA ACQUISITION (a) slice-by-slice scanning (b) Spiral (volume) scanning ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT – SPIRAL (HELICAL) SCANNING • a patient is moved 10 mm/s (24 cm / single scan) • slice thickness: 1 mm-1 cm • faster than slice-by-slice CT • no shifting of anatomical structures • slice can be reconstructed with an arbitrary orientation with (a single breath) volume ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT – SPIRAL (HELICAL) SCANNING 3 D reconstruction of the facial bones. (greater accuracy in multiplanar 3 D reconstruction) 3 D reconstruction of pulmonary vascular system ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT – MULTISLICE SYSTEM • parallel system of detectors • 4/8/16 slices at a time • generates a large data of thin slices • better spatial resolution ( better reconstruction) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT - DATA PROCESSING CT numbers (Hounsfield units) HU: • computed via reconstruction algorithm (~tissue density/ X-ray absorption) • most attenuation (bone) • least attenuation (air) • blood/calcium/contract increases tissue density ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT - DATA PROCESSING Relationship between CT numbers and brightness level ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT - IMAGE DISPLAY Human eye can perceive only a limited range gray-scale values Thoracic image: width 400 HU/level 40 HU (no lung detail is seen) width 1000 HU/level – 700 HU (lung detail is well seen; bone and soft tissue detail is Lost) window level and window width: the center of the range and the range of density, respectively (the smaller range of densities, the greatest contract) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

CT - DRAWBACKS • radiation • despite high hopes, reliable discrimination between normal and pathological tissues using CT number was unsuccessful • CT of brain inferior to MRI: contrast between white and gray matter is of order of 0. 5%, while a typical reconstruction algorithm can’t distinguish between the two • patient movements during the scan contribute to artefacts ( distortions in 3 D reconstruction) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – Magnetic Resonance Imaging • nuclear magnetic resonance (discovered by F. Bloch and E. Purcel, extended by R. Ernst) (Bloch & Purcel: Nobel Prize in Physics in 1952) • Kumar, Welti, and Ernst (1975) formed computational basis of modern MRI (Ernst: Nobel Prize in Chemistry in 1991) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – Magnetic Resonance Imaging • principal modality for image guided surgery • depends on magnetic properties of the nuclei of certain elements superb ability to discriminate between subtle differences in tissue characteristics ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – SCANNER • large strong magnet • radiofrequency transmitter and receiver coils • gradient coils to allow spatial localization of the MRI signal • ancillary equipment to convert radio signal into a digital form ( computer image) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – Magnetic Resonance Imaging • hydrogen nuclei (protons) under a strong magnetic field spin (in phase with one another) and align with the field • protons are present in large numbers in water molecules and lipids • realigned (relaxed) protons induce a (measurable) radio signal • the location of the radio signal can be detected an image representing proton density (PD) can be computed ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – T 1 and T 2 weighted images Two relaxation times can be also computed, T 1 and T 2: • T 1 depends on the time protons take to return to the axis of the magnetic field • T 2 depends on the time the protons take to dephase ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – T 1 and T 2 weighted images T 1 -weighted image T 2 -weighted image ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – Pathological Processes Clinical diagnosis is made based on changes in proton density in T 1 and T 2 relaxation times In pathological processes: • increased number of mobile protons (more signal) • prolongation of relaxation times (reduced signal intensity on T 1 -weighted image – blacker) • increases signal intensity on T 2 -weighted image (whiter) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

MRI – Magnetic Resonance Imaging To be used for: • brain & spinal cord (superior to CT) • MRA (Magnetic Resonance Angiography) with an injected contrast Drawbacks: • deviations from ideal homogeneous magnetic field result in geometric distortions of the images • artefacts caused by movements during the scan • signal voids occur in gas, bone and calcification (hydrogen proton are either absent or can’t generate a signal) ________________________ Understanding Visual Information: Technical, Cognitive and Social Factors

________________________ Understanding Visual Information: Technical, Cognitive and Social Factors