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

2. 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

3. 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…)

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

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

6. _______________________________________________ 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

7. 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

8. 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

9. 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

10. 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. 3D information is reduced to 2D (image) projection Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ X-RAY

11. 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

12. 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 3D imaging

13. 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 _______________________________________________ ANGIOGRAPHY

14. 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)

15. 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 and an image produced by a gamma camera Radionuclide bone scan

16. 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)

17. 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

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

19. 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)

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

21. 3D 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 ________________________________________________

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

23. _______________________________________________ MR image of left ventricle

24. Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ Visible Human color cryosection data

25. Sources of Images (3D) Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ 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

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

27. 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 cross- sectional image of it can be computed (reconstructed)

28. 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

29. 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

30. 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

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

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

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

34. 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)

35. 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

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

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

38. 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 3D reconstruction)

39. 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)

40. 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

41. 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)

42. 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 and an image representing proton density (PD) can be computed

43. Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ MRI – T1 and T2 weighted images Two relaxation times can be also computed, T1 and T2: T1 depends on the time protons take to return to the axis of the magnetic field T2 depends on the time the protons take to dephase

44. Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ MRI – T1 and T2 weighted images T1-weighted image T2-weighted image

45. Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________ MRI – Pathological Processes Clinical diagnosis is made based on changes in proton density in T1 and T2 relaxation times In pathological processes: increased number of mobile protons (more signal) prolongation of relaxation times (reduced signal intensity on T1-weighted image – blacker) increases signal intensity on T2-weighted image (whiter)

46. 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)

47. Understanding Visual Information: Technical, Cognitive and Social Factors _______________________________________________