C1 Medical Imaging Modalities & Characteristics Linwei Wang

Transcription

1 C1 Medical Imaging Modalities & Characteristics Linwei Wang

2 Major Types of Medical Imaging Modalities X-ray Imaging Computed Tomography (CT) Magnetic Resonance Imaging (MRI) Nuclear Imaging Positron Emission Tomography (PET) Ultrasound Imaging

3 X-Ray Photography Discovered by Wilhelm Roentgen, 1895 A cathode tube exposes paper coated with a barium compound placed at some distance away

4 X-Ray Photography Physical basis: X-rays are generated in a special tube where a beam of electrons is accelerated by high voltage, hits a metal disk, and the kinetic energy is tranformed into quanta of emitted X-rays

5 X-Ray Photography Acquisition principle: X-ray is weakened after passing through tissues The type of tissues Traveling distances Interaction types The Compton effect: main factor Meets electron. Energy reduces. Direction changes Raleigh scattering Meets atoms. Direction changes. No energy loss The photoelectric effect Photon eject a photoelectron, which is absorbed by atom! I = I 0 e "µx Distance Radiodensity (absorbance)

6 Radiodensity depends on the type of substance through which the radiation passes Expressed using the Hounsfield scale Reference value 0: water Weaker absorbance: negative down to (e.g., air in the lungs..) Greater absorbance: positive up to 1000 (e.g., bones) X-Ray Photography

7 X-Ray Photography When X-rays pass through the examined patient s body, depending on the organs and tissue lying along the path, different levels of energy reach the X-ray detector Tissues with a low Hounsfield scale: film darkens or CCD detector is excited Tissues with high Hounsfield scales: light spots on the film or detector returns a weak signal What you see on the acquired medical images are shadows of the organs through which the radiation had to pass, and the more cohesive and dense the organ, the brighter its image Add fig 2.6

8 X-Ray Photography Modern X-ray machine X-ray tube High voltage generator Detector Specialized examination Mammography Detal photographs Chest and heart diagnostics Skeletal system diagnostics

9 X-Ray Photography Modern detector can measure the differences in radiodensity of single Hounsfield units or even fractions Digital X-ray images consists of pixels represented by bits so the brightness can take one of 4,096 or even 65, 536 discrete levels Human can only differentiate 60 grey levels The portion of information that doctors can use to that obtained by X-ray imaging: 1:1000 Window functionality: cut the interval and rescale it to 256 grey levels L: middle level of the window (e.g., 0-30HU, or 30-60HU for soft tissues; HU for lungs) W: width of the window (e.g., HU for soft tissues, HU for lungs)

10 X-Ray Photography X-ray images of the same body parts may look different depending on how hard the radiation generated was The hardness or penetrating ability of X-rays is adjusted by selecting the voltage acceleration electrons in the tube Skeleton: 70 kv; chest: 50 kv; soft tissues: kv Contrast medium and subtraction graphs (angiography)

11 X-Ray Photography Details of structure (noninvasive) Organs obscure one another X-ray ionize biological tissues Potential damage and serious diseases Should be limited to the indispensible minimum

12 Computed Tomography Probing the patient many times with an appropriately shaped beam of X-rays When the beam passes the examined tissues, it gets weaker and its final intensity is measured by the detector The weakening of the beam (the radiation absorption) along the path to the detector depends on the type of substances For a single probe, we get the summary effect of beam weakening by all tissues It is possible to reconstruct the radiation absorption by a specific point on the cross-section as many images are collected It is possible to precisely identify the examined tissues and to reconstruct the shapes of organs

13 Computed Tomography Principles of CT Scan the same cross-section with sufficient images allows reconstruct of the anatomic details of the cross-section

14 Computed Tomography New generations of CT could acquire the necessary images within shorter and shorter times Adding more and more detectors

15 Computed Tomography New generations of CT could acquire the necessary images within shorter and shorter times Adding more and more detectors

16 Computed Tomography Further progress was achieved by spiral technique Possible to image the human body along any plane Volumetric / 3D CT

17 Computed Tomography Spiral scan vs. sequential (step) scan

18 Computed Tomography Mutli-row detector (vs. single-row detector)

19 Computed Tomography Key components of a CT apparatus Gantry: a ring-shaped element containing X-ray tubes and detectors Bed Computer The part of the examined body is divided into layers (cross section), every one of which consists of volume elements called voxels Series of cross-sections can be anlyazed as sequences of images on the basis of which the physician formulates the diagnosis, or by the computer to reconstruct a spatial image of the organ analyzed

20 Computed Tomography 3D reconstruction from volume dataset

21 Computed Tomography Tissue densities calculated by the CT apparatus are expressed on the Hounsfield scale Also represented with window CT images can show excellent anatomic details

22 Magnetic Resonance Imaging (MRI) Physical basis: some atomic nuclei have a spin and exhibit a magnetic moment Mainly uses hydrogen analysis: the generation of electromagnetic radiation by hydrogen atom nuclei (protons) in the organism is analyzed

23 Magnetic Resonance Imaging (MRI) Physical basis: some atomic nuclei have a spin and exhibit a magnetic moment Mainly uses hydrogen analysis: the generation of electromagnetic radiation by hydrogen atom nuclei (protons) in the organism is analyzed " Versus X-ray & CT: " MRI: Acquiring and recording a physical signal generated by the organ itself (the organs shine themselves, the source of radiation being the atomic nuclei of elements making up the molecules of the organs) " X-ray: the source of signals is external X-rays (the organs are lit up by external radiation, and we view their shadows)

24 MRI The natural atoms have to be appropriately excited Applies a strong magnetic field for imaging This field causes the nuclei of atoms to be aligned and spatially oriented When an external impulses comes to excite them, they respond with a harmonious tune, giving a clear and easily recordable signal revealing where they are abundant and where less. Manetic filed intensity: Telsa

25 MRI Principle of signal acquisition External B 0 field produces defined magnetization M Electromagentic impulse of a specific, defined frequency (Larmor frequency ω 0 ) is applied with direction perpendicular to M and B 0 A nuclear resonance occurs: the M vector transits to the plane perpendiculr to B 0 All individual nuclei spin after the excitation, in the transverse plane, at the same frequency ω 0 and in consistent phases The relationship between ω 0 and the type of nucleus means that an impulse of the right frequency will excite only one type of nuclei The receiving coil (if placed at the vicinity of the excited tissue) will receive the current with ω 0 (transverse component of M) as a signal that allows an image of the tissue to be examined

26 MRI Principle of signal acquisition Before excitation: transverse M (M t ) = 0 At the moment of excitation: longitudinal M (M l ) = 0 Relaxation: Longitudinal M is restored (T1: time for M l return 63%) Transverse M decays to 0 (T2: time for M t vanish 63%)

27 MRI MRI makes use of sequences of stimulating impulses TR (time of repetition) TE (time of echo)

28 MRI T1 weighted images: difference of T1 Tissue with shorter T1 has longer M magnetization length following the impulse, therefore brighter image TR is the key parameter: can not be too long Short TR and TE are used T2 weighted images: difference of T2 Tissue with longer T2 gives relatively stronger signal after appropriate TE time TE is the key parameter: can not be short Long TR and TE are used Proton density weighing (PD) Based on differences in the density of protons in difference tissues, disregard the differences in T1 & T2 Long TR & short TE

29 Fig 2.36 MRI

30 MRI MRI apparatus: Chamber: inside the main magnet generating the external magnetic filed B 0 to align the magnetic moments of hydrogen atoms Correcting coils induce an additional magnetic field compensating for the non-uniformities of the B 0 filed Gradient coils to produce the linear field change (gradient) Forcing and receiving coils

31 MRI

32 MRI Breath-holding or breath gating function is needed Usually 256 by 256 pixels in size CT shows better anatomical details, MRI differentiates tissue of different biological functions better Healthy tissues can easily be told apart from diseased areas CT: anatomical imaging; MRI: functional imaging

33 Nuclear Imaging Physical basis: nuclei of isotopes of biologically active elements spontaneously emit radiation. The intensity of the radiation is recorded continuously together with the location of the radiation Key: isotope-labeled, biologically active substances produced by pharmaceutical companies which specialize in such operations If properly selected and carefully examined for their radioactive level, they constitute practically no threat to the health and life of both patients and personnel Drink / Breathed in / Intravenously injected (most common)

34 Nuclear Imaging Operating principle: Radionuclide administration Gamma camera Receive radiation in tissue and organs from many points at the same time Receive very weak radiation and must be shielded very well from other radiation sources

35 Nuclear Imaging Imaging characteristics: Generally not very sharp Show not just statistic distribution of the concentration of the substance labeled with the radionuclide, but also the process of accumulating and expulsing that substance: Inaccessible in any other way

36 Positron Emission Tomography A radionuclide technique Offers good spatial resolution and allows to examine biological events that run fast Makes use of isotopes of short life: 15 O, 13 N and 11 C have half-lifes range from 2 to 20 minutes More precise examination Smaller dose of harmful radiation Extremely expensive!

37 PET PET imaging apparatus Cyclotron: produce radionuclide (very short lifetime) right before the examination Isotope emitted gamma rays that interact with human body and cause positrons to be emitted Positrons are annihilated by the emission of two gamma quanta in two opposite directions

38 PET Main advantages: If fluorodeoxyglucose (FDG) is used to carry the short halflife isotope, the compound ends up in places which at that moment exhibit an increased energy demand. It therefore offers an unequalled opportunity to follow the activity of particular organs (mainly the brain)

39 PET PET scans of the brain of the same person different situations

40 PET PET images for symptoms of diseases Drug use: the decreasing use of glucose

41 PET

42 PET Examine activity of many organs and thus to collect diagnostic information that cannot be acquired in any other way. Poor resolution of anatomic details Fusion of CT & PET

43 Ultrasound Imaging Physical basis: Recording the reflections of sound waves after they penetrate through tissues and bounce back from boundaries of structures characterized by different densities and velocities of sound-wave propagation Generation of the ultrasound echo The proportion of the energy of the wave hitting the organ and that reflected from it is determined by: R = " 2 c 2 # " 1 c 1 " 2 c 2 + " 1 c 1 Most frequently, the structures reflecting ultrasound waves are the surfaces of internal organs, whose contours can therefore be detected! and located

44 USG apparatus Ultrasound Imaging

45 Ultrasound Imaging Velocities of ultrasound waves in various media There is a very large difference in the velocity at which ultrasounds travel through the air and through body tissues. If a layer of air remained between the ultrasound transducer and the patient s skin, 90% of the wave energy would be reflected and would not penetrate the body The contact between the transducer and the patient s body is crucial: special gel or water-filled plastic bags

46 Ultrasound Imaging Different imaging methods: A-mode (Amplitude) The time until the echo returns was presented on the horizontal axis as the scale of the distance Experienced physicians could determine on the basis of the slope of the rise and fall of the echo wave, the characteristics of the tissues from which the wave bounced.

47 Ultrasound Imaging Different imaging methods: B-mode (Brightness) Display a 2D cross-section of the examined body parts The momentary value of the received signals modulates the brightness of the subsequent image points By shifting the beam, we get a 2D picture of nearby lines of modulated brightness Frequently used to examine stationary organs: typically 700 by 500 pixels or lower, and a brightness depth with 8 bits

48 Ultrasound Imaging Different imaging methods: M-mode (Motion) Listening to the echo from the same direction at two subsequent moments The echos are displayed in B-mode Echoes coming from mobile organs are represented on the time base by mobile bright dots Fundamental in examining the function and imaging the structure of the heart D-mode (Doppler) Receive the USG wave dispersed by the moving blood cells, which returns to the transceiver with a changed frequency (the Doppler effect) Depending on the velocity and direction of blood cell motion relative to the USG beam and the direction in which the wave is propagated, there is a Doppler shift of the frequency of the sent and received wave towards higher or lower frequencies Color-coded Blue for the fastest movement in one direction Red for the fastest movement in the opposite direction Intermediate colors code movement at lower velocities Stationary structure: black and white

49 Ultrasound Imaging Ultransound images may be subject to much more noises and deformations Specles: non-uniform structure of the image caused by the local interference of waves reflected within the medium Measures the difference in the return time of the echo bouncing assuming the distance traveled by the impulse over a unit of tim is the same Refraction Dampening

50 Summary CT Apply x-ray and observe shadow Different absorportion of tissues Highe resolution & anatomical details MRI Apply magnetic field and observe the radiation from the organ itself Different relaxation time or nuclei density Relatively high resolution & differentiate tissues with functional differences PET Administrate isotope and observe the radiation Track the radiation intensity and location Relatively low resolution & functional imaging (image activity) Ultrasound Apply sound wave and observe the reflection echo Different sound-wave propagation velocity and tissue density High noise & less expensive (more widely available)