Medical Imaging Modalities

Transcription

1 Image Science Introduction Medical Imaging Modalities Ho Kyung Kim Pusan National University Projection Radiography Routine diagnostic radiography Chest x rays, fluoroscopy, mammography, motion tomography Digital radiography Angiography Universal angiography & angiocardiography for blood arteries & vessels Neuroradiology For skull & cervical spine Mobile x ray systems For operating rooms or emergency vehicles 2

2 General projection radiography system Fluoroscopy system (C arm system) Mammography system 3 Instrumentation X ray tube Filters/collimators Grids Detectors 4

3 Filtration Process of absorbing low E x ray photons before they enter the patient Very undesirable for low E photons to enter the body Almost entirely absorbed w/i the body contributing to patient dose but not the image Added filtering Placing metal in the x ray beam outside of the tube Al (1 2 mm thick) typically Higher E systems: Cu + Al (to attenuate the 8 kev characteristic x rays from Cu) Beam hardening The increase in the beam s effective energy as a progressive shift in the position of the spectrum to the right due to filtering Collimators Process of absorbing the x rays outside a certain field of view To avoid exposing parts of the patient that need not be imaged 5 6

4 Contrast agents Used when difficult to visualize different soft tissues due to insufficient intrinsic contrast Chemical compounds to increase x ray absorption w/i an anatomical region Utilizing the K edge absorption effect Leading to very high differential absorption btwn the contrast agent & the surrounding tissues; enhancing contrast Iodine (Z = 53): E K = 33.2 kev Intravascular injection or ingestion Blood vessels, heart chambers, tumors, infections, kidneys, bladder Barium (Z = 56) : E K = 37.4 kev Administered as a chalky milkshake Gastrointestinal tract 7 8

5 Digital subtraction angiography After injection Before injection (mask image) After subtraction 9 Grids To reduce/remove scattered photons Thin strips of Pb alternating w/ highly transmissive interspace mat l (e.g., Al or plastic) Linear, focused grid (Airgaps) Leaving an airgap btwn the patient & the detector an effective means of scatter rejection Cons: Increased geometric magnification Blurring or unsharpness due to x ray focal spot size effect (Scanning slits) Placed in front of and/or in back of the patient Providing greater than 95% scatter reduction More complex & costly system Longer exposure times 10

6 Medical imaging modalities Projection radiography (general radiography, mammography, intra oral imaging, cephalography) Fluoroscopy: angiography Ultrasound imaging (US, echography) Magnetic resonance imaging (MRI) Computed tomography (CT) Nuclear medicine Scintigraphy Single photon emission computed tomography (SPECT) Positron emission tomography (PET) Radiation therapy (RT) Electronic portal imaging devices (EPIDs) 11 Basic Imaging Principles See inside the human body: Invasive techniques Endoscopy (put something), surgery (cut the body), Noninvasive techniques Risk free Magnetic resonance imaging (MRI), ultrasound imaging Risk associated with the radiation exposure Projection radiography, computed tomography (CT), nuclear medicine (SPECT, PET) Even more anatomic imaging: Functional MRI (fmri): organ perfusion or blood flow Positron emission tomography (PET): metabolism or receptor binding What does the human body look like on the inside? It depends on the measured signal of interest 12

7 Medical imaging physics allows us to image certain parameters (or signals) of the body s tissues: Reflectivity in ultrasound imaging Linear attenuation coefficient in CT Hydrogen proton density in MRI Input signal into an imaging system Outputs of medical imaging systems The first output: physical measurements with various imaging systems: Returning echoes in an ultrasound system X ray intensities in a CT system Radio frequency (RF) waves in an MRI system The final output: created through image reconstruction The process of creating an image from measurements of signals 13 The overall quality of a medical image is determined by "how well the image portrays the true spatial distribution of the physical parameters of interest within the body" Medical image = the spatial distribution of the measured physical parameters Dependent upon (image quality): Resolution Noise Contrast Geometric distortion Artifacts Clinical utility of medical images involves both the image quality & the medical information contained in the parameters themselves 14

8 Medical imaging signals a. Projection radiography: x ray transmission thru the body b. Planar scintigraphy: gamma ray emission from w/i the body c. Ultrasound imaging: ultrasound echoes d. Magnetic resonance imaging: nuclear magnetic resonance (NMR) induction 15 Fluoroscopy (dynamic) Slice (or tomographic) images Axial image Coronal image Sagittal image Projection image 16

9 Transverse slices, oriented perpendicular to the head & body axis CT MRI PET Why do the images look like differently? 17 Computed Tomography Liver Pitch Taken from WA Kalender's Text Material (2000) 18

10 Godfrey N. Hounsfield, the English engineer who developed the first CT scanner and received the Nobel Prize in medicine 1979 together with the physicist A. M. Cormack. Taken from WA Kalender's Text Material (2000) 19 Generations Taken from WA Kalender's Text (2000) 20

11 Detectors Taken from WA Kalender's Text Material (2000) 21 Transmission of x rays thru the body (in projection radiography & CT) Body tissues selectively attenuate the x ray intensities to form an image Projection measures the attenuated x ray beam intensity (the mean energy of the attenuated x ray spectrum) CT estimates (or reconstructs) a map of the attenuation coefficients from the measurements of ray sum data 22

12 Image reconstruction c + d = 1, a + b = 0, a + c = 1, b + d = 0... C = 1! A = B = D = 0! Light Box / holes Eyes Brain Taken from WA Kalender's Text Material (2000) 23 Simple example (iterative method)

13 Filtered backprojection (To remove or reduce the intrinsic blur artifacts) backproject after taking the highpass filter function on the projection data f x, y 1 2 p s h s dθ Taken Movies from WA Kalender's Text Material (2000) 25 Slice war, and now dose war Taken from Dr. S. Cho s Slides 26

14 Taken from the textbook (W. A. Kalender, 2000) 27 Biological effects and safety Relatively high radiation doses in CT Effective dose of a CT (a factor of 10 to 100 & more higher than a radiography): head = 1 2 msv chest, abdomen or pelvis = the order of 5 8 msv low dose lung CT = msv whole body screening = ~7 msv The dose can be limited by: a low mas a limited angle scan a high pitch a modulated tube current (a larger tube current in views with higher attenuation) 28

15 Nuclear Medicine Radiotracers: biochemically active drugs whose molecules are labeled with radionuclides that emit gamma rays Locally distributed concentration according to the body's natural uptake (the physiological behavior) functional imaging (compared with anatomical or structural imaging) Scintigraphy (conventional radionuclide imaging): 140 kev photons Single photon emission computed tomography (SPECT): 140 kev photons Positron emission tomography (PET): two 511 kev photons Courtesy: William W. Moses, LBNL 29 Small amount of radioactive labeled molecules are administered to selectively measure functional parameters of different organs (e.g., perfusion, metabolism, innervation) Single photon emitting atoms Short half life; continuously available parent; ray energy high enough to leave the body but not too high to penetrate the crystal 99m Tc (T 1/2 = 6 h), 123 I (13 h), 131 I (8 d), 111 In (3 d), 201 Tl (3 d), 67 Ga (3 d) Positron emitting tracers Produced by a cyclotron 11 C (T 1/2 = 20 min), 13 N (10 min), 15 O (2 min), 18 F (109 min) 30

16 Gamma camera & SPECT scanner Gamma camera (aka, Anger camera) Collimator + large NaI(Tl) crystal + PMT array Front end electronics (Anger logic) Calculate position coordinates (x, y), energy z, and the detection time t Scintigraphy, SPECT Photomultiplier Tubes (~50 / head) Scintillator Crystal (NaI:Tl, 50 cm square x 1 cm thick) Collimator 31 Heart 32

17 PET scanner Large ring (diameter 1 m) of BGO crystals; no detector rotation; table motion small (about 4 mm 4 mm) scintillation crystals + PMTs detection time is determined with an accuracy of about 10 ns (in 10 ns light travels about 3 m), which is very short as compared to the scintillation of BGO (300 ns) Randoms: photon pairs that do not originate from the same atom but nevertheless are considered as a coincidence the probability of a random ~ (radioactivity) 2 Schematical representation of a PET detector ring cut in half. (a) When septa are in the field of view, the camera can be regarded as a series of separate 2D systems. Coincidences along oblique projection lines between neighboring rings can be treated as parallel projection lines from an intermediate plane. This doubles the axial sampling, i.e. 15 planes can be reconstructed from 8 rings. (b) Retracting the septa, increases the number of projection lines and hence the sensitivity of the system, but fully 3D reconstruction is required. (a) (b) 33 PET detector module Array of scintillator crystals 28 mm Photomultiplier tubes Position-sensitive APD LSO Array 34

18 Collimation The source is an unknown distribution in NM, unlike the x ray imaging (e.g., projection line) W/o collimation, the detected photons do not contain information about the spatial distribution Collimation in single photon detection (SPECT) Mechanical collimator: a thick lead plate with cylindrical holes Suffering from sensitivity Collimation in PET Coincidence detection or electronic collimation Detected both photons with an electronic coincidence circuit Higher sensitivity rather than in SPECT 35 Reconstruction 36

19 PET images of cancer Courtesy: William W. Moses, LBNL Brain Heart Metastases Shown with Red Arrows Normal Uptake in Other Organs Shown in Blue Bladder 37 PET/CT & PET/MRI 38

20 Ultrasound Imaging A mode imaging (not comprising an image) Generating 1D waveform & providing very detailed information about rapid or subtle motion (of a heart valve, for example) B mode imaging Ordinary cross sectional anatomical imaging M mode imaging Generating a succession of A mode signals & not anatomical but important for measuring timevarying displacements of a heart valve Doppler imaging Generating images that are color coded by moving objects 11 week old embryo 39 Reflection of ultrasonic waves within the body (in ultrasound imaging) 1 20 MHz Firing high freq. sound into the body & receiving the echoes returning due to acoustic reflections to create images Courtesy: Jim Lacefield, RRI 40

21 A mode (amplitude) imaging Based on the pulse echo principle Measurements of the reflected waves as a function of time d (time & depth are equivalent in echography c ~1540 m/s = const.) Detected signal ~ MHz range ( called RF signal) M mode (motion) imaging Repeated A mode measurements for a moving object 41 B mode (brightness) imaging Repeated A mode measurements by translating or tilting the transducer 42

22 Doppler imaging If an acoustic source moves relative to an observer, the frequencies of the observed and transmitted waves are different the Doppler effect To visualize velocities of moving tissues 43 Ultrasound scanner Array transducer 3D imaging Rotate or wobble a 2D phased array transducer Acquire images sequentially from different scan planes (C scans) 44

23 45 Biological effects and safety Tissue heating Tissue damage due to heat converted from ultrasonic energy Monitor the thermal index based on the transmitted power not to exceed a certain threshold Heating can be used for ultrasound surgery to burn malignant tissue Cavitation Tissue damage due to the collapse of bubbles that are formed in areas of low local density resulting from a negative pressure Monitor the mechanical index based on the peak negative pressure not to exceed a certain threshold Cavitation is the basis for lithogripsers, which destroy kidney or bladder stones by means of high pressure ultrasound 46

24 Magnetic Resonance Imaging Standard MRI Echo planar imaging (EPI) Utilizing specialized apparati to generate images in real time Spectroscopic imaging Imaging other nuclei besides the hydrogen atom Functional MRI (fmri) Using oxygenation sensitive pulse sequences to image blood oxygenation in the brain Knee 47 Procession of spin systems in a large magnetic field (in magnetic resonance imaging) 64 MHz typ. for stimulation & Faraday induction of currents for signals Requiring a combination of a high strength magnetic field & radio waves to image properties of the proton nucleus of the hydrogen atom MR signal s t ρ x, y, z 1 e / e / e k r dxdydz 3D Fourier transform of an image f x, y, z F k,k,k f x, y, z e k r dxdydz The RF pulse creates a net transverse magnetization due to energy absorption and phase coherence. After the RF pulse, two distinct relaxation phenomena ensure that the dynamic (thermal) equilibrium is reached again. 48

25 No Spin, no NMR sensitivity Nucleus H H C C N N O O P S Ca Spin 1/ /2 1 1/2 0 5/2 1/2 3/2 7/2 (MHz/T) D MR angiography Courtesy: Walter F. Block, Univ. of Wisconsin-Madison 50

26 T 1 weighted ρ weighted T 2 weighted Courtesy: Walter F. Block, Univ. of Wisconsin-Madison 51 MRI systems = Magnets; gradient systems; RF systems + computer HW/SW 52

27 Gradient system Linearity for correct phase encoding (nonlinearity 1 2% in an FOV with diameter 50 cm) Higher amplitude (~50 mt/m) & its rise time for fast imaging RF system For higher sensitivity & in plane homogeneity of signal detection Anatomic specific coils to detect the weakest MR signal possible 53 Biological effects RF waves Vibrating atoms & molecules; hence resulting in increase of tissue temp. Core body temp. rise ~1 C (as a rule of thumb) Average SAR (specific absorption rate of the RF power) limits ~2 10 W/kg Magnetic gradients Magnetic flux db dt of switching gradient fields can generate currents in tissues such as blood vessels, muscles, & nerves Repetitive movement of coils due to Lorentz force (currents + magnetic fields) can cause the typical highfrequency drill noise (should be kept < 100 db) Safety Patients with heart valves, pacemakers, surgical clips, & very old prostheses or orthopedic fixation screws must not be scanned using MRI Conductive elements (& conductive loops) may produce burn lesions 54

28 Summary Courtesy: Dr. Kyeong Min Kim, KIRAMS CT MRI PET 55 Modality Radiation Role Resolution Contrast Radiation exposure WB scan time CT X ray Anat Very high Medium High few min MRI RF Anat/F/M High Medium None 10~20 min PET ray Funct/Mole Relatively low Very high Low 10~20 min Taken from the Lecture Slides (Dr. Lee Jae Sung, SNU) 56

29 Anatomical vs. functional Anatomical vs. functional Taken from the Lecture Slides (Dr. K. Mueller) 57 Remarks Radiologists look for specific patterns, which depend on the patient and the imaging modality, in medical images, and distinguish the differences in the expected signal in health and disease Engineers and scientists develop medical imaging systems to produce images that are as accurate and useful as possible; these systems depend on the physics of each modality 58