introduction [signals & (imaging)...
TRANSCRIPT
Introduction[Signals & (Imaging) Systems]
Ho Kyung [email protected]
Pusan National University
Medical Physics
Prince & Links 1
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
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• 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
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• 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
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• 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
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“Projection” image
“Axial” image “Coronal” image “Sagittal” image
Slice (or tomographic) images
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CT MRI PET
Transverse slices, oriented perpendicular to the head & body axis
Why do the images look like differently?
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Anatomical vs. functional imaging
8Taken from the Lecture Slides (Dr. K. Mueller)
History of Medical Imaging
• The first radiograph of the hand of Wilhelm Conrad Roentgen’s wife in Dec. 1895
– The first clinical use of x rays in Feb. 1896
– The first Novel Prize in Physics in 1901
• The first true CT scanner in 1972 at EMI in England by Godfrey Hounsfield
– Image reconstruction was developed a decade earlier by Allan Cormack of US
– The pair shared the Novel Prize in Medicine in 1979
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• Discovery of radioactivity by Antonie Henri Becquerrel in 1896
– Initially used in cancer therapy rather than imaging
• Use of radioactive tracers to study physiology by George de Hevesy (the farther of nuclear medicine) in 1923
• Anger scintillation camera by Hal Anger at UC Berekely in 1952
• Discovered Tc99m by Perrier & Emilio Segre in 1937; its first use in medicine 1961
• Described the interaction of acoustic waves with media by Lord John Rayleigh over 100 year ago
• Ultrasound imaging from World War II Navy sonar technology
• Modern ultrasound imaging in the 1960s
• Nuclear magnetic resonance by Felix Bloch & Edward Purcell; 1952 Nobel Prize in Physics
• Extension of NMR by Richard Ernst; Novel Prize in Chemistry in 1991
• NMR for medical imaging by Raymond Damadian in 1971 & Paul Lauterbur in 1973; Lauterbur earned Novel Prize in Medicine in 2003 w/ Peter Mansfield
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Physical Signals
• Transmission of x rays thru the body (in projection radiography & CT)
– Body tissues selectively attenuate the x‐ray intensities to form an image
• Emission of gamma rays from radiotracers in the body (in nuclear medicine)
– 140 keV & two 511 keV
– Radioactive compounds or radiotracers, injected into the body, move selectively to different regions or organs, emitting gamma rays with intensity proportional to the compound’s local concentration
• 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 reflectionsto create images
• 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
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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
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Computed Tomography
• Single‐slice CT
• Helical CT
• Multislice CT
Liver
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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)
• Single‐photon emission computed tomography (SPECT)
• Positron emission tomography (PET)
Heart
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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 time‐varying displacements of a heart valve
• Doppler imaging
– Generating images that are color‐coded by moving objects
11‐week‐old embryo
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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
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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
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