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Imaging
• Medical imaging is an important diagnostic tool
• It involves:– Image acquisition– Image reconstruction– Image Processing– Image Analysis – Image Interpretation
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Image formation
• Physical properties are different, but fundamentally a 3 D object is being imaged:– let a tissue have a distribution of some property f = q (x,y,z)– Then the image g = T(f), where T is the transformation
describing the imaging process. – Depending on the imaging modality, the distribution f only
reflects one property of the object, ie linear attenuation or water content, not the real object.
– The aim is to define some characteristics of the object using a specific T, and perhaps combine many T’s (multi-modality imaging)
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Where do we see images
• Film• Monitors
• Film properties: When images are transferred to film, the final product is affected- permanently
• Monitors: Several parameters govern the visualization
Display Issues
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Physiological Properties Mapped
Modality Property
X-ray: RadiographsComputedTomography (CT)
Linear attenuation (µ)based on tissue density,atomic number
UltrasoundDoppler
Acoustic ImpedenceMotion
Radionuclideimaging,PET, SPECT
Distribution ofchemically labelledγ emitters, receptors
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RADIOGRAPHS AND COMPUTED TOMOGRAPHY (CT)
• Based on attenuation of x-rays.– Denser the tissue > attenuation (atomic number)– Muscle, soft tissue very similar
z
Io
I = Io e -µz
For multiple thicknesses with different attenuation: I = Io e -(µ1
z1) e -(µ2
z2) e -(µ3
z3)
P(x,y) = Ln I / Io (x,y) = Integral µ (x,y,z)
Projection theorem
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RADIOGRAPHS�: WRIST
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Radiographic Image of 3D Structure
SIAP
ML
SI APML
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Examples of Radiographic Images of Trabecular Bone Pattern
Femur Sample: Density = 107.6 mg/cm3
Medial LateralSagittal
Anterior-PosteriorCoronal
Superior-InferiorAxial
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Spine Sample: Density = 59.9 mg/cm3
Medial LateralSagittal
Anterior-PosteriorCoronal
Superior-InferiorAxial
Examples of Radiographic Images of Trabecular Bone Pattern
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COMPUTED TOMOGRAPHY �
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COMPUTED TOMOGRAPHY �: SKULL RENDERING
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COMPUTED TOMOGRAPHY �: PELVIC BONE
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PRE-SURGERY
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POST - SURGERY
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COMPUTED TOMOGRAPHY �: KNEE KINEMATIC
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COMPUTED TOMOGRAPHY �: HIP KINEMATIC
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COMPUTED TOMOGRAPHY �: FINGER KINEMATIC
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DIGITAL SUBTRACTION ANGIORAPHY
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RADIONUCLIDE IMAGING
• Single Positron Emission Tomography: (SPECT)
• Positron emission tomography (PET)• Images gamma emitters, nuclides
administered to subject• Resolution governed by detectors, signal to
noise, etc.
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�SPECT
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COMPUTED TOMOGRAPHY �AND SPECT
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POSITRON EMISSION TOMOGRAPHY �: BREAST TUMOR
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POSITRON EMISSION TOMOGRAPHY �: LUNG METASTASES
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ULTRASOUND• Measures sound attributes: Acoustic
Impedence = ρc c =speed of sound)• Attenuation : Ioe-µz
• Doppler shift measures moving objects such as blood. Ifr f is frequency of the US wave, then ∆f = -2vfcosθ, v is velocity, θ angle of incidence
• For vessels: Flow volume = v Area of cross-section.
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ULTRASOUND: CAROTID ARTERY
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ULTRASOUND: CARDIAC
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Mo dality Pro perty
M agn eticResonan ce I ma gingand Spe ct ro scopy
Pro ton den sit y (wa te rcon tent ), re lax at iontimes (env iron em en t oftissues ), f low,diffu sion, metabo licfun ction
Electric I mped enc eTo mogr aph
Electrica l prop erties,condu ctanc e, re sisti vi ty
Electrica l andM agn etic Sou rceImag ing
Emi tted e lectrical andmagn etic sign als f romthe tiss ue
Op tical im ag ingAnd spectro scopy
Op tical p roper ties,molecu la r status
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MAGNETIC RESONANCE
• Water content• Biochemistry• Flow• Diffusion• Metabolic Activity
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MR: BRAIN OVERLAID WITH PET
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MR: BRAIN VOLUME RENDERED
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VISIBLE HUMAN
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VISIBLE HUMAN
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MR BASED SEGMENTATION OF PORENCEPHALIC CAVITY���
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MR AND SPECT
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MR SLICES FOR ANGIOGRAPHY
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MAXIMUM INTENSITY PROJECTIONS
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MAXIMUM INTENSITY PROJECTIONS
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SURGICAL PLANNING
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MR BASED VASCULAR LIVER MODEL
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PRE OPERATIVE HEPATIC SURGERY
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GATED CARDIAC MR
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OPTICAL MICROSCOPY
• Impact on light
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CONFOCAL MICROSCOPY:EPITHELIAL CELLS
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ELECTRO-MAGNETIC TOMOGRAPHY
• Electro-Magnetic Tomography (EMT) from EEG or MEG: – data (electric potentials in EEG or biomagnetic fields in MEG +
time) produced through an evoked potential experiment or an EEG-MEG monitoring are first acquired through a multichannelrecorder (one channel per electrode/coil).
– Accounting for the 3D location of every electrode/coil, the current density distribution inside the brain can be reconstructed in 4Dspace and time) by trying to assess the biological generators from the measurements.
– Inverse problem there is no way at this time to take accurately into account every single piece of the puzzle which affects the path between biological generators and physical measurements.
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ELECTRIC POTENTIAL TOMOGRAPHY:EPILEPSY
MAXIMUM CURRENT DENSITY
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VECTOR CURRENT DENSITY
ELECTRIC POTENTIAL TOMOGRAPHY:EPILEPSY
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Linearity of Imaging systems
• Ag = AT(f) = T(Af) Scaling the object property leads to scaling the image identically
• If Bg= BT(g) = T(Bg) then • Ag+Bg = AT(f) + BT(g) = T(Af) + T(Bg)• This is linearity, is often assumed, but films
sturate and have a curve associated and are non-linear.
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• Radionuclide imaging examines concentrations and maps directly -- this is closest to being linear
• Xray attenuation: higher the atomic number, greater the attenuation, so it should be linear, but properties of xray attenuation (wavelength dependent) change as the tissue atomic number changes.
• MR: Higher the water content the brighter the signal, yes unless the magnetic field changes due to local changes….
Linearity of Imaging systems
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Point Spread Function• All imaging systems produce a
degradation of the image • T(f) is not a delta function, it
produces a blurring.• The blurring effect is defined by
the Point Spread Function, Point Response Function (PSF, PRF).
• PSF depends on the imaging system, and noise.
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Properties of the Point Spread Function
• Point Sensitivity: Is the total signal obtained from a point object the same in space?
• Spatial Linearity: Are all points depicted identically with respect to shape and geometry?
If one knows the point spread function, and the system is position independent the system can be characterized.
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RESOLUTION
If two objects (points are close together can they be resolved.
Smallest object that can be visualized.
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If the point spread function is a gaussian, for example, the point spread function governs how small an object can be seen, as well as how close they are before they cannot be resolved.
If two Gaussians of equal intensity are placed one FWHM apart then the intensity at the midpoint is 6% less than the maximum. The two points are then resolved.
Full width at half maximum
FWHM
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RESOLUTION
• Set of parallel lines spaced different distances apart…gives resolution in line pairs per mm. With the advent with the digital imaging systems, people refer to the fullwidth at half maximum.
• Is the PSF isotropic, same in all directions?
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RESOLUTION
• 2D images are visualizations of 3D objects.– A pixel is smallest unit in a 2D image– Voxel represents the volume of a pixel taking
into account the thickness of the object (3D) that is projected onto the 2 D image
• Cross-sectional or tomographic images – Associated slice thickness– Pixel resolution
• Projection Images – Pixel resolution
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• F (x,y) = G(x,y) * H(x,y), where F is the image, G the object, and H the Point Spread Function.
• Convolution in the space is akin to multiplication in the Fourier domain.
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CONTRAST• Image Intensity in an image represents a
magnitude of a given property.
• �Difference in intensity between two tissues or entities is entitled the contrast between two entities
• No matter how high the resolution if two distinct entities have the same intensity of a given tissue property, the utility of the image is limited.
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NOISE
• Every imaging system has associated noise• This noise has different forms depending on
the imaging (Gaussian, Poisson, Risean). • The noise introduces random fluctuations in
tissue intensity which reduce the detectability of different entities in an image.
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SIGNAL TO NOISE AND CONTRAST TO NOISE
• Noise = <µ> (mean value often zero mean)
• Noise has a standard deviation σ2
• Thus signal to noise ratio: I/Sqrt(σ2 )
• Two regions have intensity I1 and I2
• Thus contrast to noise ratio: I1-I2 /Sqrt(σ2 )