Download Classical medical imaging

Medical Image Modalities
Celina Imielinska, Ph.D
Columbia University
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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
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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…)
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Understanding Visual Information: Technical, Cognitive and Social Factors
Classical medical imaging:
direct and intuitive
Contemporary Medical imaging:
indirect and often counterintuitive
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Understanding Visual Information: Technical, Cognitive and Social Factors
Sources of Images
• Structural/anatomical information (CT, MRI, US,
VH) - within each elemental volume, tissuedifferentiating properties are measured.
• Information about function (PET, SPECT, fMRI).
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Understanding Visual Information: Technical, Cognitive and Social Factors
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
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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
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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
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
X-RAY
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
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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
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
ANGIOGRAPHY
• 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)
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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)
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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
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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)
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
ULTRASOUND
Structures as fat, bone,
Structures with watery
calcification (stone) and
fluid (cyst) transmit
gas reflect a high propagation
ultrasound very well
of the ultrasound beam, appearing
(acoustic enhancement)
echogenic (acoustic shadowing)
(No echo from the cyst)
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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)
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Understanding Visual Information: Technical, Cognitive and Social Factors
Introduction of CT (1970s) transformed 2D
qualitative imaging into a quantitative 3D format
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Understanding Visual Information: Technical, Cognitive and Social Factors
3D Imaging
• All processing operations that are
applied to acquired multidimensional
data to facilitate visualization,
manipulation and analysis.
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Understanding Visual Information: Technical, Cognitive and Social Factors
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
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Understanding Visual Information: Technical, Cognitive and Social Factors
MR image of left ventricle
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Understanding Visual Information: Technical, Cognitive and Social Factors
Visible Human color cryosection data
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Understanding Visual Information: Technical, Cognitive and Social Factors
Sources of Images (3D)
•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
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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”)
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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 crosssectional image of it can be computed
(reconstructed)
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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
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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
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
CT – DATA ACQUISITION
(a) slice-by-slice scanning
(b) Spiral (volume) scanning
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
CT – SPIRAL (HELICAL) SCANNING
3D reconstruction of the facial
bones. (greater accuracy in multiplanar 3D reconstruction)
3D reconstruction of pulmonary
vascular system
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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)
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
CT - DATA PROCESSING
Relationship between CT numbers and brightness level
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Understanding Visual Information: Technical, Cognitive and Social Factors
CT - IMAGE DISPLAY
Human eye can perceive only a limited
range gray-scale values
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)
window level and window width:
the center of the range and the range of density, respectively
(the smaller range of densities, the greatest contract)
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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)
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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)
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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
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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)
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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
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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
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Understanding Visual Information: Technical, Cognitive and Social Factors
MRI – T1 and T2 weighted images
T1-weighted image
T2-weighted image
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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)
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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)
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Understanding Visual Information: Technical, Cognitive and Social Factors
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Understanding Visual Information: Technical, Cognitive and Social Factors