Download DISP-2003: Introduction to Digital Signal Processing

MEDICAL IMAGING
Dr. Hugh Blanton
ENTC 4390
• There has been an
alarming increase
in the number of
things I know
nothing about!
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ENTC 4390 --Introduction
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Lecture 1
INTRODUCTION
INTRODUCTION TO MEDICAL IMAGING
• Medical imaging of the human body
requires some form of energy.
• In radiology, the energy used to produce the
image must be capable of penetrating tissues.
• The electromagnetic spectrum outside the visible
light region is used for
• x-ray imaging,
• magnetic resonance imaging, and
• nuclear medicine.
• Mechanical energy, in the form of high-frequency
sound waves, is used in ultrasound imaging.
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INTRODUCTION TO MEDICAL IMAGING
• With the exception of nuclear
medicine, all medical imaging
requires that the energy used to
penetrate the body’s tissues also
interact with those tissues.
• Absorption,
• Attenuation, and
• Scattering.
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INTRODUCTION TO MEDICAL IMAGING
• If energy were to pass through the
body and not experience some type
of interaction (e.g., absorption,
attenuation, scattering),
• then the detected energy would not
contain any useful information regarding
the internal anatomy, and
• thus it would not be possible to
construct an image of the anatomy
using that information.
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INTRODUCTION TO MEDICAL IMAGING
• In nuclear medicine imaging,
radioactive agents are injected or
ingested, and it is the metabolic or
physiologic interactions of the agent
that give rise to the information in the
images.
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• The power levels used to make
medical images require a balance
between patient safety and image
quality.
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History, Basic Principles, & Modalities
Class consists of:
1) Deterministic Studies
- distortion
- impulse response
- transfer functions
All modalities are non-linear and space variant to
some degree.
Approximations are made to yield a linear, spaceinvariant system.
2) Stochastic Studies
SNR (signal to noise ratio) of the resultant image
- mean and variance
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Wilhelm Röntgen, Wurtzburg
Nov. 1895 – Announces X-ray discovery
Jan. 13, 1896 – Images needle in patient’s hand
– X-ray used presurgically
1901 – Receives first Nobel Prize in Physics
– Given for discovery and use of X-rays.
Radiograph
of the hand of
Röntgen’s
wife, 1895.
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Röntgen’s Setup
Röntgen detected:
• No reflection
• No refraction
• Unresponsive to mirrors or lenses
His conclusions:
• X-rays are not an EM wave
• Dominated by corpuscular behavior
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Projection X-Ray
attenuation
coefficient
Id  Ioe
μ ( x, y, z )  f (electron density, z)
  μ( x,y,z )dl
Measures line integrals of attenuation
Film shows intensity as a negative ( dark areas, high x-ray detection
Disadvantage: Depth information lost
Advantage:
Cheap, simple
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Sagittal
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Coronal
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Body Structure
Directional Terms
• Anatomical position
•
•
•
•
•
Beginning reference point
Body upright
Facing front
Arms at side, palms forward
Feet parallel
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Directional Terms
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Planes of Division
• Frontal plane
• Coronal plane
• Divides body into anterior, posterior
parts
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Planes of Division
• Sagittal plane
• Divides body into right, left portions
• If plane cuts midline, called midsagittal
or medial plane
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Planes of Division
• Transverse plane
• Divides body into superior, inferior parts
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Anatomical Directions
• Anterior (ventral) = toward front of
body
• Posterior (dorsal) = toward back of
body
• Medial = toward midline of body
• Lateral = toward side of body
• Proximal = nearer to reference point
• Distal = farther from reference point
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Body Cavities
• Dorsal cavity contains:
• Cranial cavity
• Spinal cavity
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Body Cavities (cont’d)
• Ventral cavity contains:
• Thoracic cavity
• Diaphragm
• Separates
• thoracic cavity and
• abdominal cavity
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Body Cavities (cont’d)
• Abdominopelvic cavity:
• Abdominal cavity
• Pelvic cavity
• Peritoneum
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Body Regions
• Imaginarily divided
into 9 regions
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Body Regions
• Midline sections:
• Epigastric = above
stomach
• Umbilical = umbilicus
or navel
• Hypogastric = below
the stomach
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Body Regions (con’t)
• Lateral sections:
• Right and left
hypochondriac
• Positioned near
ribs, specifically
cartilages
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Body Regions (con’t)
• Right and left
lumbar
• Positioned
near small of
back (lumbar
region)
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Body Regions (con’t)
• Right and left
iliac
• Named for
upper bone of
hip (ilium)
• Also called
inguinal region
(referring to
groin)
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Body Positions
• Anatomical
• Standing erect, facing forward, arms at sides,
palms forward, toes pointed forward
• Prone
• Lying face down
• Supine
• Lying face up
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X-Ray
Early Developments
• Intensifying agents, contrast agents all developed
within several years.
• Creativity of physicians resulted in significant
improvements to imaging.
- found ways to selectively opacify regions of interest
- agents administered orally, intravenously, or via
catheter
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Later Developments
More recently, physicists and engineers have initiated new
developments in technology, rather than physicians.
1940’s, 1950’s
Background laid for ultrasound and nuclear medicine
1960’s
Revolution in imaging – ultrasound and nuclear medicine
1970’s
CT (Computerized Tomography)
- true 3D imaging
(instead of three dimensions crammed into two)
1980’s
MRI (Magnetic Resonance Imaging)
PET ( Positron Emission Tomography)
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Computerized Tomography (CT)
Result:
ID ( x, y)  μ( x, y)
1972 Hounsfield announces findings at British Institute of Radiology
1979
Hounsfield, Cormack receive Nobel Prize in Medicine
(CT images computed to actually display attenuation coefficient m(x,y))
Important Precursors:
1917 Radon:
1961 Oldendorf:
Dr. Blanton
Characterized an image by its projections
Rotated patient instead of gantry
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First Generation CT Scanner
Acquire a projection (X-ray)
Translate x-ray pencil
beam and detector across
body and record output
Rotate to next angle
Repeat translation
Assemble all the projections.
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Reconstruction from Back Projection
1.Filter each projection to account for sampling data on polar grid
2. Smear back along the “line integrals” that were calculated by
the detector.
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Modern CT Scanner
From Webb, Physics of Medical Imaging
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Computerized Tomography (CT), continued
Early CT Image
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Current technology
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Inhalation
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Exhalation
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Nuclear Medicine
-
Grew out of the nuclear reactor research of World War II
Discovery of medically useful radioactive isotopes
1948 Ansell and Rotblat: Point by point imaging of thyroid
1952 Anger: First electronic gamma camera
a) Radioactive tracer is selectively taken up by organ of interest
b) Source is thus inside body!
c) This imaging system measures function (physiology)
rather than anatomy.
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Nuclear Medicine, continued
Very specific in imaging physiological function - metabolism
- thyroid function
- lung ventilation: inhale agent
Advantage:
Direct display of disease process.
Disadvantage: Poor image quality (~ 1 cm resolution)
Why is resolution so poor?
Very small concentrations of agent used for safety.
- source within body
Quantum limited:
CT
109 photons/pixel
Nuclear ~100 photons/pixel
Tomographic systems:
SPECT: single proton emission computerized tomography
PET:
positron emission tomography
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Combined CT / PET Imaging
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Comparison of Modalities
Why do we need multiple modalities?
Each modality measures the interaction between energy and
biological tissue.
- Provides a measurement of physical properties of tissue.
- Tissues similar in two physical properties may differ in a
third.
Note:
- Each modality must relate the physical property it measures
to normal or abnormal tissue function if possible.
- However, anatomical information and knowledge of a large
patient base may be enough.
- i.e. A shadow on lung or chest X-rays is likely not good.
Other considerations for multiple modalities include:
- cost
- safety
- portability/availability
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X-Ray
Measures attenuation coefficient
μ ( x, y , z )
Safety: Uses ionizing radiation
- risk is small, however, concern still present.
- 2-3 individual lesions per 106
- population risk > individual risk
i.e. If exam indicated, it is in your interest to get
exam
Use: Principal imaging modality
Used throughout body
Distortion: X-Ray transmission is not distorted.
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Ultrasound
Measures acoustic reflectivity R( x, y, z )
Safety: Appears completely safe
Use: Used where there is a complete soft tissue and/or
fluid path
Severe distortions at air or bone interface
Distortion:
Reflection: Variations in c (speed) affect depth
estimate
Diffraction: λ ≈ desired resolution (~.5 mm)
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Magnetic Resonance (MR)
Multiparametric
M(x,y,z) proportional to ρ(x,y,z) and T1, T2.
(the relaxation time constants)
Velocity sensitive
Safety: Appears safe
Static field - No problems
- Some induced phosphenes
dB
Higher levels
- Nerve stimulation
 10 T/s
dt
RF heating:
body temperature rise < 1˚C - guideline
Use:
Distortion:
Dr. Blanton
Some RF penetration effects
- intensity distortion
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Clinical Applications - Table
Chest
Abdomen
Head
X-Ray/
CT
+ widely used
+ CT - excellent
– needs contrast
+ CT - excellent
+ X-ray - is good
for bone
– CT - bleeding,
trauma
Ultrasound
– no, except for
+ heart
+ excellent
– problems with
gas
– poor
Nuclear
+ extensive use
in heart
Merge w/ CT
+ PET
MR
+ growing
cardiac
applications
+ minor role
+ standard
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Clinical Applications - Table
Cardiovascular
Skeletal / Muscular
X-Ray/
CT
+ X-ray – Excellent, with
catheter-injected
contrast
+ strong for skeletal system
Ultrasound
+ real-time
+ non-invasive
+ cheap
– but, poorer images
– not used
Nuclear
+ functional information
on perfusion
+ functional - bone marrow
MR
+ getting better
High resolution
Myocardium viability
+ excellent
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Economics of modalities:
X-Ray:
Cheapest
Ultrasound: ~ $100K – $250K
CT: $400K – $1.5 million (helical scanner)
MR: $350K (knee) - $4.0 million
Service: Annual costs
Hospital must keep uptime
Staff:
Scans performed by technologists
Hospital Income: Competitive issues
Significant investment and
return
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