Transcript 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!
Dr. Blanton
ENTC 4390 --Introduction
2
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.
Dr. Blanton
ENTC 4390 --Introduction
4
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.
Dr. Blanton
ENTC 4390 --Introduction
5
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.
Dr. Blanton
ENTC 4390 --Introduction
6
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.
Dr. Blanton
ENTC 4390 --Introduction
7
• The power levels used to make
medical images require a balance
between patient safety and image
quality.
Dr. Blanton
ENTC 4390 --Introduction
8
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
Dr. Blanton
ENTC 4390 --Introduction
9
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.
Dr. Blanton
ENTC 4390 --Introduction
10
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
Dr. Blanton
ENTC 4390 --Introduction
11
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
Dr. Blanton
ENTC 4390 --Introduction
12
Sagittal
Dr. Blanton
Coronal
ENTC 4390 --Introduction
13
Body Structure
Directional Terms
• Anatomical position
•
•
•
•
•
Beginning reference point
Body upright
Facing front
Arms at side, palms forward
Feet parallel
Dr. Blanton
ENTC 4390 --Introduction
15
Directional Terms
Dr. Blanton
ENTC 4390 --Introduction
16
Planes of Division
• Frontal plane
• Coronal plane
• Divides body into anterior, posterior
parts
Dr. Blanton
ENTC 4390 --Introduction
17
Planes of Division
• Sagittal plane
• Divides body into right, left portions
• If plane cuts midline, called midsagittal
or medial plane
Dr. Blanton
ENTC 4390 --Introduction
18
Planes of Division
• Transverse plane
• Divides body into superior, inferior parts
Dr. Blanton
ENTC 4390 --Introduction
19
Dr. Blanton
ENTC 4390 --Introduction
20
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
Dr. Blanton
ENTC 4390 --Introduction
21
Body Cavities
• Dorsal cavity contains:
• Cranial cavity
• Spinal cavity
Dr. Blanton
ENTC 4390 --Introduction
22
Body Cavities (cont’d)
• Ventral cavity contains:
• Thoracic cavity
• Diaphragm
• Separates
• thoracic cavity and
• abdominal cavity
Dr. Blanton
ENTC 4390 --Introduction
23
Body Cavities (cont’d)
• Abdominopelvic cavity:
• Abdominal cavity
• Pelvic cavity
• Peritoneum
Dr. Blanton
ENTC 4390 --Introduction
24
Body Regions
• Imaginarily divided
into 9 regions
Dr. Blanton
ENTC 4390 --Introduction
25
Body Regions
• Midline sections:
• Epigastric = above
stomach
• Umbilical = umbilicus
or navel
• Hypogastric = below
the stomach
Dr. Blanton
ENTC 4390 --Introduction
26
Body Regions (con’t)
• Lateral sections:
• Right and left
hypochondriac
• Positioned near
ribs, specifically
cartilages
Dr. Blanton
ENTC 4390 --Introduction
27
Body Regions (con’t)
• Right and left
lumbar
• Positioned
near small of
back (lumbar
region)
Dr. Blanton
ENTC 4390 --Introduction
28
Body Regions (con’t)
• Right and left
iliac
• Named for
upper bone of
hip (ilium)
• Also called
inguinal region
(referring to
groin)
Dr. Blanton
ENTC 4390 --Introduction
29
Body Positions
• Anatomical
• Standing erect, facing forward, arms at sides,
palms forward, toes pointed forward
• Prone
• Lying face down
• Supine
• Lying face up
Dr. Blanton
ENTC 4390 --Introduction
30
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
Dr. Blanton
ENTC 4390 --Introduction
32
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)
Dr. Blanton
ENTC 4390 --Introduction
33
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
ENTC 4390 --Introduction
34
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.
Dr. Blanton
ENTC 4390 --Introduction
35
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.
Dr. Blanton
ENTC 4390 --Introduction
36
Modern CT Scanner
From Webb, Physics of Medical Imaging
Dr. Blanton
ENTC 4390 --Introduction
37
Computerized Tomography (CT), continued
Early CT Image
Dr. Blanton
Current technology
ENTC 4390 --Introduction
38
Inhalation
Dr. Blanton
ENTC 4390 --Introduction
39
Exhalation
Dr. Blanton
ENTC 4390 --Introduction
40
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.
Dr. Blanton
ENTC 4390 --Introduction
41
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
Dr. Blanton
ENTC 4390 --Introduction
42
Combined CT / PET Imaging
Dr. Blanton
ENTC 4390 --Introduction
43
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
Dr. Blanton
ENTC 4390 --Introduction
44
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.
Dr. Blanton
ENTC 4390 --Introduction
45
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)
Dr. Blanton
ENTC 4390 --Introduction
46
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
ENTC 4390 --Introduction
47
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
Dr. Blanton
ENTC 4390 --Introduction
48
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
Dr. Blanton
ENTC 4390 --Introduction
49
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
Dr. Blanton
ENTC 4390 --Introduction
50