MEDICAL IMAGING

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MEDICAL IMAGING

• Dr. Hugh Blanton
• ENTC 4390

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• There has been an alarming increase in the number
  of things I know nothing about!

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Lecture 1

• INTRODUCTION

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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 bodys 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
• 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,
  space-invariant system.
• 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 patients 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 Röntgens wife, 1895.
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Röntgens 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
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
Coronal
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Body Structure
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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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(No Transcript)
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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 (contd)

• Ventral cavity contains
• Thoracic cavity
• Diaphragm
• Separates
• thoracic cavity and
• abdominal cavity

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Body Cavities (contd)

• 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 (cont)

• Lateral sections
• Right and left hypochondriac
• Positioned near ribs, specifically cartilages

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Body Regions (cont)

• Right and left lumbar
• Positioned near small of back (lumbar region)

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Body Regions (cont)

• 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
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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.
• 1940s, 1950s
• Background laid for ultrasound and nuclear
  medicine
• 1960s
• Revolution in imaging ultrasound and nuclear
  medicine
• 1970s
• CT (Computerized Tomography)
• - true 3D imaging
• (instead of three dimensions crammed into
  two)
• 1980s
• MRI (Magnetic Resonance Imaging)
• PET ( Positron Emission Tomography)

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Computerized Tomography (CT)
Result

• 1972 Hounsfield announces findings at British
  Institute of Radiology
• Hounsfield, Cormack receive Nobel Prize in
  Medicine
• (CT images computed to actually display
  attenuation coefficient m(x,y))
• Important Precursors
• 1917 Radon Characterized an image by its
  projections
• 1961 Oldendorf 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
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

• Radioactive tracer is selectively taken up by
  organ of interest
• Source is thus inside body!
• 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/availabili
  ty

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X-Ray

• Measures attenuation coefficient
• Safety Uses ionizing radiation
• - risk is small, however, concern still
  present.
• - 2-3 individual lesions per 106
• - population risk gt 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
• 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 ? desired resolution (.5 mm)

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Magnetic Resonance (MR)

• Multiparametric
• M(x,y,z) proportional to ?(x,y,z) and T1, T2.
• (the relaxation time constants)
• Velocity sensitive
• Safety Appears safe
• Static field - No problems
• - Some induced phosphenes
• Higher levels - Nerve stimulation
• RF heating body temperature rise lt 1C -
  guideline
• Use
• Distortion 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