INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS

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INTRODUCTION TO IMAGING PHYSICSCAPABILITIES AND
LIMITATIONS

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DAVID B. CHALPIN, MD

• ASSISTANT PROFESSOR OF
• CLINICAL RADIOLOGY
• LSU HEALTH SCIENCES CENTER
• NEW ORLEANS, LA

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GOALS

• TO BECOME FAMILIAR WITH THE BASICS OF IMAGE
  GENERATION USING X-rays, CT, AND MRI
• TO BECOME FAMILIAR WITH THE LIMITATIONS OF
  IMAGING AS PRACTICALLY APPLIED

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TEST-TAKER TOPICS

• KNOW THE WISHFUL THINKING PITFALLS!
• REVIEW THE TAKE-HOME MESSAGES FOR EACH IMAGING
  MODALITY!
• (denoted by a RED asterisk - )

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OVERVIEW

• RADIOGRAPHY, FLUOROSCOPY, DSA
• COMPUTED TOMOGRAPHY
• MAGNETIC RESONANCE IMAGING

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SPECTRUM OFE-M RADIATION

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GENERATION OF X-Rays

• VACUUM TUBE
• Electric current is passed through a filament,
  leading to e- emission, then striking target (W
  or Mb), leading to X-ray emission.

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mAs and kVp

• e- current through filament (expressed in mAs for
  milliAmperes) at Cathode generates a
  proportionate amount of X-Ray photons
• kVp kiloVoltage peak relates to the Voltage
  potential between the Anode Cathode and
  reflects a SPECTRUM of emitted X-ray photon
  energies

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X-Rays 3 Fates

• Photons can be ABSORBED
• Photons can be SCATTERED with some exposing the
  film ? degrading the image, aka FOGGING, OR
• Photons can proceed directly through subject to
  EXPOSE film.

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SCATTERING

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How reduce X-ray SCATTERING?

• ASK YOUR PATIENTS TO
• LOSE WEIGHT?

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TO ? SCATTERING

• COLLIMATION of X-ray Beam
• Use of GRIDS in cassettes

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X-ray Collimation

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X-ray GRID

• Tradeoff
• Grids require ?
• mAs compared with XR studies done w/o grids

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How Improve Spatial Resolution Decrease
Image Distortion?

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• Center the Area
• of Interest!

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AP versus PA

• Direction of emitted beam from the X-ray tube
  ?Patient?Cassette
• AP Anterior to Posterior
• PA Posterior to Anterior

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PORTABLE X-RAYS

• HOW CONVENIENT!!
• DECREASED QUALITY (sometimes) due to limited kVp
  mAs, ? tube to subject distance, positioning
  ROI
• Is it FEASIBLE that the patient could have had
  the X-ray study done in the Radiology Department?
  If so, ..

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FIRST APHORISM

• DONT MAKE GOOD CALLS FROM BAD FILMS !!

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DONT MAKE GOOD CALLS FROM BAD FILMS !!!

• Bad
• can mean Suboptimal Quality OR the study as
  ordered was NOT dedicated for evaluation of
  Region or Organ of Interest.

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SECOND APHORISM

• YOU CANNOT CALL WHAT YOU DONT SEE!
• HOWEVER, IF YOU
• SUSPECT SOMETHING,
• GET ANOTHER VIEW!!

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DIGITAL/COMPUTED RADIOGRAPHY

• IMAGES CAN BE MANIPULATED POST-ACQUISITION TO
  OPTIMIZE VIEWING OF ONE PART OF HD Curve.

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WISHFUL THINKING IN RADIOGRAPHY

• QUALITY OF PORTABLE STUDIES
• PATIENT THICKNESS SIZE
• Table Weight limits
• COOPERATIVENESS OF PATIENT

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WISHFUL THINKING IN RADIOGRAPHY

• QUALITY OF PORTABLE STUDIES
• PATIENT THICKNESS SIZE
• Table Weight limits
• COOPERATIVENESS OF PATIENT

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X-ray COMPUTED AXIAL TOMOGRAPHY

• aka CAT scan (archaic,) now CT
• STEP AND SHOOT mode
• 1st Gen CT Scanner 45 min/slice

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2nd Generation CT scanner

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TransmissionSpecial Case

• For monochromatic Photon energy
• log T a 1/linear attenuation

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What data generates an image as a slice?

• The Transmission of Photon energy received by
  detectors is recorded at multiple projections
  around the subject the data is then
  reconstructed to create a cross-sectional image

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X-ray AttenuationRevisited
Transmission of photon energy received by
detectors is recorded at multiple projections
around the subject the data is reconstructed
to create a cross-sectional image
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X-ray ATTENUATION

• µ - the intrinsic X-ray coefficient
• a function of
• kVp
• Atomic Mass
• electron density

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ATTENUATION VALUE CT

• Hounsfield Units (H.U.)
• of sample S
• (µS - µH2O) x 1000
• µH2O

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CT ADVANTAGES I

• COMPARED WITH X-rays, U/S, MRI
• Better Soft Tissue Contrast Resolution than XR
  usually Ultrasound (except reproductive organs,
  in general)
• Along with Fluoroscopy using Barium, CT best for
  Intestinal Tract Evaluation (though not so
  dynamic as fluoro.)

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CT ADVANTAGES II

• Easier Quicker than MRI but not always better
  tissue contrast resolution
• BEST for detection characterization of
  CALCIFICATION

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CT BEST FOR Calcification

• e.g. a Bony Sequestrum Involucrum
• of Osteomyelitis

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CT - DISADVANTAGES

• IONIZING RADIATION!!
• EACH SERIES OF IMAGES TOGETHER IS ONLY ONE
  SNAPSHOT IN TIME
• ARTIFACTS Partial Volume
• Scattering (Obesity)
• Beam Hardening
• Metal Streaking

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PARTIAL VOLUMEEFFECT

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EFFECT OF THICK SLICES

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BEAM-HARDENING

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Metallic streaking

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AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS

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3RD GENERATION CT

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HELICAL CT

• 3rd GENERATION CT SCANNER
• ADVENT OF
• SLIP RING TECHNOLOGY TO CREATE HELICAL ACQN!

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ORIGIN OF MultiDetector CT

• TWIN DETECTOR concept done with conventional
  STEP SHOOT technique
• MARRIAGE OF MULTIDETECTOR DESIGN WITH HELICAL
  DESIGN
• ? MDCT !

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THIN SLICES ? ISOTROPIC VOXELS

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IV Contrast - TIMING of Image Acquisition

• X-ray, U/S, but ESPECIALLY CT MRI!
• CONTRAST ENHANCEMENT PHASES
• Arterial Hepatic Arterial
• Portal Venous Renal Capillary
• Renal Excretion, etc.

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Hypervascular Metonly seen on Hepatic Arterial
phase

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RESOLUTION IN IMAGING

• THERE ARE 3 COMPETING FORMS OF RESOLUTION
  SPATIAL, CONTRAST, AND TEMPORAL!
• SUCH COMPETITION IS GREATEST IN MRI, WHILE IN
  CT IT CAN BE TRADED OFF THROUGH CHOICE OF A
  RECONSTRUCTION KERNEL BUT ESCALATED BY HIGHER
  RADN DOSE USE OF IV CONTRAST.

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SPATIAL RESOLUTION

• Improves with THINNER SLICES
• But need ? mAs to compensate
• Improves with choice of reconstruction KERNEL
  emphasizing spatial resolution when facilitated
  by great inherent differences in attenuation
  within region or organ of interest

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CONTRAST RESOLUTION

• MAY IMPROVE WITH INHERENT DIFFERENCES IN TISSUE
  ATTENUATION, e.g. IV contrast
• IMPROVES WITH MORE mAs
• IMPROVES WITH USE OF SOFT TISSUE KERNEL

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TEMPORAL RESOLUTION

• IMPROVES BY SCANNING FASTER
• Useful for Freezing or Evaluating
  RAPIDLY-MOVING STRUCTURES, e.g. the HEART OR
• MULTIPHASIC Imaging for assessing Contrast
  Enhancement over time within Organ(s) or
  Lesion(s) ? Pt. Increased Radiation Dose if
  using CT

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WISHFUL THINKING IN CT

• PATIENT SIZE WEIGHT LIMIT OF SCANNER TABLE
• PATIENT BODY HABITUS OBESITY ? ?SCATTER
  PRETZEL CONFIGURATION
• RESIDUAL DENSE GI Contrast

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WISHFUL THINKING IN CT

• (rhetorical negatives)
• NO INCREASED BEAM HARDENING ARTIFACT AT SHOULDERS
  HIPS
• NO EFFECT 2 to UE position
• PT. COOPERATION NO PROB!

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MRI 1

• CURRENTLY, CLINICAL MRI INVOLVES PRIMARILY
  HYDROGEN NUCLEI
• 1 TESLA 10,000 gauss
• Earth Magnetic Field Strength 0.5g

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MRI 2

• TWO SPIN STATES FOR PROTONS EXIST - PARALLEL TO
  APPLIED MAIN MAGNETIC FIELD AND ANTIPARALLEL
• THE ANTIPARALLEL STATE HAS A HIGHER ENERGY LEVEL
  (Q.M.)
• AT EQUILIBRIUM, 100,000 NUCLEI ARE ANTI-// AND
  100,001 ARE //.

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MRI 3

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MRI 4

• RF (radiofrequency) Energy added to system,
  flipping protons from parallel to higher energy
  antiparallel state.
• The excitation frequency required, ?, to flip
  the protons is governed by the LARMOR EQUATION
  ? ? ? Bo

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The NMR Phenomenon

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MAGNETIC FIELD GRADIENTS

• MANIPULATION (OF THE RF ENERGY DEPOSITED) BY
  MAGNETIC FIELD GRADIENTS IS DONE TO ENCODE
  SPATIAL INFORMATION
• ADDITIONAL GRADIENTS MAY BE USED TO CREATE IMAGES
  BASED ON DIFFUSION, DIFFERENCES IN FLOW VELOCITY,
  etc.

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MR Signal Reception

• When RF turned off, the excess of protons in
  antiparallel state returns to the ground state
  and emit either heat or RF, i.e. the patient is
  essentially turned into a little radio
  station!!

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PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 1

• An RF coil is used to receive the emitted
  signal, like an antenna.

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PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 2

• The larger the coil used, the greater the volume
  of coverage.
• BUT, the Larger the Coil, the Lower the
  Signal-to-Noise (aka S/N)

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PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 3

• AND, the Further the Region of Interest is from
  the coil,
• the Lower the S/N !!

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WHAT IS THE SIGNIFICANCE?

• USE THE SMALLEST POSSIBLE COIL NECESSARY TO SCAN
  THE REGION ANSWER THE CLINICAL QUESTION!
• THUS, STATING THE CLINICAL QUESTION(S) CLEARLY
  MAY AID NOT ONLY IMAGE INTERPRETATION, BUT MAY
  DETERMINE HOW THE STUDY IS CONDUCTED!!

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IMAGE CONTRAST POSSIBILITIES

• Processing of emitted RF signal yields Spatial
  Information as
• well as various forms of
• Image Contrast

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Forms of MRI contrast

• T1
• T2
• T2
• Balanced (Proton Density)
• Contrast administration effects

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Forms of MRI contrast

• Selective 1H excitation or presaturation in
  lipid, free H2O, bound H2O, or Si-hyd
• Flow velocity or rate
• Differential O2 (aka BOLD)
• Diffusion
• Diffusion Tensor
• Multi-nuclear Spectroscopy, e.g. 1H, 13C, 19F,
  31P

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MRI 7 WISHFUL THINKING

• PATIENTS MUST -
• LIE FLAT!
• BE STILL!
• FIT INSIDE MAGNET!
• Have SAFETY SCREENING Done!
• FOLLOW INSTRUCTIONS (prn) !

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ACKNOWLEDGEMENTS

• ILLUSTRATIONS COURTESY OF
• MRI in Practice, 3rd ed. Westbrook
• Clinical MRI Atlas, 2nd ed. Runge
• Radiologic Physics, 4th ed. Christenson
• Fundamentals of Radiology, LF Squire