Imaging physics and limitations
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Imaging physics and limitations

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Imaging physics and limitations Imaging physics and limitations Presentation Transcript

  • INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS
  •  
  • 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
    View slide
  • TEST-TAKER TOPICS
    • KNOW THE WISHFUL THINKING PITFALLS!
    • REVIEW THE “TAKE-HOME” MESSAGES FOR EACH IMAGING MODALITY!
    • (denoted by a RED asterisk - * )
    View slide
  • OVERVIEW
    • RADIOGRAPHY, FLUOROSCOPY, & DSA
    • COMPUTED TOMOGRAPHY
    • MAGNETIC RESONANCE IMAGING
  • SPECTRUM OF E-M RADIATION
  • 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.
  • 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
  • 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.
  • SCATTERING
  • How reduce X-ray SCATTERING ? ASK YOUR PATIENTS TO LOSE WEIGHT?
  • TO  SCATTERING
    • COLLIMATION * of X-ray Beam
    • Use of GRIDS * in cassettes
  • X-ray Collimation
  • X-ray GRID Tradeoff Grids require  mAs compared with XR studies done w/o grids
  • How Improve Spatial Resolution & Decrease Image Distortion?
  • Center the Area of Interest!
  • AP versus PA
    • Direction of emitted beam from the X-ray tube  Patient  Cassette
    • AP = Anterior to Posterior
    • PA = Posterior to Anterior
  • 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, ………..
  • FIRST APHORISM DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!
  • DON’T 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.
  • SECOND APHORISM YOU CANNOT CALL WHAT YOU DON’T SEE! * HOWEVER, IF YOU SUSPECT SOMETHING, GET ANOTHER VIEW!! *
  • DIGITAL/COMPUTED RADIOGRAPHY IMAGES CAN BE MANIPULATED POST-ACQUISITION TO OPTIMIZE VIEWING OF ONE PART OF H&D Curve.
  • WISHFUL THINKING IN RADIOGRAPHY
    • QUALITY OF PORTABLE STUDIES *
    • PATIENT THICKNESS & SIZE *
    • Table Weight limits *
    • COOPERATIVENESS OF PATIENT *
  • WISHFUL THINKING IN RADIOGRAPHY
    • QUALITY OF PORTABLE STUDIES *
    • PATIENT THICKNESS & SIZE *
    • Table Weight limits *
    • COOPERATIVENESS OF PATIENT *
  • X-ray COMPUTED AXIAL TOMOGRAPHY
    • aka CAT scan (archaic,) now CT
    • “ STEP AND SHOOT ” mode
    • 1 st Gen CT Scanner – 45 min/slice
  • 2 nd Generation CT scanner
  • %Transmission Special Case For monochromatic Photon energy – log %T α 1/linear attenuation
  • 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
  • X-ray Attenuation Revisited * %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
  • X-ray ATTENUATION
    • µ - the intrinsic X-ray coefficient
    • a function of:
    • kVp *
    • Atomic Mass *
    • electron density *
  • ATTENUATION VALUE – CT * Hounsfield Units (H.U.) * of sample S = ( μ S - μ H2O ) x 1000 μ H2O
  • 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.)
  • CT – ADVANTAGES II
    • Easier & Quicker than MRI * but not always better tissue contrast resolution
    • ~BEST for detection & characterization of CALCIFICATION *
  • CT BEST FOR Calcification e.g. a Bony Sequestrum & Involucrum of Osteomyelitis
  • CT - DISADVANTAGES
    • IONIZING RADIATION!! *
    • EACH SERIES OF IMAGES TOGETHER IS ONLY ONE SNAPSHOT IN TIME *
    • ARTIFACTS : Partial Volume *
    • Scattering (Obesity) *
    • Beam Hardening *
    • Metal Streaking *
  • PARTIAL VOLUME EFFECT
  • EFFECT OF THICK SLICES
  • BEAM-HARDENING
  • Metallic streaking
  • … AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS
  • 3 RD GENERATION CT
  • HELICAL CT
    • 3 rd GENERATION CT SCANNER +
    • ADVENT OF
    • SLIP RING TECHNOLOGY TO CREATE HELICAL ACQ’N!
  • ORIGIN OF MultiDetector CT
    • TWIN DETECTOR concept done with conventional “STEP & SHOOT” technique
    • MARRIAGE OF MULTIDETECTOR DESIGN WITH HELICAL DESIGN
    • -> MDCT !
  • THIN SLICES  ISOTROPIC VOXELS
  • 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.
  • Hypervascular Met only seen on Hepatic Arterial phase
  • 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 RAD’N DOSE & USE OF IV CONTRAST .
  • 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
  • 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
  • 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
  • WISHFUL THINKING IN CT *
    • PATIENT SIZE – WEIGHT LIMIT OF SCANNER TABLE
    • PATIENT BODY HABITUS OBESITY ->  SCATTER ; “PRETZEL” CONFIGURATION
    • RESIDUAL DENSE GI Contrast
  • WISHFUL THINKING IN CT *
    • (rhetorical negatives)
    • NO INCREASED BEAM HARDENING ARTIFACT AT SHOULDERS & HIPS
    • NO EFFECT 2 ° to UE position
    • PT. COOPERATION – NO PROB!
  • MRI 1
    • CURRENTLY, CLINICAL MRI INVOLVES PRIMARILY HYDROGEN NUCLEI
    • 1 TESLA = 10,000 gauss
    • Earth Magnetic Field Strength = 0.5g
  • 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 //.
  • MRI 3
  • MRI 4
    • RF (radiofrequency) Energy added to system, “flipping” protons from parallel to higher energy antiparallel state.
    • The excitation frequency required, ω , to “flip” the protons is governed by the LARMOR EQUATION: ω = γ  B o
  • The NMR Phenomenon
  • 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.
  • 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”!!
  • PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 1 *
    • An RF coil * is used to receive the emitted signal, like an antenna.
  • 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) *
  • PRINCIPLE CONCEPTS OF COIL USAGE IN MRI - 3 *
    • AND , the Further the Region of Interest is from the coil,
    • the Lower the S/N !! *
  • 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 !! *
  • IMAGE CONTRAST POSSIBILITIES Processing of emitted RF signal yields Spatial Information as well as various forms of Image Contrast
  • Forms of MRI contrast
    • T1
    • T2
    • T2*
    • Balanced (“Proton Density”)
    • Contrast administration effects
  • Forms of MRI contrast
    • Selective 1 H excitation or presaturation in lipid, free H 2 O, bound H 2 O, or Si-hyd
    • Flow velocity or rate
    • Differential [O 2 ] (aka BOLD)
    • Diffusion
    • Diffusion Tensor
    • Multi-nuclear Spectroscopy, e.g. 1 H, 13 C, 19 F, 31 P
  • MRI 7 * WISHFUL THINKING
    • PATIENTS MUST -
    • LIE FLAT!
    • BE STILL!
    • FIT INSIDE MAGNET!
    • Have SAFETY SCREENING Done!
    • FOLLOW INSTRUCTIONS (prn) !
  • ACKNOWLEDGEMENTS ILLUSTRATIONS COURTESY OF: MRI in Practice , 3 rd ed. Westbrook… Clinical MRI Atlas , 2 nd ed. Runge… Radiologic Physics , 4 th ed. Christenson… Fundamentals of Radiology , LF Squire