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INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS. DAVID B. CHALPIN, MD. ASSISTANT PROFESSOR OF CLINICAL RADIOLOGY LSU HEALTH SCIENCES CENTER NEW ORLEANS, LA. GOALS. TO BECOME FAMILIAR WITH THE BASICS OF IMAGE GENERATION USING X-rays, CT, AND MRI

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INTRODUCTION TO IMAGING PHYSICS CAPABILITIES AND LIMITATIONS

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INTRODUCTION TO IMAGING PHYSICSCAPABILITIES AND LIMITATIONS


DAVID B. CHALPIN, MD

ASSISTANT PROFESSOR OF

CLINICAL RADIOLOGY

LSU HEALTH SCIENCES CENTER

NEW ORLEANS, LA


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


TEST-TAKER TOPICS

  • KNOW THE WISHFULTHINKING PITFALLS!

  • REVIEW THE “TAKE-HOME” MESSAGES FOR EACH IMAGING MODALITY!

    (denoted by a RED asterisk - *)


OVERVIEW

  • RADIOGRAPHY, FLUOROSCOPY, & DSA

  • COMPUTED TOMOGRAPHY

  • MAGNETIC RESONANCE IMAGING


SPECTRUM OFE-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 mAsfor 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 beABSORBED

  • Photonscan beSCATTERED with some exposing the film degrading the image, akaFOGGING, OR

  • Photonscan 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

  • HOWCONVENIENT!!

  • 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 QualityOR 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

  • 1st Gen CT Scanner – 45 min/slice


2nd Generation CT scanner


%TransmissionSpecial 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 AttenuationRevisited

*%Transmission of photon energyreceived 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 =

(μS - μH2O) x 1000

μH2O


CT – ADVANTAGES I

COMPARED WITH X-rays, U/S, & MRI

  • Better Soft Tissue ContrastResolutionthan 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 ONLYONE SNAPSHOT IN TIME*

  • ARTIFACTS: Partial Volume*

    Scattering (Obesity)*

    Beam Hardening*

    Metal Streaking*


PARTIAL VOLUMEEFFECT


EFFECT OF THICK SLICES


BEAM-HARDENING


Metallic streaking


… AND IMAGES DERIVED FROM THOSE w/ ARTIFACTS


3RD GENERATION CT


HELICAL CT

  • 3rd GENERATION CT SCANNER +

  • ADVENT OF

    SLIP RING TECHNOLOGY TO CREATE HELICAL ACQ’N!


ORIGIN OF MultiDetector CT

  • TWIN DETECTORconcept 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 Metonly 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Bo


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 Lowerthe S/N !!*


WHAT IS THE SIGNIFICANCE?

  • USE THE SMALLEST POSSIBLE COILNECESSARY TO SCAN THE REGION & ANSWER THE CLINICAL QUESTION!*

  • THUS, STATING THE CLINICAL QUESTION(S) CLEARLY MAYAIDNOT ONLY IMAGE INTERPRETATION,BUTMAY DETERMINE HOW THE STUDY IS CONDUCTED!! *


IMAGE CONTRAST POSSIBILITIES

Processing of emitted RF signal yields SpatialInformationas

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 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


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, 3rd ed. Westbrook…

Clinical MRI Atlas, 2nd ed. Runge…

Radiologic Physics, 4th ed. Christenson…

Fundamentals of Radiology, LF Squire


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