Transcript introduction to imaging physics capabilities and limitations

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
TO BECOME FAMILIAR WITH THE
LIMITATIONS OF IMAGING AS
PRACTICALLY APPLIED
TEST-TAKER TOPICS
KNOW THE WISHFUL
THINKING 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 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

1st Gen CT Scanner – 45 min/slice
2nd 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 =
(μS - μH2O) x 1000
μH2O
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
RD
3
GENERATION CT
HELICAL CT
3rd 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  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 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



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Selective 1H excitation or presaturation
in lipid, free H2O, bound H2O, or Sihyd
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