imaging physics and limitations
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INTRODUCTION INTRODUCTION TO IMAGING TO IMAGING
PHYSICSPHYSICS
CAPABILITIES AND CAPABILITIES AND LIMITATIONSLIMITATIONS
GOALS TO BECOME FAMILIAR WITH THE TO BECOME FAMILIAR WITH THE
BASICS OF IMAGE GENERATION BASICS OF IMAGE GENERATION USING X-rays, CT, AND MRIUSING X-rays, CT, AND MRI
TO BECOME FAMILIAR WITH THE TO BECOME FAMILIAR WITH THE LIMITATIONS OF IMAGING AS LIMITATIONS OF IMAGING AS PRACTICALLY APPLIEDPRACTICALLY APPLIED
TEST-TAKER TEST-TAKER TOPICSTOPICS
KNOW THE WISHFUL THINKING PITFALLS!
REVIEW THE “TAKE-HOME” MESSAGES FOR EACH IMAGING MODALITY!
(denoted by a RED asterisk - *)
OVERVIEWOVERVIEW
RADIOGRAPHY, FLUOROSCOPY, & DSA
COMPUTED TOMOGRAPHY
MAGNETIC RESONANCE IMAGING
SPECTRUM OFSPECTRUM OFE-M RADIATIONE-M RADIATION
GENERATION GENERATION OF X-RaysOF X-Rays
VACUUM TUBEElectric 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 FatesX-Rays – 3 Fates**
Photons can be Photons can be ABSORBED Photons can be Photons can be SCATTERED
with some exposing the film with some exposing the film degrading the image, aka degrading the image, aka FOGGING, OR
Photons can proceed directly Photons can proceed directly through subject to through subject to EXPOSE film.film.
SCATTERINGSCATTERING
How reduce X-ray How reduce X-ray SCATTERINGSCATTERING??
ASK YOUR PATIENTS TO LOSE WEIGHT?
TO TO SCATTERING SCATTERING
COLLIMATION** of X-ray Beam
Use of GRIDS** in cassettes
X-ray CollimationX-ray Collimation
X-ray GRIDX-ray GRID
TradeoffTradeoff Grids require
mAs
compared with XR studies done w/o grids
How Improve
Spatial Resolution & Decrease Image
Distortion?
Center the Area Center the Area
of Interest!of Interest!
AP versus PA
Direction of emitted beam from the X-ray tube
PatientCassette
AP = Anterior to Posterior PA = Posterior to Anterior
PORTABLE X-RAYSPORTABLE 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 APHORISMFIRST APHORISM
DON’T MAKE GOOD CALLS FROM “BAD” FILMS !!
DON’T MAKE GOOD DON’T MAKE GOOD CALLS FROM “BAD” CALLS FROM “BAD”
FILMS !!!FILMS !!!**
“Bad”
can mean Suboptimal Quality OR the study as ordered was NOT dedicated for evaluation
of Region or Organ of Interest.
SECOND SECOND APHORISMAPHORISM
YOU CANNOT CALL WHAT YOU CANNOT CALL WHAT YOU DON’T SEE!YOU DON’T SEE!**
HOWEVER, IF YOU HOWEVER, IF YOU
SUSPECT SOMETHING,SUSPECT SOMETHING,
GET ANOTHER VIEW!!GET ANOTHER VIEW!!**
DIGITAL/COMPUTED RADIOGRAPHY
IMAGES CAN BE IMAGES CAN BE MANIPULATED MANIPULATED POST-POST-ACQUISITIONACQUISITION TO TO OPTIMIZE OPTIMIZE VIEWING OF ONE VIEWING OF ONE PART OF H&D PART OF H&D Curve.Curve.
WISHFUL THINKING IN WISHFUL THINKING IN RADIOGRAPHYRADIOGRAPHY
QUALITY OF QUALITY OF PORTABLEPORTABLE STUDIESSTUDIES**
PATIENT THICKNESS & SIZEPATIENT THICKNESS & SIZE** Table Table Weight limits** COOPERATIVENESSCOOPERATIVENESS OF OF
PATIENTPATIENT**
WISHFUL THINKING IN WISHFUL THINKING IN RADIOGRAPHYRADIOGRAPHY
QUALITY OF QUALITY OF PORTABLEPORTABLE STUDIESSTUDIES**
PATIENT THICKNESS & SIZEPATIENT THICKNESS & SIZE** Table Table Weight limits** COOPERATIVENESSCOOPERATIVENESS OF OF
PATIENTPATIENT**
X-ray COMPUTED X-ray COMPUTED AXIAL TOMOGRAPHYAXIAL TOMOGRAPHY
aka aka CATCAT scan (archaic,) now CT
“STEP AND SHOOT” mode
1st Gen CT Scanner – 45 min/slice
22ndnd Generation CT Generation CT scannerscanner
%Transmission%TransmissionSpecial CaseSpecial Case
For For monochromatic Photon energymonochromatic Photon energy ––
log %T log %T αα 1/linear attenuation 1/linear attenuation
What data What data generates an generates an
image as a slice?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 energy received by detectors is recorded at multiple projections around the subject & the data is reconstructed to create a cross-sectional image
X-ray ATTENUATIONX-ray ATTENUATION
µ - the intrinsic X-ray coefficient
a function of:
kVp* Atomic Mass* electron density*
ATTENUATION ATTENUATION VALUE – CTVALUE – CT**
Hounsfield Units (H.U.)Hounsfield Units (H.U.)**
of sample S =of sample S =
((μμSS - μ - μH2OH2O)) x x 10001000
μμH2OH2O
CT – ADVANTAGES CT – ADVANTAGES II
COMPARED WITH X-rays, U/S, & COMPARED WITH X-rays, U/S, & MRIMRI
• Better Soft Tissue Better Soft Tissue ContrastContrast ResolutionResolution than XR & usually than XR & usually Ultrasound (except reproductive Ultrasound (except reproductive organs, in general)organs, in general)**
• AlongAlong with Fluoroscopy using with Fluoroscopy using Barium, CT best for Barium, CT best for Intestinal Intestinal Tract EvaluationTract Evaluation* * (though not so (though not so “dynamic” as fluoro.)“dynamic” as fluoro.)
CT – ADVANTAGES CT – ADVANTAGES IIII
• Easier & Quicker than MRIEasier & Quicker than MRI* * but but not always better not always better tissue contrast tissue contrast resolutionresolution
• ~BEST~BEST for detection & for detection & characterization of characterization of CALCIFICATIONCALCIFICATION**
CT BEST FOR CT BEST FOR CalcificationCalcification
e.g. a Bony Sequestrum & e.g. a Bony Sequestrum & InvolucrumInvolucrum
of Osteomyelitisof Osteomyelitis
CT - CT - DISADVANTAGESDISADVANTAGES
IONIZING RADIATION!!IONIZING RADIATION!!** EACH SERIES EACH SERIES OF IMAGES OF IMAGES
TOGETHER IS TOGETHER IS ONLYONLY ONE ONE SNAPSHOT IN TIMESNAPSHOT IN TIME**
ARTIFACTSARTIFACTS: Partial Volume*Scattering
(Obesity)*Beam Hardening*Metal Streaking*
PARTIAL VOLUMEPARTIAL VOLUMEEFFECTEFFECT
EFFECT OF EFFECT OF THICK SLICESTHICK SLICES
BEAM-HARDENINGBEAM-HARDENING
Metallic streakingMetallic streaking
… … AND IMAGES DERIVED AND IMAGES DERIVED FROM THOSE w/ FROM THOSE w/
ARTIFACTSARTIFACTS
33RDRD GENERATION GENERATION CTCT
HELICAL CTHELICAL CT
33rdrd GENERATIONGENERATION CT SCANNER CT SCANNER ++
ADVENT OF ADVENT OF
SLIP RINGSLIP RING TECHNOLOGY TECHNOLOGY TO CREATE TO CREATE HELICAL HELICAL ACQ’N!ACQ’N!
ORIGIN OF ORIGIN OF MultiDetector CTMultiDetector CT
TWIN DETECTOR concept concept done with conventional “STEP done with conventional “STEP & SHOOT” technique& SHOOT” technique
MARRIAGE OF MARRIAGE OF MULTIDETECTOR MULTIDETECTOR DESIGN DESIGN WITH WITH HELICALHELICAL DESIGN DESIGN
→ → MDCT !
THIN SLICESTHIN SLICES ISOTROPICISOTROPIC
VOXELSVOXELS
IV Contrast - TIMING IV Contrast - TIMING of Image Acquisitionof Image Acquisition
X-ray, U/S, but X-ray, U/S, but ESPECIALLY CT & MRI!
CONTRAST ENHANCEMENT PHASES:
Arterial; Hepatic Arterial; Portal Venous; Renal Capillary; Renal Excretion, etc.
Hypervascular MetHypervascular Metonly seen on only seen on
Hepatic Arterial Hepatic Arterial phasephase
RESOLUTION IN RESOLUTION IN IMAGINGIMAGING
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 RESOLUTIONSPATIAL RESOLUTION
Improves with Improves with THINNER SLICESTHINNER SLICES
But need But need mAs mAs to compensate to compensate Improves with choice of Improves with choice of
reconstruction reconstruction KERNELKERNEL* emphasizing spatial resolution emphasizing spatial resolution when facilitated by great inherent when facilitated by great inherent differences in attenuation within differences in attenuation within region or organ of interestregion or organ of interest
CONTRAST CONTRAST RESOLUTIONRESOLUTION
MAY IMPROVE WITH MAY IMPROVE WITH INHERENT DIFFERENCES IN INHERENT DIFFERENCES IN TISSUE ATTENUATION, e.g.TISSUE ATTENUATION, e.g. IV IV contrastcontrast
IMPROVES WITH IMPROVES WITH MORE mAsMORE mAs IMPROVES WITH IMPROVES WITH USE OF SOFT USE OF SOFT
TISSUE KERNELTISSUE KERNEL
TEMPORAL TEMPORAL RESOLUTIONRESOLUTION
IMPROVES BY IMPROVES BY SCANNING FASTERSCANNING FASTER Useful for “Freezing” or Useful for “Freezing” or
Evaluating Evaluating RAPIDLY-MOVING RAPIDLY-MOVING STRUCTURESSTRUCTURES, e.g. the HEART , e.g. the HEART OROR
MULTIPHASICMULTIPHASIC Imaging for Imaging for assessing Contrast Enhancement assessing Contrast Enhancement over time within Organ(s) or over time within Organ(s) or Lesion(s) → Lesion(s) → Pt.Pt. Increased Increased Radiation Dose if using CTRadiation Dose if using CT
WISHFUL WISHFUL THINKING THINKING
IN CTIN CT** PATIENT SIZEPATIENT SIZE – WEIGHT – WEIGHT
LIMIT OF SCANNER TABLELIMIT OF SCANNER TABLE PATIENT BODY HABITUSPATIENT BODY HABITUS
OBESITY OBESITY → → SCATTERSCATTER;; “PRETZEL” “PRETZEL” CONFIGURATIONCONFIGURATION
RESIDUAL RESIDUAL DENSEDENSE GI GI ContrastContrast
WISHFUL WISHFUL THINKING THINKING
IN CTIN CT**(rhetorical negatives)(rhetorical negatives)
NO NO INCREASED INCREASED BEAM BEAM HARDENINGHARDENING ARTIFACT AT ARTIFACT AT SHOULDERS & HIPSSHOULDERS & HIPS
NO EFFECT NO EFFECT 22° to ° to UE UE positionposition
PT. COOPERATION – PT. COOPERATION – NO NO PROB!PROB!
MRI 1MRI 1 CURRENTLY, CLINICAL MRI CURRENTLY, CLINICAL MRI
INVOLVES PRIMARILY HYDROGEN INVOLVES PRIMARILY HYDROGEN NUCLEINUCLEI
1 TESLA = 10,000 gauss1 TESLA = 10,000 gauss Earth Magnetic Field Strength = Earth Magnetic Field Strength =
0.5g0.5g
MRI 2MRI 2
TWO SPIN STATES FOR PROTONS TWO SPIN STATES FOR PROTONS EXIST - EXIST - PARALLELPARALLEL TO APPLIED TO APPLIED MAIN MAGNETIC FIELD AND MAIN MAGNETIC FIELD AND ANTIPARALLELANTIPARALLEL
THE ANTIPARALLEL STATE HAS A THE ANTIPARALLEL STATE HAS A HIGHER ENERGY LEVEL (Q.M.)HIGHER ENERGY LEVEL (Q.M.)
AT EQUILIBRIUM, 100,000 NUCLEI AT EQUILIBRIUM, 100,000 NUCLEI ARE ANTI-// AND 100,001 ARE //.ARE ANTI-// AND 100,001 ARE //.
MRI 3MRI 3
MRI 4MRI 4
RF (radiofrequency) Energy RF (radiofrequency) Energy added added to system, “flipping” protons from to system, “flipping” protons from parallel to higher energy parallel to higher energy antiparallel state.antiparallel state.
The excitation frequency The excitation frequency
required, required, ωω, to “flip” the protons , to “flip” the protons is governed by the is governed by the LARMOR LARMOR EQUATION: EQUATION:
ω = γ ω = γ BBoo
The NMR The NMR PhenomenonPhenomenon
MAGNETIC FIELD MAGNETIC FIELD GRADIENTSGRADIENTS
MANIPULATION (OF THE RF MANIPULATION (OF THE RF ENERGY DEPOSITED) BY ENERGY DEPOSITED) BY MAGNETIC FIELD GRADIENTSMAGNETIC FIELD GRADIENTS IS DONE TO ENCODE IS DONE TO ENCODE SPATIAL SPATIAL INFORMATIONINFORMATION
ADDITIONAL GRADIENTS MAY BE ADDITIONAL GRADIENTS MAY BE USED TO CREATE IMAGES BASED USED TO CREATE IMAGES BASED ON DIFFUSION, DIFFERENCES IN ON DIFFUSION, DIFFERENCES IN FLOW VELOCITY, etc.FLOW VELOCITY, etc.
MR Signal MR Signal ReceptionReception
When When RFRF turned off, the turned off, the excess # of protons in excess # of protons in antiparallel state returns to antiparallel state returns to the ground state and emit the ground state and emit either heat or either heat or RFRF, , i.e. the i.e. the patient is essentially turned patient is essentially turned into a “little radio station”!!into a “little radio station”!!
PRINCIPLE CONCEPTS PRINCIPLE CONCEPTS OF COIL USAGE IN MRI OF COIL USAGE IN MRI
- 1- 1** An An RF coilRF coil** is used to receive is used to receive
the emitted signal, like an the emitted signal, like an antenna.antenna.
PRINCIPLE CONCEPTS PRINCIPLE CONCEPTS OF COIL USAGE IN MRI OF COIL USAGE IN MRI
- 2- 2** The The largerlarger the coil used, the the coil used, the greatergreater the the volumevolume of coverage. of coverage.**
BUTBUT, the , the LargerLarger the Coil, the the Coil, the LowerLower the the Signal-to-Noise (aka Signal-to-Noise (aka S/N)S/N)**
PRINCIPLE CONCEPTS PRINCIPLE CONCEPTS OF COIL USAGE IN MRI OF COIL USAGE IN MRI
- 3- 3** ANDAND, the , the Further the Region of Further the Region of
Interest Interest is from the coil, is from the coil,
the the LowerLower the S/N !!the S/N !!**
WHAT IS THE WHAT IS THE SIGNIFICANCE?SIGNIFICANCE?
USE THE SMALLEST POSSIBLE USE THE SMALLEST POSSIBLE COILCOIL NECESSARY TO SCAN THE NECESSARY TO SCAN THE REGION & ANSWER THE CLINICAL REGION & ANSWER THE CLINICAL QUESTION!QUESTION!**
THUS, STATING THE CLINICAL THUS, STATING THE CLINICAL QUESTION(S) CLEARLY QUESTION(S) CLEARLY MAYMAY AIDAID NOT ONLY IMAGE NOT ONLY IMAGE INTERPRETATION,INTERPRETATION, BUTBUT MAY MAY DETERMINE DETERMINE HOWHOW THE STUDY IS THE STUDY IS CONDUCTEDCONDUCTED!!!! * *
IMAGE CONTRAST IMAGE CONTRAST POSSIBILITIESPOSSIBILITIES
ProcessingProcessing of emitted RF of emitted RF signal yields signal yields SpatialSpatial
InformationInformation as as
well as various forms of well as various forms of
Image Contrast Image Contrast
Forms of MRI Forms of MRI contrastcontrast
T1T1 T2 T2 T2*T2* Balanced (“Proton Density”)Balanced (“Proton Density”) Contrast administration Contrast administration
effectseffects
Forms of MRI Forms of MRI contrastcontrast Selective Selective 11H excitation or H excitation or
presaturation in lipid, free Hpresaturation in lipid, free H22O, O, bound Hbound H22O, or Si-hydO, or Si-hyd
Flow velocity or rateFlow velocity or rate Differential [ODifferential [O22] (aka BOLD)] (aka BOLD) DiffusionDiffusion Diffusion TensorDiffusion Tensor Multi-nuclear Spectroscopy, e.g. Multi-nuclear Spectroscopy, e.g.
11H, H, 1313C, C, 1919F, F, 3131P P
MRI 7MRI 7** WISHFUL THINKINGWISHFUL THINKING
PATIENTS PATIENTS MUSTMUST - - LIE FLAT!LIE FLAT! BE STILL!BE STILL! FIT INSIDE MAGNET!FIT INSIDE MAGNET! Have SAFETY SCREENING Have SAFETY SCREENING
Done!Done! FOLLOW INSTRUCTIONS FOLLOW INSTRUCTIONS
(prn) !(prn) !
ACKNOWLEDGEMENTACKNOWLEDGEMENTSS
ILLUSTRATIONS COURTESY OF:ILLUSTRATIONS COURTESY OF:
MRI in PracticeMRI in Practice, 3, 3rdrd ed. Westbrook… ed. Westbrook…
Clinical MRI AtlasClinical MRI Atlas, 2, 2ndnd ed. Runge… ed. Runge…
Radiologic PhysicsRadiologic Physics, 4, 4thth ed. ed. Christenson…Christenson…
Fundamentals of RadiologyFundamentals of Radiology, LF , LF SquireSquire