Imaging physics and limitations

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