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Imaging Principles
Objectives for this lecture
To teach the basic principles of diagnosticimaging with
X-rays (planar and CT)
Magnetic Resonance
Ultrasound
Radionuclide (SPECT and PET)
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Imaging Principles
Imaging employs
electromagnetic radiation
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Imaging Principles
Medical Imaging modalitiesMedical Imaging modalities
XX--ray CTray CT (X(X--ray computed tomography) uses ionising radiation,ray computed tomography) uses ionising radiation,
source is external to the body. In some cases, contrast agentssource is external to the body. In some cases, contrast agents areare
injected. Anatomical imagesinjected. Anatomical images
MRIMRI (Magnetic resonance imaging)(Magnetic resonance imaging) uses magnetic fields anduses magnetic fields and
radiofrequency pulses to produce anatomical images. In someradiofrequency pulses to produce anatomical images. In some
cases, contrast agents are injected. Also,cases, contrast agents are injected. Also, fMRIfMRIUSUS (Ultrasound imaging)(Ultrasound imaging) uses high frequency sound waves anduses high frequency sound waves and
the pulse echo effect (which is the basis of radar) to give anatthe pulse echo effect (which is the basis of radar) to give anatomicalomical
information.information.
Nuclear medicine imagingNuclear medicine imaging uses unsealed radioactivity touses unsealed radioactivity toproduce functional imagesproduce functional images
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Imaging Principles
Planar - X-rayModern direct capture Radiography
Early X-ray apparatus ~ 1920s
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Imaging Principles
X-ray tube
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Imaging Principles
Production of characteristic X-rays
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Imaging Principles
Production of Bremsstrahlung X-rays
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Process of Image Production
X-rays produced
X-ray photons are either: Attenuated,
Absorbed, Scattered, Transmitted
air < fat < fluid < soft tissue < bone < metal Transmitted X-ray photons (+some scatter)
reaches the cassette and may interact with:
Intensifying screens (produce light) or Film Latent image (i.e. undeveloped) produced
which is then processed.
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Imaging Principles
Producing a Radiograph
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Imaging Principles
Digital images
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Imaging Principles
Direct Capture Radiography
Direct capture Imaging System No Cassettes
Amorphous Silicate
used as detector material
Similar to digital simulator/
treatment setup
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Imaging Principles
Factors affecting Radiograph
Scatter Distance
Movement kVp and mAs settings
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The normal CXR
First of all is the filmtechnically adequate ?
Correct area imaged
Inspiratory effort
Penetration
Rotation
Annotation
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FUNGAL PNEUMONIA
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TUMOUR
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Aggressive- fibrosarcoma
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Non- aggressive- aneurysmal bone cyst
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Aggressive- Ewings tumour
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Imaging Principles
Fluoroscopy
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Imaging Principles
Computerised X-ray Tomography
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Imaging Principles
Computerised X-ray Tomography
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Imaging Principles
CT numbersLinear attenuation coefficient
= Fraction of energy absorbed Tissue approx CT number
dense bone 1000
Muscle 50
white matter 45
grey matter 40Blood 20
CSF 15
Water 0
Fat -100
Lungs - 200Air - 1000
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Imaging Principles
Radiation doses
CT head 2.5 mSv
CT chest 8 mSv
CT abdomen 10 mSv
CT pelvis 10 mSv
chest radiograph PA 0.02 mSv
abdomen radiograph AP 0.7 mSv
pelvis radiograph AP 0.7 mSv
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Imaging Principles
Applications of Imaging in Cancer
Diagnosis
Staging
Monitoring response Detection of recurrence
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Imaging Principles
Diagnosis
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Staging local spread
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Imaging Principles
Staging local spread
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Imaging Principles
Staging lymph nodes
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Staging distant spread
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Magnetic Resonance Imaging
The newest imaging modality
Principle used in spectroscopy since1950s
First human scan 1977
Adopted for clinical use ~ 1988
Approximately 300 in the UK (compared
with approximately 500 CT scanners -which have been around since 1971!)
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Magnetic Resonance Imaging
MRI gives superior soft tissue
discrimination compared with CT: largedifferences in signals emitted from different
soft tissues
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Principle of MRI
The spinning single proton in a hydrogen atom creates a
magnetic field and each hydrogen atom acts like a tiny magnet
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Principle of MRI
In the absence of an external magnetic
field Hydrogen nuclei magnetic momentsare randomly oriented and have a net
magnetization of zero.
In the presence of an external
magnetic field hydrogen protons align
themselves in one of two directions,
parallel or anti-parallel to the netmagnetic field producing a net
magnetic field (Mo)
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Imaging Principles
Precession
The hydrogen atoms are not still but wobble or precess like a
spinning top in the direction of the external magnetic field
Larmor (or precessional) frequency (wO) = B0 x l
Where B0 is the magnetic field and l is the gyromagnetic ratio
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Imaging Principles
Principles of MRI
When the RF is switched off, the protons:
1. Give up the energy they have absorbed and startto return to their previous direction
2. Start to precess out of frequency
With the result that Longitudinal magnetization gradually increases -
called T1 recovery
Transverse magnetization gradually decreases -called T2 decay
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T1 and T2
The rate at which these processes occur vary from tissue to tissue
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Signal intensities on T1
Imaging Principles
High: Fat, bone marrow, contrast agents
Intermediate: Soft Tissues
Low: Water (urine, CSF)
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Signal intensities on T2
Imaging Principles
High: Fat, Water
Intermediate: Soft tissue
Low: Tendons
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Imaging Principles
MR contrast agents
The most common contrast agents are Gadolinium chelates (DOTA,
DTPA, DO3A etc) which interact with the water molecules in its vicinity
to produce white areas in T1 weighted images
T2 T1 +Gd
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Ovarian Cancer within endometrial cyst
Imaging Principles
Pre -Gd Post Gd
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Iron-oxide particles-darken on T2
Imaging Principles
Benign
Malignant
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Mn-DPDP brightens liver on T1
Imaging Principles
T1 T1 + Teslascan
Manganese(II)-dipyridoxal diphosphate (Mn-DPDP)
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Di i l d
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Imaging Principles
Diagnostic ultrasound
Ultrasound imaging uses ultra-high-frequency sound waves (3-10 MHz).Human hearing - 20 to 20 000 Hz
a Piezoelectric transducer ( a "crystalline" material such as quartz thatchanges shape when an electric current is applied creating sound wavesand when struck by sound waves creates electrical currents)
ultrasonic waves are emitted by the transducer and travel through human
tissues at a velocity of 1540 m s-1. When the wave reaches an object orsurface with a different texture or acoustic nature, a wave is reflected
back
these echoes are received by the apparatus, changed into electric currentand a 2-D image is produced
more than 20 frames can be generated per second, giving a smooth, real-time image
Di i Ul d
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Imaging Principles
Diagnostic Ultrasound
The stronger the returning signal, the more white it will beon the grey-scale image (hyperechoic = white or light greye.g. fat containing tissues)
hypoechoic = dark grey (e.g. lymphoma, fibroadenoma ofthe breast)
pure fluid gives no echoes, appearing black (anechoic)leading to acoustic enhancementof tissues distal to e.g.gallbladder and urinary bladder
acoustic shadow is the opposite effect where tissues distalto e.g. gas containing areas, gallstones, renal stones receivelittle sound and thus appear as black
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Ult d di d t
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Ultrasound - disadvantages
interactive modality, operator
dependent
ultrasound waves are greatly
reflected by air-soft tissue and
bone-soft tissue interfaces, thuslimiting its use in the head, chest
and musculoskeletal system
Ultrasound image of gallstone (G) causing accoustic
shadow (S). L = liver
D l Ult d
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Imaging Principles
Doppler Ultrasound
Doppler effect: the influence of a moving object on sound waves
object travelling towards listener causes compression of sound waves (higher
frequency)
object travelling away from listener gives lower frequency
flowing blood causes an alteration to the frequency of the sound wavesreturning to the ultrasound probe, allowing quantitation of blood flow
Colour Doppler shows blood
flowing towards the transducer
as red, blood flowing away as
blue - particularly useful in
echocardiography and
identifying very small blood
vessels
N l M di iN l M di i
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Imaging Principles
.the clinical application.the clinical application
ofof unsealedunsealed radioisotopesradioisotopes
oror radiopharmaceuticalsradiopharmaceuticals
Nuclear MedicineNuclear Medicine
The discovery of RadioactivityThe discovery of Radioactivity
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Imaging Principles
In 1896, Henri Becquerel discovered that uraniumIn 1896, Henri Becquerel discovered that uranium
(and its salts) emitted radiation(and its salts) emitted radiation
2 years later, Pierre and Marie Curie showed that2 years later, Pierre and Marie Curie showed thaturanium rays were an atomic phenomenonuranium rays were an atomic phenomenon
characteristic of the element, and notcharacteristic of the element, and not
related to its chemical or physical state.related to its chemical or physical state.
They called this phenomenonThey called this phenomenon radioactivityradioactivity
Becquerel and the Curies shared the NobelBecquerel and the Curies shared the Nobel
PrizePrize for Physicsfor Physics -- 19031903
y yy y
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Imaging Principles
In 1931, Ernest Lawrence invented theIn 1931, Ernest Lawrence invented the
cyclotron and it became possible tocyclotron and it became possible toproduce artificial radioisotopesproduce artificial radioisotopes
99m99mTc was first produced by a 37 inchTc was first produced by a 37 inch
cyclotron in 1938cyclotron in 1938
the first nuclear medicine scan (the first nuclear medicine scan (131131II--
thyroid) was carried out in 1948 (point bythyroid) was carried out in 1948 (point by
point)point)
Ernest LawrenceErnest Lawrence
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planar imaging using an Anger cameraplanar imaging using an Anger camera
-- 19571957
1967 SPET with Anger camera1967 SPET with Anger camera
(rotating the patient on a chair in front of(rotating the patient on a chair in front of
the camera)the camera)
19781978 -- first commercial gammafirst commercial gamma--cameracamera--based SPECT systemsbased SPECT systems
The beginnings of PET (the techniqueThe beginnings of PET (the technique
of counting gammas from positronof counting gammas from positronannhilationannhilation) had come about in 1951) had come about in 1951
and images were produced in 1953and images were produced in 1953
Hal Anger with hisHal Anger with his
invention, theinvention, the
scintillation camerascintillation camera
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Nuclear Medicine ImagingNuclear Medicine Imaging
Three types of emissions from radioactiveThree types of emissions from radioactive
isotopes:isotopes: particles,particles, particles andparticles and --rays (alsorays (alsosome associated Xsome associated X--rays)rays)
onlyonly --rays are useful for radioisotope imagingrays are useful for radioisotope imaging(high energy photons)(high energy photons)
In radioisotope imaging, source is inside the bodyIn radioisotope imaging, source is inside the body
(X(X--ray CTray CT source is external).source is external).
Nuclear Medicine
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Imaging Principles
Nuclear Medicine
Radiolabelled tracer (Radiopharmaceutical) is administered
-rays (high energy photons) emitted by the radioisotope aredetected outside the body on a Gamma camera
NaI crystal
Lead collimator
Photomultiplier
tubes
Object
Lead collimators are used to
absorb scattered -rays
-rays impinge on sodium iodide
crystals (dense enough to stop the
photons) and converted into light
which is detected byphotomultipliers.
Photon Detection
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Imaging Principles
Photon Detection
photon is converted by
scintillation crystal to flash oflight
Crystal is coupled to
Photomultiplier Tube
Photocathode converts light toelectron.
Electron avalanche leads to
electronic pulse
HV
Crystal
PM
tube
Gamma-camera Principle
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Patient
CollimatorCrystal
Photomultiplier
Acquisition module
Image processor
Gamma camera Principle
Gamma radiation
Functional Imaging
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Functional Imaging
Normal distribution of bone function Abnormal distribution
Quantitative
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Quantitative
Dynamic acquisition
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Dynamic MAGDynamic MAG--3 kidney transplant study3 kidney transplant study
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Tomographic acquisition (SPECT)
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Tomographic acquisition (SPECT)
Myocardial perfusion
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Myocardial perfusion
3-D Rendering
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3 D Rendering
SYSTOLE DIASTOLE
Beating mouse heart
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g
Imaging Principles
Positron Emission Tomography (PET)
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Imaging Principles
g p y ( )
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PET coincidence detection
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bismuth germanate (BGO) or
Lutetium Oxyorthoscilicate (LSO) crystals
No collimators
High sensitivity
Picomolar concentrations
Absolute quantification (moles per microlitre)
Fluorodeoxyglucose -FDG
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yg
Substrate for glucose transporters
undergoes phosphorylation
No further metabolism
FDG shows increased tumour uptake
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Head and Neck
Lung cancer
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Glucose metabolism is very low on the first PET studyGlucose metabolism is very low on the first PET study
GdGd contrast MRIcontrast MRI FDGFDG--PETPET Image overlayImage overlay
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FDGFDG--PET uptake has increased three months later.PET uptake has increased three months later.
This suggestsThis suggests tumortumorrecurrence, and effectively rules outrecurrence, and effectively rules out
radiation necrosis.radiation necrosis.
GdGd contrast MRIcontrast MRI FDGFDG--PETPET Image overlayImage overlay
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Comparison of PET and SPECTComparison of PET and SPECT
Biological isotopes can be used for PETBiological isotopes can be used for PET
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High sensitivity (arising from coincidence detection) and betterHigh sensitivity (arising from coincidence detection) and better
image resolutionimage resolution
Collimators essential for SPECT (much of signal is lost)Collimators essential for SPECT (much of signal is lost)
Attenuation correction in PET is simpleAttenuation correction in PET is simple -- in SPECT it isin SPECT it is v.complexv.complex
PET can be quantitativePET can be quantitative
FastFast -- detector ring in PET collects much more of the signal anddetector ring in PET collects much more of the signal andno need for gantry rotationno need for gantry rotation
HoweverHowever
SPECT is much more commonplace and is cheaper than PETSPECT is much more commonplace and is cheaper than PET
Access to a local cyclotron essential in PETAccess to a local cyclotron essential in PET
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PET-CT - The best of both worlds
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Combines functionalinformation from PET
with anatomical
location provided byCT
PET-CT
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PET/CT shows an area of increased
uptake in the left nasopharynx and
physiologic increased uptake inferior
oral cavity and tongue.
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