the principles of diagnostic imaging
TRANSCRIPT
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Imaging Principles
The Principles of Diagnostic
ImagingStephen J Mather
Barts and the London School of Medicine and Dentistry,
Queen Mary University of London.
Khuloud T Al-Jamal
Institute of Pharmaceutical Sciences
University College London
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Imaging Principles
Objectives for this lecture
• To teach the basic principles of diagnostic
imaging 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 agents source is external to the body. In some cases, contrast agents are are
injected. Anatomical imagesinjected. Anatomical images
MRIMRI (Magnetic resonance imaging) (Magnetic resonance imaging) –– uses magnetic fields and uses magnetic fields and
radiofrequency pulses to produce anatomical images. In some radiofrequency pulses to produce anatomical images. In some
cases, contrast agents are injected. Also, cases, contrast agents are injected. Also, fMRIfMRI
USUS (Ultrasound imaging) (Ultrasound imaging) –– uses high frequency sound waves and uses 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 anatomical omical
information.information.
Nuclear medicine imagingNuclear medicine imaging –– uses unsealed radioactivity to uses unsealed radioactivity to
produce functional imagesproduce functional images
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Imaging Principles
• when experimenting with cathode ray tubes
in a darkened room, he noticed a faint
fluorescent glow emanating from a plate he
had left on the bench
• when he moved to pick it up, he was
amazed to see the image of the bones from
his hand cast onto the plates
• the prospects for x-ray diagnosis were
immediately recognised but Roentgen
refused to patent his discovery
• Won first Nobel Prize in Physics for his
discovery - 1901
The beginnings of RadiologyNovember 1895 - Roentgen discovered X-rays
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Imaging Principles
Planar - X-ray
Modern direct capture Radiography
Early X-ray apparatus ~ 1920’s
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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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Imaging Principles
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 film
technically 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 numbers
Linear attenuation coefficient µ
= Fraction of energy absorbed Tissue approx CT number
dense bone 1000
Muscle 50
white matter 45
grey matter 40
Blood 20
CSF 15
Water 0
Fat -100
Lungs - 200
Air - 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
• substances with high atomic numbers have high density which is
useful for X-ray contrast. Appear bright white in X-ray exams
• e.g. Barium (atomic number 56) causes considerable attenuation of
X-rays compared with the soft tissues of the body (used for barium
meals and barium enema’s for diagnosis in the gastrointestinal
tract) (Barium sulfate - inert) used mainly for plain radiographs
• Salts of iodine (atomic no. 53) are used as water soluble CT
contrast agents. Can be injected intravascularly or into any cavity,
sinus or tract. Can also give an indication of function e.g. filtration
by the kidney. Can be toxic- allergic side effects.
X-ray Contrast Agents
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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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Imaging Principles
Staging – distant spread
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Imaging Principles
Magnetic Resonance Imaging
• The newest imaging modality
• Principle used in spectroscopy since 1950s
• 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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Imaging Principles
Magnetic Resonance Imaging
• MRI gives superior soft tissue
discrimination compared with CT: large
differences in signals emitted from different
soft tissues
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Imaging Principles
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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Imaging Principles
Principle of MRI
In the absence of an external magnetic
field Hydrogen nuclei magnetic moments
are 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 net
magnetic 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
Resonance
If an RF pulse at the Larmor frequency is applied to the
nucleus of an atom, the protons will absorb some energy and
alter their alignment away from the direction of the main magnetic field .
As well as changing direction the protons also begin to precess ‘in phase’
resulting in a net magnetic moment transverse to the external field which
induces a current and is detected in the transiever coil
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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 start to 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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Imaging Principles
T1 and T2
The rate at which these processes occur vary from tissue to tissue
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Imaging Principles
T1 weighted T2 weightedProton density weighted
Imaging Parameters
The duration, repetition, timing and amplitude of RF pulse
sequences are varied to produce signals which can be analysed
in different ways in order to ‘weight’ the image.
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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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Imaging Principles
Magnetic resonance spectroscopy
• allows examination of individual molecules within a sample
• MRS can be used to study the biochemical nature of disease
• looks at concentrations of different substances in tissue to identify
disease
• e.g. brain spectra can give concentrations of N-acetyl aspartate
(NAA), creatine/phosphocreatine and choline. In patients with
temporal lobe epilepsy, the levels of NAA are reduced and the
levels of creatine/phosphocreatine and choline are increased in the
diseased lobe
• e.g. lipid concentration can be used to grade tumours
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Imaging Principles
Ultrasound imaging
• Ultrasound imaging is based on the pulse-echo principle, which is also the basis of radar
• It only came into use as a medical imaging technique after WW2 during which fast electronic pulse technology was developed
• first 2-D ultrasound scan in a living subject (of a myoblastoma in the leg) was carried out in 1951
• 1961 - first scan of pregnant abdomen
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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 that changes shape when an electric current is applied creating sound waves and 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 or surface with a different texture or acoustic nature, a wave is reflected back
• these echoes are received by the apparatus, changed into electric current and a 2-D image is produced
• more than 20 frames can be generated per second, giving a smooth, real-time image
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Imaging Principles
Diagnostic Ultrasound
• The stronger the returning signal, the more white it will be on the grey-scale image (hyperechoic = white or light grey e.g. fat containing tissues)
• hypoechoic = dark grey (e.g. lymphoma, fibroadenoma of the breast)
• pure fluid gives no echoes, appearing black (anechoic) leading to acoustic enhancement of tissues distal to e.g. gallbladder and urinary bladder
• acoustic shadow is the opposite effect where tissues distal to e.g. gas containing areas, gallstones, renal stones receive little sound and thus appear as black
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Imaging Principles
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Imaging Principles
Ultrasound - disadvantages
• interactive modality, operator
dependent
• ultrasound waves are greatly
reflected by air-soft tissue and
bone-soft tissue interfaces, thus
limiting its use in the head, chest
and musculoskeletal system
Ultrasound image of gallstone (G) causing accoustic
shadow (S). L = liver
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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 waves returning 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
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Imaging Principles
GGGG.the clinical application .the clinical application
of of ‘‘unsealedunsealed’’ radioisotopes radioisotopes
or or ‘‘radiopharmaceuticalsradiopharmaceuticals’’
Nuclear MedicineNuclear Medicine
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Imaging Principles
•• In 1896, Henri Becquerel discovered that uranium In 1896, Henri Becquerel discovered that uranium
(and its salts) emitted radiation(and its salts) emitted radiation
•• 2 years later, Pierre and Marie Curie showed that 2 years later, Pierre and Marie Curie showed that
uranium rays were an atomic phenomenon uranium rays were an atomic phenomenon
characteristic of the element, and not characteristic of the element, and not
related to its chemical or physical state.related to its chemical or physical state.
•• They called this phenomenon They called this phenomenon ““radioactivityradioactivity””
•• Becquerel and the Curies shared the NobelBecquerel and the Curies shared the Nobel
Prize Prize for Physics for Physics -- 19031903
The discovery of RadioactivityThe discovery of Radioactivity
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Imaging Principles
•• In 1931, Ernest Lawrence invented the In 1931, Ernest Lawrence invented the
cyclotron and it became possible to cyclotron and it became possible to
produce artificial radioisotopesproduce artificial radioisotopes
•• 99m99mTc was first produced by a 37 inch Tc 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 by thyroid) was carried out in 1948 (point by
point)point)
Ernest LawrenceErnest Lawrence
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Imaging Principles
••planar imaging using an Anger camera planar imaging using an Anger camera
-- 19571957
••1967 SPET with Anger camera 1967 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)
••1978 1978 -- first commercial gammafirst commercial gamma--
cameracamera--based SPECT systemsbased SPECT systems
••The beginnings of PET (the technique The beginnings of PET (the technique
of counting gammas from positron of counting gammas from positron
annhilationannhilation) had come about in 1951 ) had come about in 1951
and images were produced in 1953and images were produced in 1953
Hal Anger with his Hal Anger with his
invention, the invention, the
scintillation camera scintillation camera
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Imaging Principles
Nuclear Medicine ImagingNuclear Medicine Imaging
•• Three types of emissions from radioactive Three types of emissions from radioactive
isotopes: isotopes: αα particles, particles, ββ particles and particles and γγ--rays (also rays (also
some associated Xsome associated X--rays)rays)
•• only only γγ--rays are useful for radioisotope imaging rays are useful for radioisotope imaging
(high energy photons)(high energy photons)
•• In radioisotope imaging, source is inside the body In radioisotope imaging, source is inside the body
(X(X--ray CT ray CT –– source is external). source is external).
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Imaging Principles
Nuclear Medicine
• Radiolabelled tracer (Radiopharmaceutical) is administered
• γ-rays (high energy photons) emitted by the radioisotope are detected outside the body on a ‘Gamma camera’
NaI crystal
Lead collimator
Photomultiplier
tubes
Object
• Lead ‘collimators’ are used to
absorb scattered γ-rays
• γ-rays impinge on sodium iodide
crystals (dense enough to stop the
photons) and converted into light
which is detected by
photomultipliers.
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Imaging Principles
Photon Detection
• photon is converted by
scintillation crystal to flash of
light
• Crystal is coupled to
Photomultiplier Tube
• Photocathode converts light to
electron.
• Electron avalanche leads to
electronic pulse
HV
Crystal
PM
tube
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Imaging Principles
Patient
CollimatorCrystal
Photomultiplier
Acquisition module
Image processor
Gamma-camera Principle
Gamma radiation
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Imaging Principles
Functional Imaging
Normal distribution of bone function Abnormal distribution
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Imaging Principles
Quantitative
Dynamic acquisition
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Imaging Principles
RenogramRenogram with absent Left kidney functionwith absent Left kidney function
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Imaging Principles
Dynamic MAGDynamic MAG--3 kidney transplant study3 kidney transplant study
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Imaging Principles
Tomographic acquisition (SPECT)
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Imaging Principles
Myocardial perfusion
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Imaging Principles
3-D Rendering
SYSTOLE DIASTOLE
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Beating mouse heart
Imaging Principles
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Imaging Principles
Positron Emission Tomography (PET)
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Imaging Principles
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Imaging Principles
PET coincidence detection
bismuth germanate (BGO) or
Lutetium Oxyorthoscilicate (LSO) crystals
• No collimators
• High sensitivity
• Picomolar concentrations
• Absolute quantification (moles per microlitre)
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Imaging Principles
Fluorodeoxyglucose -FDG
• Substrate for glucose transporters
• undergoes phosphorylation
• No further metabolism
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Imaging Principles
FDG shows increased tumour uptake
Head and Neck
Lung cancer
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Imaging Principles
FDG-whole body PET showed increased glucose metabolism, highly
suspicious for metastatic breast carcinoma. Fine-needle aspiration
revealed metastatic adenocarcinoma. NOTE: MRI is negative
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Imaging Principles
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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Imaging Principles
FDGFDG--PET uptake has increased three months later. PET uptake has increased three months later.
This suggests This suggests tumortumor recurrence, and effectively rules out recurrence, and effectively rules out
radiation necrosis. radiation necrosis.
GdGd contrast MRIcontrast MRI FDGFDG--PETPET Image overlayImage overlay
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Imaging Principles
Biologically relevant Biologically relevant radionuclidesradionuclides
From From KaschtenKaschten et alet al., JNM, ., JNM, 3939 (1998), 778(1998), 778
NH2
C11
OS
CH3
OH
OH
OH
OH
O
OH
F18
C-11 methionine
FDG
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Imaging Principles
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Imaging Principles
Comparison of PET and SPECTComparison of PET and SPECT
Biological isotopes can be used for PETBiological isotopes can be used for PET
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 simple Attenuation correction in PET is simple -- in SPECT it is in SPECT it is v.complexv.complex
PET can be quantitativePET can be quantitative
Fast Fast -- detector ring in PET collects much more of the signal and detector ring in PET collects much more of the signal and
no 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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Imaging Principles
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Imaging Principles
PET-CT - The best of both worlds
Combines functional
information from PET
with anatomical
location provided by
CT
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Imaging Principles
PET-CT
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Imaging Principles
PET/CT shows an area of increased
uptake in the left nasopharynx and
physiologic increased uptake inferior
oral cavity and tongue.