Nuclear Imaging Gamma CameraPhysics & Instrumentation : Nuclear Imaging Gamma CameraPhysics & Instrumentation Dr. Swagat Dash Introduction : Introduction Nuclear imaging Gamma Camera produces images of the distribution(site and no. of radioactive atoms) of radionuclides within patient .
Uses gamma rays or characteristic x-rays to form images.
less than 1% of emitted gamma rays are used to form image .
Vast majority (over 99.95%) of emitted photons is wasted in nuclear imaging gamma camera. Gamma Camera Imaging : Gamma Camera Imaging Rectilinear Scanner
Anger Scintillation Camera (single crystal)
Multiple crystal Gamma Camera
Mobile Gamma Camera
Single Photon Emission Computed Tomography(SPECT) Rectilinear scanner : Rectilinear scanner Rectilinear scanner was developed by Benedict cassen-1950s
A single moving radiation detector sampled the photon fluence at a small region of the image plane at a time
the detector was scanned mechanically in a raster-like pattern over the area of interest.
The image was a pattern of dots imprinted on a sheet of paper by a mechanial printer that followed the scanning motion of the detector ,printing the dots as the ?-ray were detected Anger scintillation camera : Anger scintillation camera Developed by Hal Anger at Berkeley from 1952 to 1958
Anger Scintillation Camera replaced rectilinear scanners in the late 1960s
Permits more rapid acquisition of images and enables dynamic studies.
More flexible in its positioning, permitting images to be obtained from almost any angle Anger Scintillation Camera : Anger Scintillation Camera SystemComponents
Detector Crystal -NaI(Tl)
Preamplifier & Amplifier
Pulse height analyser-PHA
Dislay System overview : System overview Detector System : Detector System Characteristics of detector-
High detection efficiency- ? large
High stopping power- high Z Types of detectors : Low detection efficiency
low stopping power
it requres very low temp for good working. Low detection efficiency
Low stopping power for imaging ?-ray range 70-300KeV Gas-filled detector Semicondutor detector Types of detectors Scintillation detector : Scintillation detector Scintillator is a transparent material that converts the energy lost by ionization into pulse of light.
amount of light produced following the interaction of a ?-ray is proportional to the energy deposited by the incident radiation Desirable properties for scintillator : Desirable properties for scintillator High conversion efficiency
Decay times of excited states should be short
Color of emitted light should match with spectral sensitivity of the light receptor
Material transparent to its own emissions
High detection efficiencies
Rugged, unaffected by moisture ,inexpensive to manufacture. Scintillator detectors- : Scintillator detectors- NaI(Tl) Crystal : NaI(Tl) Crystal It is relatively dense (d=3.67g/cm3), High Z of Iodine (Z=53) so good absorber and very efficient detection 50-300KeV range sensitivity>85%@140KeV
Moderate energy Resolution ~9-10%
Yield-one light photon/30eV
Transparent to its own scintillation emission
Can be grown relatively inexpensively in large plates
Well matched in ? to peak response of PMT Advantage Disadvantage : Disadvantage Is quite fragile and easily fractured by mechanical or thermal stresses(rapid temp. changes)
is hygroscopic-yellowish surface discoloration that impairs light transmission to PMT
At higher ?-ray energies (>=300 KeV) , needs large volume of NaI(Tl) for adequate detection efficiency NaI(Tl) Crystal (cont..) Crystal parameters : Crystal parameters Pure NaI is scintillator only at liquid N2 Temp, so Tl(0.1-0.4mole%) add to create activator center to scintillate at Room Temp
Single ,large area usually 6-12.5mm(~9.5mm) thick (sens. increases with thickness but Resn. decreases) and 25-50cm diameter
Modern camera have Rectangular crystal with 60*40cm which provide increased FOV.
is surrounded by highly reflective material-TiO2 / Mag. Oxide to maximize light output.
Is hermetically sealed inside a thin Al to protect it form moisture
Array of PMT is coupled optically to the back face of the crystall with a silicon based adhesive or grease.
Light Guides- employed between the detector crystal and PM tubes. It increase light collection efficiency , improve the uniformity of light collection as a function of position. Photomultiplier tubes(PMTs) : Photomultiplier tubes(PMTs) Is an electronic evacuated glass tube containing a light sensitive photocathode , typically 10 to 12 electrodes called dynodes and an anode.
It performs two functions-
-Conversion of light photons into an electrical signal
-Signal amplification(10^6 -10^8) Slide 17: photocathode- is coated with photoemissive substance~Alloy of CsSb(bialkali)….releases e- after absorption of light photons
for every 3-5photon - one photoelectron is released by PEE on photocathode
Electrons emitted by the photocathode are attracted to the first dynode and are accelerated to kinetic energies equal to the potential difference between the photocathode and the first dynode
When these electrons strike the first dynode, about 3- 5 electrons are ejected from the dynode for each electron hitting it
These electrons are attracted to the second dynode, and so on, finally reaching the anode Photomultiplier tubes(PMTs) PMT Amplification : PMT Amplification Total amplification of the PMT is the product of the individual amplifications at each dynode
If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately 10,000,000
Amplification can be adjusted by changing the voltage applied to the PMT
PMT requires high voltage supply(~1300V) first dynode(300),and then increment of voltage is 100V ,finally reach to ANODE.
Amplitude of pulse is proportional to the no of light photons received by the photocathode / E of y ray photon absorbed at the detector PMTs size :-1-7.5cm(5cm)Number of PMTs-30-100(37,55) : PMTs size :-1-7.5cm(5cm)Number of PMTs-30-100(37,55) Preamplifier : Preamplifier Amplify the signals produced by detector (small)
Match the impedance level between the detector and subsequent component circuits
Shape the signal pulse for optimum signal processing by subsequent components
provide driving force so that pulse will not lost in several feet of cable
Gain factor-5-20, but NaI(Tl) employ 1(no gain)
Is placed as close to the detector as possible to maximize the performance (maximize SNR) Function:- Amplifier : Amplifier Amplify the still relatively small pulses from preamp (mV to V), Linear amplification (increased in amplitude by a constant factor)
Reshape the slow decaying pulse from the preamp. into narrow one to avoid the pulse pile-up at high counting rates
Gain factor / Amplifier Gain -1*1000( adjustable)
pulse shaping – specifically pulse shortening is an essential function, which increases SNR ratio and provides an output of cleanly separated pulses Function:- Pulse height analyser (PHA) : Pulse height analyser (PHA) PHA is an electronic device which differentiate the primary radiation (image carrying) from secondary radiation ie scatter radiation
is a device used to process
only those pulses from amplifier falling within selected voltage amplitude intervals or channels
it selects only required amplitude range and to discriminate against scattered, Bkg etc. PHA Cont... : PHA Cont... Scatter photon either from patient or crystal in image formation degrade resolution and contrast.
Scatter photon from primary radiation is discriminated by Z pulse (energy)
PHA is used to set a window around the photopeak having lower and upper level so that energy fall within this range only go for further processing
window has a finite width so some scattered photon may be accepted (e.g. 50 degree angle scatter in in 140KeV at 15%window) PHA (cont..) : PHA (cont..) SCA-single channel analyser
has a lower and upper level discriminator and produce an output logic pulse whenever an input pulse between the discriminator levels
MCA-Multichannel analyser Position calculation : Position calculation Each PMT is assigned a weighting factor which is determined by its distance from the center of the crystal.
=Summation of W*O
=-2 Position weighting factor
-3 -2 -1 +1 +2 +3
PMT PMT PMT PMT PMT PMT
1 2 3 4 5 6
CRYSTAL Position calculation : Position calculation The position is determined by spliting the signal from each PMT onto four output lines, denotes as X+,X-,Y+,Y-
fraction of PMT current goes to each output line is determined by value of register(R)
A separate circuit sums the outputs of all the PMT to form Z-signal,used for PHA
The X+,X-,Y+,Y- are combined to obtain X-position and Y-position
X=X+ - X-/X+ + X-
Y=Y+-Y-/Y++Y- Analog camera : Analog camera Position circuit received pulses from individual preamps and, by determining the centroid of these pulses, produced an X-position pulse and a Y-position pulse
Summing circuit added the pulses from individual preamps to produce an energy (Z) pulse proportional to total energy deposited in the crystal
Z pulse sent to a SCA; produced a logic pulse only if Z pulse within a preset range of energies Analog camera (cont.) : Analog camera (cont.) Analog X- and Y-position pulses and the logic pulse from each interaction sent to a cathode ray tube (CRT)
CRT produced a momentary dot of light on its screen at a position determined by the X and Y pulses if a logic pulse from a SCA received simultaneously
Photographic camera aimed at CRT recorded the flashes of light, forming an image on film, dot by dot Hybrid camera : Hybrid camera Position and Summing circuits are Analog
Analog X, Y, and Z pulses converted to digital signals by analog-to-digital converters (ADCs)
Digital signals sent to digital correction circuits
Corrected digital X, Y, and Z signals converted back to analog voltage pulses
Energy discrimination done in the analog domain by SCAs
Output to CRT as with analog camera or digitized again for display on a computer monitor Digital camera : Digital camera Pulses from individual PMTs are digitized
Position signals and energy signal formed using digital circuitry
Digital X, Y, and Z signals are corrected, using digital correction circuits, and energy discrimination applied in digital domain
Signals formed into digital images within a computer Collimator : Collimator Is a device which collimate the beam of photons (is direct the primary photons to form an image).
?-rays can not be focused so “principle of absorptive collimation” is used.
An absorptive collimator projects an image of the source distribution onto the detector by allowing only those ?-rays traveling along certain direction to reach the detector. Basic principle : Basic principle A collimator consists of hole in the absorptive medium (Lead).
Walls of the holes are called “SEPTA”
historically holes have been circular,square & hexagonal. Collimator materials : Collimator materials Main interaction in “collimating” photons is photoelectric attenuation.
PE attenuation depends on Z of material
need to have highest density and largest Z as possible to keep septal thickness to a minimum Lead(Z=82,d=11.3g/cc)
cheap,machinable but fragile
at 140 KeV,HVL is 0.23mm
has usefull properties but harder to workwith and is more expensive. Used in special applications.
At 140 KeV,HVLis 0.19mm
Gold &Uranium are also but are not common.
Au -Z=79 ,d=19.3 HVL=0.14
U -Z=92,d=18.7 HVL=0.14 Collimator materials Basic consideration : Basic consideration Septal thickness.
Without appropriate thickness of septa result a “STAR” pattern for a point source . Septal thickness : Septal thickness Effective length(te) Is the distance travels by photon through the septa (for photon striking the septa at an angle) important aspect is the
steep angle- te & at less steep angle ,strike more holes
For higher energy photons, septal thickness must increases General characterization : General characterization sensitivity : sensitivity Is the efficiency of collimator
is the no. of ratio of photons that pass through the collimator to the number emitted.
Greater “hole” space =higher sensitivity
larger hole=greater sinsitivity
thinner septa provides greater sensitivity Sens is dependent on area seen by hole-
Resn is better as Rg but sens is much worse as Rg (sens~(Rg)2)
Coice of collimator is a tradeoff between spatial resn. and sens. Collimator types : Collimator types Pinhole collimator
parallel hole Collimator
Fan Beam Collimator Pinhole collimator : Pinhole collimator Consists of small pinhole aperture in a piece of Lead, (most common),Tungston, platinum
aperture is loated at the end of a lead cone , typically 20-25cm from detector
size of pinhole can be varied by using removabe inserts
Used primerily for magnification imaging of small objects
inverted image Parallel hole collimator : Parallel hole collimator Have an array of parallel holes perpendicular to crystal face.
Resolution is worse with distance but sensitivity is independent with distance between source and detector.
FOV is increases with distance.
Most widely used collimator. Parallel hole collimator(cont..) : Parallel hole collimator(cont..) Slant-hole collimator
is a parallel hole collimator in which all of holes are parallel to each other but angled by about 25-30 degrees
used for cardiac imaging Converging collimator : Converging collimator Have an array of tapered holes that converge at a point at some distance in front of collimator (Focal point)
Resolution(high at surface)
Sensitivity- slowly increases as source is moved from collimator face to focal plane and then decreases.
Good for imaging smaller objects Diverging collimator : Diverging collimator Is upside down converging collimator
have an array of tpered holes that diverge from hypothetical point behind crystal.
Used for imaging large objects on smaller field Specialized collimator : Specialized collimator Fan beam collimator:-is a combination of parallel hole collimator(along one axis) and a converging collimator (along other axis)
used for tomography of small organs
Single axis-diverging collimator:-has diverging holes in transvese direction but has parallel holes in axial direction
used when transverse tansverse FOV of camera is smaller than patient’s width Energy Rating of Available Collimator : Energy Rating of Available Collimator Image formation : Image formation Photons from each point in the patient are emitted isotropically
Some photons escape the patient without interaction, some scatter within the patient before escaping, and some are absorbed in the patient
Many of the escaping photons are not detected because they are emitted in directions away from the detector
Collimator absorbs majority of photons which reach it Image formation (cont.) : Image formation (cont.) Only a tiny fraction of emitted photons has trajectories permitting passage through the collimator holes
Of those reaching the crystal, some are absorbed in the crystal, some scatter from the crystal, and some pass through the crystal without interaction
Relative probabilities of these events depends on the energies of the photons and the thickness of the crystal Image formation( cont..) : Image formation( cont..) In analog cmera (older camera) - photographic image are directly formed during acquisition
camera may be set up to acquire an image for predetermined time interval (preset time mode) until a predetermined amount of activity was being detected (preset count mode ) or until a no. of counts /cm2 has been reached (preset information density mode)
For each Z-pulse that passes through PHA, is associated x&y pulses are used to position a finely dot of light on the CRT face.
Collection of these light dots over time produces image Digital image fomation : Digital image fomation In digital , x&y signal from positioning circuit is directly entered into the computer in real time during image acquisition.
X&Y signal are stored in two-way basesd on mode-
FRAME MODE-In this mode face is represented by matrices of pixels , eah of which correspond to certain area of camera face & is designed by a specific range of x,y signal values
when computer receives an x,y signal from camera , pixel associated with concern x,y signal value is increased and when data acquisition is complete, images are immediately available for display. LIST MODE : LIST MODE X ,y signals are transferred directly to computer memory in the form of x,y coordinates and to this signal , other type of data can be inserted in the list typically-time markers(every 1-10msec) , physiologic triggers-R wave from an ECG of patient’s heart
No matrices or images are formed within computer memory during acquisition and also no images are produced immediately.
This mode is useful because x,y signals are permanantly recorded in computer memory allow flexible control over subsequent formatting into digital matrices MULTIPLE GATED ACQUISITION : MULTIPLE GATED ACQUISITION It is an extension form of FRAME MODE
Data from camera are distributed to a series of matrices on computer memory
A trigger signal (usually physiologically-R wave ) controls the distribution of data among the matrices.
Immediately after the trigger , data from camera are placed in the first matrces for a fixed interval when the interval has elapsed then placed in the 2nd matrix for same interval. This process continues until Types of data : Types of data Static:-produced by acquiring a single frame mode or by ignoring of time markers and formatting list mode data into single matrix
Dynamic:-produced by acquiring a series of frame mode images over time or by formatting list mode data into a series of matrices .
A cycle gated:- study produced by multiple gated frame mode acquisition or by formatting list mode data into a series of images representing a single average cycle with refernce to be embedded trigger markers on the list Display device : Display device Cathode ray tube:-
is most commonly used display device
is an evacuated tube containing-Electron gun,Deflection plates phosphor coated Display screen Focus and Brightness controls,Color cathode ray tube. Oscilloscope:- is an electronic display device containing a CRT used to produce visible patterns that are graphical representations of electrical signals
Computer monitor-is basically CRTs in which horizontal and vertical deflection plates are controlled by constant frequency time-sweep generators. Multiple crystal camera : Multiple crystal camera Is developed by Bender and Bala
Small physically separate crystals were arranged in a matrix.
Each Crystal was coupled to two PMTs
One PMT determine X position and the other determine Y position directly. Multiple crystal camera : Multiple crystal camera At present, a single block of NaI(TI) divided into ‘400’ detector element in a 20*20 Matrix used.
PMTs are coupled to block without any light pipe is a pixelated Camera
In early state, CZT is used
presently, CsI(Tl) and silicon photodiode is used (Efficiency-80%)
has a size of 21*21cm and 4096 CsI -3*3 mm
Good for Small organs and Cardiac Imaging. Solid state cameras Mobile scintillation camera : Mobile scintillation camera Specially designed for sick patients/Breast imaging.
have smaller detectors .
Detector size ~10*10 to 20*20 cm2
detector- NaI(Tl)/PMT or CsI(Tl) with silicon photodiode arrays
some systems employ CZT for direct detection of gamma rays. Slide 63: Thank You