CT Physics Lecture 3

Data Acquisition: Major Components Gantry Generator Xray Tube Collimator + Filter Detector Array DAS – Data Acquisition System Patient table

3rd Generation CT Discussed generations of CT scanners – modern day scanner is 3rd generation with rotating x-ray tube and detectors.

3rd Generation CT System

What do we mean by cross section?

Gantry Ring-shaped part of the CT scanner Houses Slip rings Generator Cooling system X-ray source Filtration Collimation Detectors

Gantry Aperture (The Hole) Tilt capabilities Laser Commonly 70 cm Large Bore – 80 – 90 cm (Toshiba Acquilon LB) Tilt capabilities Varies from system to system but usually between +/- 12 to +/- 30 degrees in 0.5 degree increments Laser Positioning ISOCENTER

Gantry X Y Z

Gantry Important to recognize that especially in the imaging of a bariatric patient that despite a listed aperture size that it does not account for the table height for which has to advance into the bore

Couch Carbon Fiber Top Weight limit Strong & rigid Low absorption Floats and Rests on pedestal Vertical and Horizontal movement Weight limit Generally 450 lbs. Philips Brilliance Large Bore = 650 lbs.

Couch Typical Maximum Scan Range Not comparable to the length of the couch Generally can cover approximately 162 cm of scan length Most modern scanners do have a general increase in scan length

What is a Slip Ring? Its what made helical scanning possible by providing continuous rotation without conventional cables Enables the transmission of power and electrical signals from a stationary to a rotating structure. Consists of conductive rings and brushes which facilitates the transfers Allows for faster scan times and continuous acquisitions without cable worry

What is a Slip Ring? Brushes enable transmission of power by sliding in and out of grooves on the stationary ring 2 Brush designs Wire Brush – Conductive wire as contact Composite Brush – Conductive material as contact

What is a Slip Ring?

What is a Slip Ring? High Voltage Slip Ring AC power delivered to high voltage generator Supplies Slip Ring which powers tube GENERATOR DOES NOT ROTATE WITH TUBE Low Voltage Slip Ring More Common today AC Power is transferred to slip rings by brushes Provides power to high voltage (step up) transformer then to tube POSITIONED TO ROTATE WITH TUBE

Generator Produce high voltage for the creation of x-ray photons Modern scanners use High Frequency Generator Parameters we can control: kVp (80, 100, 120, 140) mA (25-1000) Time (0.5 – 2 sec) mA * time = mAs

Heat Capacity Modern CT units can accommodate 3 – 5 million HU Heat Unit Product of kVp, mA and seconds – the heat generated. Eg 75kVp X 100 mA x 2 s = 15000 heat units When heated to capacity machinery will automatically compensate Decrease kV Decrease mA Decrease time End result will result in imaging that is sub-optimal due to the increased presence of noise on the images

X-Ray Source Tube: Getter: Lead lined cast steel: NOT PYREX GLASS Lead lined to further contain potential leakage or off focus radiation Greater cooling properties More efficient isolation of high voltages Getter: Barium Helps ensure vacuum by absorbing any air molecules potentially released by the target during operation

CT Unique Components Bow Tie Filter Shaped in order to compensate for the attenuations within both the head and the body Effectively hardens the beam and equalizes the amount of radiation reaching the image receptor

CT Unique Components Collimator 2 kinds Pre-Patient Beam Collimation - Single Detector Detector Collimation – Muti- Detector Pre-Patient Adjustable; influenced by focal spot size (Penumbra) Ensures constant beam width at the detectors Detector (Pre-detector) Assist in shaping the beam Remove any scattered radiation from reaching the detectors

Collimation Restricts the x-ray beam to a specific area Reduces scatter radiation Improves contrast resolution Decreases patient dose

Collimation Specified in number of detector rows x detector width. Eg. 16 x 1.25 (20 mm)

Detectors Purpose – capture attenuated radiation from the patient and convert it into an electrical signal Signal then converted into digital data

Detector Characteristics Efficiency How well the detector receive attenuated photons Capture Efficiency Efficiency in which detectors can obtain attenuated photons Influenced by detector size and distance between detectors Absorption Efficiency Number of photons absorbed by detectors Atomic #; Density of material; Size and Thickness of the face

Detector Characteristics Stability Detector response How often is calibration needed? Response Time How quickly the detector can detect a photon and how quick can it recover in order to record the next event Dynamic Range Ratio of the largest signal to measured to the precision of the smallest that is discriminated; …Accuracy… Product of Capture Efficiency, Absorption Efficiency, and Conversion Efficiency Afterglow continuous luminescence after event

Types of Detectors 1.) Scintillation Detectors Scintillation crystal coupled to a photodiode tube X-Ray falls onto crystal which upon interaction creates photons of light Light gets directed to the photomultiplier increasing the light Light strikes photocathode which emits electrons Electrons pass through dynodes which are arranged and maintained resulting in a signal

Scintillation Detector Solid State Photodiode Scintillation Crystals (Solid State) Photodiode – calcium tungstate and ceramic to which the crystals are bonded by fiber optics Allows current flow when exposed to light Current is proportional to the amount of light Extremely fast response time Conversion Efficiency = 99 % Capture Efficiency = 99 % Dynamic Range = 1,000,000 to 1

Types of Detectors 2.) Gas Ionization Chamber Think AEC… Series of individual highly pressurized gas (Xenon) filled chambers which are separated by metal plates (Tungsten) X-Ray strikes gas filled chambers Gas becomes ionized Plates are charged + and -, e- migrate to the positive plate, positive ions to the negative plate As ions move produces a small signal depending on the number of photons

Gas Ionization Detectors Pressure 30 Atm to increase probability of event Excellent stability Zero Afterglow Detection efficiency less than scintillation detector 50 – 60 % compared to 95 – 100 % No longer utilized…

Multi-slice Detectors Dual Row Introduced in 1992 Allowed for faster volume coverage than single slice Dynamic Focal Spot – position of focal spot switched by a computer system during each scan in order to double sampling and measurements Produced two contiguous slices with great resolution con·tig·u·ous adjective \kən-ˈti-gyə-wəs, -gyü-əs\ —used to describe things that touch each other or are immediately next to each other

Multi-slice Detectors Multi-row(slice) Detectors Purpose = increase volume coverage speed of both single and dual slice scanners IE. 16 slice scanner will be 16 times faster than single slice Commonly used for machines which acquire from 4 to 320 slices per rotation Consists of multiple separate detector rows which can image simultaneously up to the maximum amount of slices per 360 degree rotation 16 slice scanner = 16 slices per rotation…Duh

ARRAY TYPES

Multi-slice Detectors Two Different Types Matrix Array Detectors AKA Fixed Array All detector cells are equal in dimension along an array Perfect Cubes one right after the other Adaptive Array Detectors Detector cells have different sizes along an array

Multi-slice Detectors Number of slices is dependent on configuration used Data collection “channels” Configuration can use each individual detector to produce that size slice Ie. 16-1.25 mm detector can produce 16-1.25 mm slices Or….can use combination to produce larger slices Ie. 16-1.25 mm detector can produce 8-2.5 mm slices or 4-5 mm slices

Multi-slice Detectors

DAS Data Acquisition System Refers to all the electronic components which lie after the detectors but before the computer Performs 3 functions Measures attenuated radiation Converts measurements to digital Transmits digital data set to computer

Attenuation = log transmission * thickness…or DAS DAS Components: Pre-amplifier – takes the weak electrical signal from the detectors and boosts it so that it can be more easily converted Logarithmic amplifier – performs the conversion of attenuated transmission data to logarithmic data which is sent across to ADC Conversion of transmission to attenuation and thickness Attenuation = log transmission * thickness…or µ1 + µ2 + µ3 = (ln I0 / 1) (I / x) µ = linear attenuation coefficient I0 = Original intensity I = Transmitted intensity x = Thickness of Object

DAS ADC Optoelectronics Analog to Digital Converter – divides the electrical signals into multiple portions The more portions the better the digital signal…now how do we get more portions again? Hmmmmmm… Help determine the bit depth (grey scale) of image Optoelectronics Series of lens and light diodes which moves data transmissions from the ADC to the computer at tremendously high speed 50 million bits / sec.

Data Acquisition In order to ensure that an adequate amount of measurements (samples) are obtained for optimal quality… Utilize thin slices to reduce sample artifact… Closely Packed Detector Array – closer they are, the more are available to receive radiation, the more samples obtained… Quarter Shifted Detector Arc – provides 2 sets of data which can be individually reconstructed Siemens developed

References Image from of Sprawls.com Stewart Bushong “Radiologic Science for Technologists” Bushberg et al., “The Essential Physics of Medical Imaging” Wikipedia Impact.org