Ch. 5: Data Acquisition Concepts. What is Data Acquisition? Data Acquisition Process

Transcription

1 Ch. 5: Data Acquisition Concepts CLRS 408: Introduction to CT What is Data Acquisition? The method by which the patient is scanned to obtain enough data for image reconstruction Basic scheme for data acquisition: o Beam geometry size, shape, and motion of the x-ray beam and its path o Components physical devices that shape/define the x-ray beam, measure its transmission through the patient, and convert info into digital data Data Acquisition Process 1. Tube emits x-ray beam 2. Beam is shaped 3. Beam is collimated 4. Beam is attenuated 5. Transmitted x-rays are detected and converted to electrical signal 6. ADC converts electrical signal to digital data 7. Digital data is sent to computer 1

2 A Few Definitions Ray part of beam that falls on one detector View collection of rays for one translation Profile electrical signal representing the variation in attenuation of the transmitted beam Data sample each transmission measurement Acquisition Geometries Parallel beam Fan beam Spiral First Generation Scanners Used parallel beam geometry o Defined by a set of parallel rays that generates a projection profile o Translate-rotate principle o Rectilinear pencil beam scanning 180-degree rotation around the patient o 4.5 to 5.5 minutes per scan 2

3 Second Generation Scanners Fan beam geometry Translate-rotate principle Different from first generation scanners by the addition of the linear detector array and multiple pencil beams o Resulted in fan beam effect o 180-degree rotation o 20 seconds to 3.5 minutes per scan Third Generation Scanners Fan beam geometry o Continuously rotating fan beam scanning o X-ray tube and detectors all rotate o 360-degree rotation o Curved detector array o Scan time = seconds Fourth Generation Scanners Rotating fan beam within a circular detector array o X-ray tube rotates and detectors are stationary o Nutating detector ring Nutating refers to how the detector ring tilts to expose an array of detectors to the x-ray beam No longer manufactured In a fourth generation scanner, each detector position gives rise to a fan 3

4 Spiral and Multislice Spiral/helical Continuous tube rotation and couch movement Slip ring technology Collects data for volume of tissue Single breath hold Multislice CT Multiple slices are imaged during one revolution of tube 2 64; slices cone beam instead of fan beam Cone beam requires different algorithms Cone beam produces multiple slices per revolution of the x-ray tube and detectors 5 th Generation Scanners High speed CT scanners o Scan time = milliseconds o Electron Beam CT (EBCT) Cardiovascular CT o Dynamic spatial reconstructor (DSR) Figure 5-8 The essential components of an EBCT scanner. The data acquisition geometry is a fan beam of x-rays produced by the electron beam striking the tungsten targets. EBCT vs Conventional Scanner Based on electron beam technology - no x-ray tube is used No mechanical motion of the components Acquisition geometry of the EBCT scanner is fundamentally different compared with conventional systems Marketed as "ultrafast CT" or a special type of high-speed CT scan designed to detect plaque in the coronary arteries 4

5 Sixth Generation Scanners Dual source CT scanner (DSCT) o Two x-ray tubes and two sets of detectors - offset by 90º o Created to deal with artifacts created in CT angiography studies o Research shows better image quality without need for heart control Seventh Generation Scanners Flat-panel digital detectors o Consists of a cesium iodide (CsI) scintillator coupled to an amorphous silicon thinfilm transistor (TFT) array o Excellent spatial resolution but low contrast resolution o May be used for angiography or breast imaging, where image sharpness is of primary importance Slip-Ring Technology Electromechanical devices consisting of circular electrical conductive rings and brushes that transmit electrical energy across a rotating interface o Allows for continuous gantry rotation o Faster scan times and minimal interscan delays o Capacity for continuous acquisition protocols o Elimination of the start-stop process found in conventional scanners o Removal of cable wraparound process Figure 5-13 Slip ring based on the cylindrical design characteristic of the Picker PQ-2000 CT scanner. The brushes glide in contact grooves on the stationary slip ring. (Courtesy Picker International, Cleveland, Ohio.) 5

6 X-ray System in CT X-ray tube Generator Filter Collimators X-ray System: Tubes Anode Cathode X-ray tube window X-ray System: Tubes High speed electrons from cathode Bombard positively charged anode Kinetic energy is converted into heat and x-ray photons ma = milliamperage= quantity of x-rays kvp = kilovoltage peak = quality of x-rays (penetrability) 6

7 Exposure Factors Kilovoltage = tube potential o Penetrating power of the photons coming from x-ray tube o Higher kvp higher energy photon more penetrating of thicker parts o Generally, high kvp techniques are used 120 kvp for adults o Radiation dose is proportional to the square of kvp (Bushberg et al, 2004) As the kvp increases, the dose increases Exposure Factors - kv Exposure Factors Milliamperage tube current + Exposure time seconds = mas o mas determines the quantity of photons incident on the patient for the duration of the exposure dose Dose is directly proportional to mas if mas is doubled, dose will be doubled Constant Milliamperage-Seconds o Selection of ma and time (sec) or mas before the scan begins, keeping all other technical factors constant Effective Milliamperage-Seconds o Denotes mas per slice used for MSCT Effective mas = True mas / pitch To keep the effective mas constant, as the pitch increases, the true mas must be increased as well o Ex: Increase pitch from 1 to 2, increases the mas per rotation from 100 to 200 while the relative CTDI volremains the same mas Manual or Automatic 7

8 Exposure Factors - mas 75mAs 58mAs 160mAs 200mAs X-ray System: Tubes Metal envelope Rotating anodes o Require high heat-loading / rapid heat dissipation o 3,600-10,000 rpm Anode disk o Rotating anode tubes made of rhenium, tungsten, and molybdenum (RTM) alloy o Tubes for spiral imaging may include graphite base body for high thermal capacity o Small target angle (12º) Straton X-Ray Tube Created to deal with heat dissipation problems Useful for MSCT scanners Anode is immersed in oil, resulting in high cooling rates o Allows for high ma and long exposure times for increasing anatomical coverage Cathode consists of an electron beam that is deflected to strike the anode at two focal spots 8

9 X-ray System: Generator CT scanners use three-phase power High-frequency generators o Located within the CT gantry Attached to rotating frame in CT gantry or in gantry and doesn t rotate o Low-voltage, low-frequency AC current (60 Hz) is converted to high-voltage, high-frequency DC current ( Hz) for use by the x-ray tube o Voltage ripple of less than 1% o Power ratings range from 20 to 100 kw, allowing around: kv = 80, 120,140 kv ma = ma X-ray System: Filtration o Removes long-wavelength x-rays that do not play a role in CT image formation Produces a harder beam Reduces patient dose o Shapes the energy distribution across the radiation beam to produce uniform beam hardening CT beam must appear monochromatic to detectors o Bow-tie filter X-ray System: Collimation Restriction of beam size Effects o Decreases patient dose o Increases image quality 2 sets of collimators in CT o Pre-patient located as beam exits tube Reduces unsharpness due to size of focal spot o Post-patient (a.k.a. Pre-detector) located just before the radiation detector but after patient Rreduces scatter reaching detector Defines thickness of slice 9

10 Bow-tie filter Post-patient (pre-detector) collimator CT Detectors Purpose: receive radiation and convert it into electrical signal Characteristics to be considered: o Efficiency o Stability o Response time o Dynamic range Detector Efficiency Efficiency ability to capture, absorb, and convert x-ray photons to electrical signals o CT detectors must possess high capture efficiency, absorption efficiency, and conversion efficiency! Capture efficiency o How efficiently the detectors can obtain photons transmitted from the patient o Determined by size of detector area facing beam and distance between 2 detectors Absorption efficiency o Number of photons absorbed by the detector o Depends on atomic number, physical density, size, and thickness of the detector face Conversion efficiency o How well detector converts x-ray to electrical signal 10

11 Detector Stability Steadiness of the detector response Ability to consistently respond to similar levels of radiation If the system is not stable, frequent calibrations are required to render the signals useful Detector Response Time Speed detector can detect an x-ray event and recover to detect another event o Response times should be very short (microsec) o The faster the better to avoid afterglow and pile-up Dynamic Range of Detector Ratio of largest signal to be measured to the precision of the smallest signal it can discriminate o Largest signal to the smallest signal o DR for most CT scanners = ~1 million to 1 The total detector efficiency, or dose efficiency, is the product of the capture efficiency, absorption efficiency, and conversion efficiency 11

12 Types of Detectors Scintillation detector -- o x-rays light electricity o The more x-rays absorbed, the more light emitted, the greater the electrical current o Solid-state detectors scintillation crystal coupled to photodiode tube Scintillation Detectors o When x-rays fall on crystal, flashes of light produced scintillation! o The light is then directed to the photomultiplier tube o Light from the crystal strikes photocathode of the PM tube, which then releases electrons cascade through a series of dynodes that are carefully arranged and maintained at different potentials to result in a small output signal o Crystals: Sodium iodide crystals coupled to PM tubes Other crystals: calcium fluoride and bismuth germanate were used Afterglow & limited dynamic range o Today, solid-state photodiode multiplier scintillation crystal detectors are used Scintillation Detectors o Photodiode is a semiconductor (silicon) whose p-n junction allows current flow when exposed to light Lens is used to focus light from the scintillation crystal to the p-n junction Electron hole pairs are generated and electrons move to the n side of the junction while the holes move to the p side The amount of current is proportional to the amount of light o Response time is extremely fast (about 0.5 to 250 ns) o Currently used cadmium tungstate and a ceramic material made of high-purity, rare earth oxides based on doped rare earth compounds such as yttria and gadolinium oxysulfide ultrafast ceramic, optically bonded to photodiodes o Cadmium tungstate: Conversion efficiency = 99%; photon capture efficiency = 99%; dynamic range is 1 million to 1 o Ceramic rare earth oxide: Absorption efficiency = 99%; scintillation efficiency =3x of cadmium tungstate 12

13 Types of Detectors Gas ionization detector o The more x-rays absorbed, the greater the ionization of the gas, producing a movement of electrons (to positively charged plate) o Series of individual gas chambers (usually xenon) separated by tungsten plates With MSCT no longer in use Gas Ionization Detectors o When x-rays fall on the individual chambers, ionization of the gas results and produces positive and negative ions o + ions migrate to negatively charged plate; - ions are attracted to the positively charged plate Causes small signal current that varies directly with # photons absorbed o Gas chambers are enclosed by thick ceramic substrate material because the xenon gas is pressurized to about 30 atmospheres to increase the number of gas molecules available for ionization o Xenon detectors have excellent stability, fast response times, and no afterglow problems o Quantum detection efficiency (QDE) is less than that of solid-state detectors 98% vs 50% Multirow/Multislice Detectors Major problem with single-slice, single-row detectors length of time needed to acquire data Dual-slice, dual-row detector introduced to increase volume coverage speed and decrease the time for data collection CT scanners now use multirow detectors to image multislices during a 360-degree rotation 13

14 Multirow/Multislice Detectors Dual-row/dual-slice detectors o Twin-beam technology results in simultaneous scan of 2 contiguous slices Multirow/multislice detectors o More detector rows = more slices per revolution o Acquire slices per 360-degree rotation o Array consisting of multiple detector rows Figure 5-33 The basic structure of a multislice or multirow detector used in multislice volume CT scanners. Figure 5-31 B, Twin-beam geometry from Elscint's dynamic focal spot system. C, Dual-row, dual-detector system. Detector Electronics Data Acquisition System (DAS) Detector electronics positioned between detector array and computer Measures transmitted signal from detectors and amplifies Encodes these measurements into binary data Transmits binary data to computer DAS Components Detector measures transmitted x-rays from patient and converts into electrical energy This electrical signal is so weak that must be amplified by preamplifier before it can be analyzed further Transmission measurement data must be changed into attenuation & thickness data o Performed by logarithmic amplifier signals are directed to the ADC where electrical signals divided into multiple parts (16-bit ADC) Data transmitted to computer 14

15 Data Acquisition and Sampling We have to obtain enough info to get good image quality! Each detector samples the beam intensity incident in it Artifacts may result if not enough samples are obtained Ways to increase sampling o Thinner slice thickness more slices, more samples o Closely packed detectors more detectors available for data o Increase sampling shift focal spot to sample twice o Be careful about oversampling! radiation dose! The End of Ch. 5 15