RMI310 Principles of CT Techniques PDF

# RMI310: Principles of CT Techniques ## Lecture 3: Main Components of the CT Scanner **What are the main components of the CT Scanner?** There are three main parts of a CT system: - **Power or Computer Room:** Includes processor, transformer and voltage systems. - **Scanner Room:** Includes tabl...

# RMI310: Principles of CT Techniques ## Lecture 3: Main Components of the CT Scanner **What are the main components of the CT Scanner?** There are three main parts of a CT system: - **Power or Computer Room:** Includes processor, transformer and voltage systems. - **Scanner Room:** Includes table, gantry and couch control unit. - **Operator Room:** Includes archiving and display image systems. **Our Region of Interest** - **Scanner Room:** Where the patient is located and scanned. - **Operating Room:** Where the images are processed and controlled. Both rooms are separated by a lead shielded wall. **Scanner Room (Table & Gantry)** - **Isocenter Point:** The point where the x, y, and z axes meet. - **Table:** With two motion directions, y and z. **Gantry External View** * Ring shape with an aperture of 70-90 cm. * X-ray hole. * Internal microphone. * Control panel (laser light for patient adjustment, patient in/out, gantry tilting) **What are the main parts in the gantry (Gantry Internal View)?** - **X-ray Tube** - Metal Target (high atomic number): Tungsten, cadmium, gold - Rotating Anode (electron beam): Interacts with whole metal target, increasing the emitted x-ray photons. - **Generator:** With Peak Power 60-100kW/Various Voltage 80, 100, 120, and 140 kV. **Regarding X-ray Tube and Generator** State-of-the-art X-ray tube/generator combinations provide a peak power of 55-100kW, usually at various, user-selectable tube voltages, such as 80, 100, 120, and 140 kV. Different clinical applications require different x-ray spectra and hence different kilovolt settings for optimal image quality and/or best possible signal to noise ratio at the lowest dose. Furthermore, the tube current (mA) can be freely selected to adjust the dose to the patient. Lower tube voltages require over-proportionately higher tube currents to achieve the same dose to the scanned object. In a conventional tube design, an anode plate of typically 160-220 mm diameter rotates in a vacuum housing. The heat storage capacity of the anode plate and tube housing measured in Mega Heat Units (MHU) determines the performance level: the bigger the anode plate, the larger the heat storage capacity and the more scan seconds can be delivered until the anode plate reaches its temperature limit. A state-of-the-art X-ray tube has a heat storage capacity of typically 5-9 MHU. ## 3-Filters X-ray beam is a polychromatic photons (photons with varying energy). **Why?** Because it emitted from different energy levels or shells, so we have high and low penetration energy photons. **Main Function of Filter** - Reduction dose radiation by removing or not allowing low photons energy to pass (not in CT image), allowing only high penetration energy photons. - Uniform of x-ray beam. ## 4-Collimator (2 collimators) **Pre-patient collimator:** - Mounted to X-ray tube - Protection by limiting radiation exposure - Controlling Slice Thickness by limiting beam width - Is called Slice Collimator **Post-Patient Collimator:** - Mounted to Detector - Reduce Scatter Radiation & Noise. X-Ray Tissue Interaction - Proper beam width as it enters the detector i.e. protect the slice thickness - Is called Detector Collimator ## 5-Detector **Conventional X-ray (Panel Detector or cassette with silver film)** - Panel detector converting penetrating photons into electrical charges, which in turn converted into a digital image. - Degree Attenuation = Degree of Densities= Silver deposition=Degree blackening **Computed Tomography Detector** - Detectors Rotate simultaneously with x-ray tube - Receive & Collect all emitted X-ray photons **Consist of Elements** - Detector with single element (Individual Detector) - Detector with more than one element (Array Detector) - These elements situated in (Linear, Arc or Ring) - Single row (it is called single detector) - Multiple rows (multi detector) ## What are Types of Detectors according to (FUNCTION?) 1. **Xenon Gaze Detector** - Enclosure glass filled with xenon gas. - Emitted photons interact with xenon gas producing ions. Or charges amplified and accelerated to produce electrical signals which called (Row Data). 2. **Solid Crystal Detector** - Consists of Crystals. - When the emitted photons strike crystals lighten. - Crystal contact with photo-diode which transfer the optical signal into electrical signals (Row Data). ## What are types of Detectors according to (DESIGN)? **Single Detector System (SDCT)** - **A- Single Detector with one Element** - **B- Single Detector with Multiple Elements arranged in (Linear, Arc &Ring).** **N.B Each Gantry rotation produce data for a Single Slice** **Multi Detector System (MDCT)** - Detector with Multiple Parallel Rows or Arrays (4-64-128) - **N.B Each Gantry rotation produce data for a Multiple Slice** - Ex 4 slices (4 Rows) or 64 slices (64 rows). ## 6-Data Acquisition System DAS under Detector **Main Function:** Transfer the Electrical Signals into Digital signals (Computer Language processor). Analog to Digital Conversion (ADC) By Fourier Transformation Analysis FTA ## 7-**Slip Ring** Electro Mechanical Device Like brush to provide continues electrical power and connection with gantry cables to rotate continuously avoiding the twisting of gantry cables. **What is slip-ring gantry?** Slip rings are electromechanical devices consisting of circular electrically conductive rings and brushes that transmit electrical energy across a moving interface. All power and control signals from the stationary parts of the scanner system are communicated to the rotating frame through the slip ring. The slip-ring design consists of sets of parallel conductive rings concentric to the gantry axis that connects to the tube, detectors, and control circuits by sliding contractors. These "sliding contractors" allow the scan frame to rotate continuously with no need to stop between rotations to rewind system cables. ## Summary and main points of Lecture 3 The overall performance of a multi-detector MDCT system depends on several key components. These components include the gantry, X-ray source, a high-powered generator, detector and detector electronics, data transmission systems (slip rings), and the computer system for image reconstruction and manipulation. ## What does gantry rotation speed mean? It's the speed at which the CT gantry rotates once around the patient. It is often called "scan time." ## What is the difference between SDCT and MDCT? The principal differences between single and multiple-row detector helical scanners are: - In both cases, both the x-ray tube and the detectors rotate around the patient to collect multiple projection data while the patient table is translated through the CT gantry. - The differences are in the number of detector rows in the z-direction of the patient and the number of multiple slices obtained at the Isocenter. - By late 1998, all major CT manufacturers launched MDCT scanners capable of providing at least four slices per section per rotation with minimum gantry rotation times of 0.5 second. Again, the difference between SDCT and MDCT is the number of detector arrays available in the z-direction. - In SDCT, there was only single row of detectors arrays in the z direction, and therefore, it yielded only single axial slice per each CT gantry rotation. - On the other hand, in MDCT, there are multiple rows of detector arrays yielding multiple axial slices per each CT gantry rotation. ## What is MDCT? MDCT is a commonly used acronym that stands for multiple-row detector computed tomography. The other commonly used term is MSCT, or multiple slices computed tomography. ## What are the different detector arrays designs available in four-section MDCT scanners? A maximum of four slices per gantry rotation, even though the detector design consists of multiple rows of detectors, the maximum number of slices obtained per gantry rotation was limited by the number of data acquisition channels (DAS). In this case, the maximum number of slices was four due to the availability of 4 DAS channels. ## What are the advantages of MDCT? The clinical advantages of multiple-row detector technology can be broadly divided into three categories: 1. **The ability to obtain large number of thin slices resulting in higher spatial resolution in both axial and longitudinal direction.** This is important in terms of obtaining isotropic spatial resolution, that is, cubic voxels scanned volume. This capability is reasonably obtained with multiple sections of sub-millimeter thickness. 2. **The speed can be utilized for fast imaging of large volume of tissue with variable slice thickness.** This is particularly useful in studies in which patient motion is a limiting factor. 3. **The other main advantage of MDCT systems is their ability to cover large volumes in short scans times.** The volume coverage and speed performance in MDCT scanners are better than their counterparts in SDCT without compromises in image quality. The fast rotation times and large volume coverage provide improved multiplanar reconstruction and 3D images with reduced image artifacts. ## What are the various detector array designs in four-slice MDCT scanners? 1. **Uniform element arrays:** In this type of detector array designs, several solid-state small detectors of same dimension are arranged in rows of identical thickness (e.g., 8 rows of 1.25 mm). The image acquired depends on the x-ray beam width, selection of detector rows, and how the two are coupled. It is possible to acquire four simultaneous slices of 1.25 mm each, or to increase the slice thickness by coupling rows of detectors. 2. **Nonuniform element arrays:** In this type of detector arrays, the detector width gradually increases in thickness as they move away from the center of axis of rotation. The two detector rows in the center of the array are 1 mm each, whereas the detectors adjacent to the central row are of increasing thickness with the outermost detector row of 5 mm thick. 3. **Hybrid element arrays:** The third type of design incorporated features of uniform and nonuniform design. This detector arrays is composed of 4 thin detectors of 0.5 mm at center and 15 detectors of 1 mm width on either side of the central detectors for the total z-axis coverage of 32 mm per x-ray tube rotation around the gantry. ## Detector Electronics X-ray photons that strike the detector must be measured, converted to a digital signal, and sent to the computer. This is accomplished by the data-acquisition system (DAS), which is positioned within the gantry near the detectors. Signals emitted from the detectors are analog (electric), whereas computers require digital signals. Therefore, one of the tasks of the DAS is to convert the analog signal to a digital format. This is accomplished with the aptly named analog-to-digital converter or ADC. **Note:** To measure the x-ray photons that have penetrated the patient, the detectors are sampled many times, as many as 1,000 times per second by the DAS. The number of samples taken per second from the continuous signal emitted from the detector is known as the sampling rate, sample rate, or sampling frequency. Artifacts, such as streaking, can appear on the image if the number of samples is insufficient. **What is the streak artifact phenomenon?** These phenomena produce dark streaks between metal, bone, iodinated contrast, barium, and other high-attenuation materials. Bright streaks are seen adjacent to the dark streaks. ## End of Document