Computed Tomography

Computed Tomography:

Computed Tomography

Room With CT Scanner:

Room With CT Scanner

Console Room:

Console Room

History Of CT Scan :

History Of CT Scan Godfrey Hounsfield first demonstrated the technique in 1970. CT was invented in 1972 by British engineer Godfrey Hounsfield of EMI Laboratories . The first clinical CT scanners were installed between 1974 and 1976 .

Introduction:

Introduction Computed Tomography (CT) imaging, also known as "CAT scanning" (Computed Axial Tomography). Tomography is from the Greek word "tomos" meaning "slice" or "section" and graph meaning "describing". A technique to help locate a problem and confirm diagnosis in order to give the correct treatment.

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Continued It combines the use of a digital computer together with a rotating x-ray device to create detailed cross sectional images or "slices" of the different organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis, extremities, brain, spine, and blood vessels . The x-rays tube rotates around the body, the x-rays passes through the body and cross sectional images (pictures) of tissues are formed

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Continued CT Scan has the unique ability to image a combination of soft tissue, bone, and blood vessels. For example, conventional x-ray imaging of the head can only show the dense bone structures of the skull

How Does Ct Scan Work?:

How Does Ct Scan Work? Computed Tomography is based on the x-ray principal: As x-rays pass through the body, they are absorbed or attenuated (weakened) at differing levels. This x-ray profile is registered on film, thus creating an image. In the case of CT, the film is replaced by a banana shaped detector which measures the x-ray profile.

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Continued A CT scanner looks like a big, square doughnut. The patient aperture (opening) is 60 cm to 70 cm (24" to 28") in diameter. Inside the covers of the CT scanner is a rotating frame which has an x-ray tube mounted on one side and the banana shaped detector mounted on the opposite side. A fan beam of x-ray is created as the rotating frame spins the x-ray tube and detector around the patient.

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Diagram shows the inside view of modern CT system, the x-ray tube is on the top at 1 o’clock position an the arc-shaped CT detector is on the bottom at the 7 o’clock position. The frame holding the x-ray tube and detector rotate around the patient as the data is gathered

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Continued Each time the x-ray tube and detector make a 360⁰ rotation, an image or "slice" has been acquired. This "slice" is collimated (focused) to a thickness between 1 mm and 10 mm using lead shutters in front of the x-ray tube and x-ray detector. As the x-ray tube and detector make this 360⁰ rotation, the detector takes numerous snapshots (called profiles) of the attenuated x-ray beam.

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Continued Typically, in one 360⁰ lap, about 1,000 profiles are sampled. Each profile is subdivided spatially (divided into partitions) by the detectors and fed into about 700 individual channels. Each profile is then backwards reconstructed (or "back projected") by a dedicated computer into a two-dimensional image of the "slice" that was scanned.

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Continued Multiple Computers are used to control the entire CT system. The main computer that orchestrates the operation of the entire system is called the “host computer.” There is also a dedicated computer that reconstructs the “raw CT data” into an image. A workstation with a mouse, keyboard and other dedicated controls allows the technologist to control and monitor the exam.

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Continued The CT gantry and table have multiple microprocessors that control the rotation of the gantry, movement of the table (up/down and in/out), tilting of the gantry for angled images, and other functions such as turning the x-ray beam on an off.

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Diagram showing relationship of x-ray tube, patient, detector, and image reconstruction computer and display monitor

Generations Of CT Scanner:

Generations Of CT Scanner First Generation: Parallel-Beam Geometry. Second Generation: Fan Beam, Multiple Detectors. Third Generation: Fan Beam, Rotating Detectors. Fourth Generation: Fan Beam, Fixed Detectors. Fifth Generation: Scanning Electron Beam.

First Generation: Parallel-Beam Geometry.:

First Generation: Parallel-Beam Geometry. In the first CT scanner design, a single X-ray source and a single X-ray detector cell collect all the data for a single slice. The x–ray head and detector were rotated at arc of 180°. The source and detector are rigidly coupled and the pencil beam is translated across the patient to obtain a set of parallel projection measurements at one angle.

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Continued The source/detector pair is then rotated slightly and a subsequent set of measurements are obtained during a translation past the patient. This process is repeated once for each projection angle. Some scanners had two x–ray beams and a second detector to allow the scanner to acquire two anatomical section scans at the same time.

Second Generation: Fan Beam, Multiple Detectors:

Second Generation: Fan Beam, Multiple Detectors A fan–shaped x–ray beam was projected onto a linear array of approximately 30 detectors. The x–ray head and detectors could rotate 180 ° around the patient. The increased number of detectors reduced the number of linear scans required during the 180° arc. The time required for a scan was reduced to less than 90 seconds.

Third Generation: Fan Beam, Rotating Detectors:

Third Generation: Fan Beam, Rotating Detectors A wider fan–shaped x–ray beam and a curved array of 250–750 detectors. The wider beam and larger detector array allowed the scanner to include the entire body in a single exposure. Eliminated the need for linear scanning to be combined with the x–ray head rotation. Third generation scanners would acquire approximately 700,000 measurements per anatomical section.

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Continued Scan times reduced to less than 12 seconds. 360° scan arcs were possible and the scan arc could be varied. Shorter scan times allowed sequential scans to approximated dynamic functions with approximately 4 scans per minute. The single detector array made third generation scanners prone to ring artifact.

Fourth Generation: Fan Beam, Fixed Detectors:

Fourth Generation: Fan Beam, Fixed Detectors A single projection fan–shaped x–ray beam and 600–2000 stationary detectors. The x–ray head rotates more than 360o around the patient. The detectors are fixed in a circular ring around the patient and x–ray head. Alignment of the x–ray beam to each detector is essential. The x–ray head needs to travel more than 360° in order to provide an acceleration and deceleration zone.

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Continued Scanning takes place as the x–ray head travels in either direction, clockwise and counter clockwise. Scan rates of approximately 15 scans per minute are achieved. Scan rates are limited by the inter scan time used to change the direction of the x–ray head travel. Dynamic scanning modes are available. Over scanning modes exist and use scanning arcs of greater than 360°.

Fifth Generation: Scanning Electron Beam:

Fifth Generation: Scanning Electron Beam Fifth-generation scanners are unique in that the x-ray source becomes an integral part of the system design. The detector array remains stationary, while a high-energy electron beams is electronically swept along a semicircular tungsten strip anode. X-rays are produced at the point where the electron beam hits the anode, resulting in a source of x-rays that rotates about the patient with no moving parts.

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Continued An alternative fifth-generation design, called the dynamic spatial reconstructor (DSR) scanner. This machine is a research prototype and is not available commercially. It consists of 14 x-ray tubes, scintillation screens, and video cameras. Volume CT images can be produced in as little as 10 ms.

Helical/Spiral CT Scanners :

Helical/Spiral CT Scanners Advanced slip ring technology is used to allow the x–ray head to travel in one direction indefinitely. The x–ray head does not have to be decelerated and accelerated in between scans. Scanners returned to using moving detector arrays. Approximately 1000 detectors are aligned opposite the x–ray tube.

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Continued The primary advantage to helical scans is the reduced scan time. The entire abdomen scanned in less than 40 seconds. This results in the use of less contrast media, less motion artifact and greater patient through put. The primary disadvantage is that the entire 360o scan information is not acquired for every anatomical cross section.

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Continued Computer reconstruction is required to fill in the missing data. The reconstruction algorithms are more sophisticated because they must accommodate the spiral or helical path traced by the x-ray source around the patient .

Slide 33:

Spiral scanning causes the focal spot to follow a spiral path around the patient as indicated

Components Of CT Scanner:

Components Of CT Scanner Gantry Patient Table Power Distribution Unit Operator Console

Gantry:

Gantry The gantry maintains the alignment of the x–ray head and detectors while moving the x–ray head through the scan arc. A circular aperture in the center of the gantry frame allows the patient to be positioned within the x–ray field by the table assembly. The gantry aperture diameter limits the maximum patient size.

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Continued Each scanner has a maximum table weight limit that is design to prevent patients that are too large for the gantry from being forced into the scanner. The entire gantry can be tipped or angled up to 30° to allow the acquisition of partial coronal images. Three positioning lights or lasers within the gantry assure centering of the patient in the sagittal, coronal, and transverse planes.

Gantry:

Gantry The Gantry includes the The X-ray tube The Detector Array The High-Voltage generator The patient Support Couch

The X-Ray Tube:

The X-Ray Tube CT x-ray tubes are very large. They have an anode heat storage capacity of 8 MHU or more. They have anode-cooling rates of approx 1MHU per minute because the anode disc has a larger diameter, ant it is thicker.

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Continued The x-ray tube can be energized upto 60 sec. Some x-ray tubes operate are relatively low tube current, for many the instantaneous power capacity must be high.

The Detector Array:

The Detector Array CT imaging system have multiple detectors in an array that numbers up to tens of thousands. Previously gas-filled detectors were used, but now, all are scintillation, solid state detectors. Early scintillation detector arrays contained scintillation crystal-photodiode assemblies because photodiodes convert the light into an electronic signal..

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Continued The combination of a scintillation crystals and the light detector is called a scintillation detectors. Cadmium tungstate (CdW04) and special ceramics are the current crystals. The concentration of detectors is an important characteristic of a CT imaging system that affects the spatial resolution of the system.

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Continued Scintillation detectors have high x-ray detection efficiency. Approx 90% of the x-rays incident on the detector are absorbed, and thus contribute the output signal. Over all detection efficiency approaches 90%. The efficiency of x-ray detector array reduces the patient dose, allows faster imaging time, and improves image quality.

Collimation:

Collimation Proper collimation reduces patient dose restricting the volume of tissue irradiated. More important is the fact that it improves image contrast by limiting scatter radiation. In CT imaging, two collimators are used. One collimator is mounted on the x-ray tube housing or adjacent to it.

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Continued This is called Prepatient collimator that determines dose profile and patient dose. Second collimator is called predetector collimator. This collimator reduces scatter radiation that reaches the detector array, thereby improving the image contrast and also determines the slice thickness.

The High Voltage Generator:

The High Voltage Generator All CT imaging system operate on high frequency power. A high-frequency generator is small because the high-voltage step-up transformer is small, so it can be mounted on the rotating gantry. Tube potential (80-140 KV). Tube current (20-500 mA).

The Patient Support Couch:

The Patient Support Couch To support the patient comfortably, the patient couch must be constructed of low Z-material, such as carbon fiber. It does not interface with x-ray beam transmission and patient imaging. When the patient couch positioning is not exact, the same tissue can be imaged twice , thus doubling the dose, or it can be missed altogether.

Patient Table:

Patient Table The table is a flat or slightly curved platform that the patient lies on during the scan. The table is responsible for positioning the patient with the gantry. Up/down positioning movements are available as well as the longitudinal positioning of the patient into the gantry aperture.

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Continued The tabletop is usually made of a carbon fiber composite to reduce x – ray beam attenuation. It is very important not to exceed the patient weight limit of the table.

Slide 50:

Figure shows the numerous sub-systems that are required for a CT Scanning Table

Power Distribution Unit:

Power Distribution Unit The power distribution unit is the power supply for the x – ray tube, scanning movement drive motors, table, and the detectors. The x-ray tube requires very high voltages, kV, supplied at very high currents. These high voltage, high current demands also require very fast switching, sort rise and fall times.

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Continued The power distribution unit is housed in its own room or cabinet. Often the high current, high voltage drivers are water-cooled.

Operator Console:

Operator Console CT imaging systems can be equipped with two or three consoles. One consoles is used by the CT radiologic technologist to operate the imagine system. Another console may be available for a technologist to post-process images for filming and filing. A third console may be available for the physician to view the images and manipulate image contrast,size, and general visual appearance.

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Continued The operating console contains meters and controls for selection of proper imaging factors, for proper mechanical movement of the gantry and the patient couch, and fro the use of computer commands that allow image reconstruction and transfer. The physician’s viewing console accepts the reconstructed image from the operator’s console and displays it for viewing and diagnosis.

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Continued A typical operating console contains controls and monitors for the various technique factors. The operating console usually includes two monitors. One is provided for the operator to annotate patient data on the image. The second monitor allows the operator to view the resulting image before transferring it to hard copy or to physician,s viewing console.

Physician’s Work Station:

Physician’s Work Station This console allows the physician to call up any image and manipulate that image to optimize diagnostic information. The manipulative controls provide for contrast and brightness adjustments, magnification techniques and use of online computer software packages.

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Operating Console Workstation

How is the CT Scan Performed?:

How is the CT Scan Performed? The CT scanner is located in a large room. Patient should wear comfortable, loose-fitting clothing to your exam. You may be given a gown to wear during the procedure. Patient will lie on a narrow table that slides into the hollow tube-shaped scanner. The CT physician and staff will be in an adjacent room where the equipment controls are located.

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Continued CT physician and staff will be able to see patient through a large window and will be monitoring him/her constantly during the procedure Once the procedure begins, patient will need to remain very still at all times so that movement will not negatively affect the quality of the images. Patient have an intravenous (IV) line for contrast medication. The contrast medication may be injected prior to the procedure or during the procedure.

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Continued At intervals, he/she will be instructed to hold his/her breath, if possible, for a few seconds. He/she will then be told when to breathe. Young children who cannot hold still for the procedure will be given medication to help them relax or sleep during the CT scan. Once the procedure is finished, the table will slide out of the scanner.

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Continued If the patient received medication for relaxation or sleep, he/she will be monitored until the medication wears off and he/she is awake again. You may be asked to wait for a short time while the radiologist reviews the scans to make sure they are clear and complete. If the scans are not sufficient to obtain adequate information, additional scanning may be done. The test normally takes approximately 30 to 60 minutes .

Application Of CT Imaging:

Application Of CT Imaging CT imaging is used for studying the chest, abdomen and pelvis because it provides detailed, cross-sectional views of all types of tissue. CT imaging detect many different cancers, including lung, liver and pancreatic cancer and the tumors of brain and head. Since the image allows a physician to confirm the presence of a tumor and measure its size, precise location and the extent of the tumor's involvement with other nearby tissue.

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Continued CT shows blood clots and blood vessel defects. It shows enlarged ventricles (caused by a build up of cerebrospinal fluid). It also shows image and abnormalities such as those of the nerves or muscles of the eye. CT scanning help in diagnosis and treatment of vascular diseases that can lead to stroke, kidney failure or even death.

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Continued CT quickly identify injuries to the lungs, heart and vessels, liver, spleen, kidneys, bowel or other internal organs in cases of trauma. CT scan assess the results of surgery, such as organ transplants or gastric bypass. CT helps in diagnosing and treating spinal problems and injuries to the hands, feet and other skeletal structures because it can clearly show even very small bones as well as surrounding tissues such as muscle and blood vessels.

Benefits Of CT Imaging:

Benefits Of CT Imaging CT scanning is painless, noninvasive and accurate. A major advantage of CT is its ability to image bone, soft tissue and blood vessels all at the same time. Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue as well as the lungs, bones, and blood vessels.

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Continued CT examinations are fast and simple; in emergency cases, they can reveal internal injuries and bleeding quickly enough to help save lives. CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems. CT is less sensitive to patient movement than MRI.

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Continued CT imaging provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspirations of many areas of the body, particularly the lungs, abdomen, pelvis and bones.

Risks Of CT Imaging :

Risks Of CT Imaging There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk. Women should always inform their physician and x-ray or CT technologist if there is any possibility that they are pregnant. CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the baby.

Limitations of CT Scanning :

Limitations of CT Scanning Soft-tissue details in areas such as the brain, internal pelvic organs, knee or shoulder can be more readily and clearly seen with magnetic resonance imaging (MRI). A person who is very large may not fit into the opening of a conventional CT scanner or may be over the weight limit for the moving table.

Radiation Dose :

Radiation Dose The total amount of x–rays energy delivered to a patient during a radiographic procedure is called the patient dose. X–ray radiation is capable of harming humans. Exposure to x–rays can result in both somatic and genetic effects. Genetic effects are not generally evident on the individual that has been irradiated. Genetic effects become evident in the children of the irradiated individual.

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Continued The potential for x–rays to produce a somatic or genetic effect in a human is dependent on the total amount of x–ray energy absorbed by the body. Patient dose represents a hazard to the patient and must be kept to the minimum level required to produce the necessary diagnostic information. Patient dose is measured in roentgen (R), radiation absorbed dose (RADs), or kerma (cGy) units.

CT Patient Dose :

CT Patient Dose The patient dose delivered during a CT scanning procedure cannot be accurately calculated using conventional radiography techniques. Two CT specific patient dose measurements have be developed to indicate the total x–ray energy absorbed by the patient during a CT procedure. Computed Tomography Dose Index (CTDI).

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Continued CTDI is a measure of the x–ray energy delivered in the primary x–ray beam during one CT scan. Multiple Scan Average Dose (MSAD), is a measure of the average dose a patient receives during an examination that contains multiple CT scans or a helical moving scan. The patient dose delivered during a CT scan is much higher than the dose delivered during any conventional x–ray imaging procedures.

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Continued This is a result of the immense amount of diagnostic information acquired during a CT scan.

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Continued Reduced scan times allowed the patient to hold their breath for the entire scan allowing the radiographer to scan the thoracic region of the body. The fan shaped beam increased scatter radiation artifact. This forced the use of lead masks on the detectors to reduce the scatter radiation artifact.

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Continued The detector array travels in a circle around the patient at the same time as the x–ray head moves. The patient can be advanced through the CT gantry as the continuously revolving x–ray head circles the patient. The computer acquires measurements from the detectors that result in data representing a continuous helical scan.

Image Characteristics:

Image Characteristics Image obtained in CT is different from that obtained in conventional radiograph. In radiography, x-rays form an image directly on the image receptor. With the CT imaging systems, the x-rays form a stored electronic image that is displayed as a matrix of intensities.

Image Matrix:

Image Matrix CT image format consists of many cells, each assigned a number and displayed as an optical density or brightness level. CT imaging system provide matrices of 512×512, resulting in 262,144 cells of information. Each cell of information is a pixel (picture element), and the numeric information contained in each pixel is a CT number, or Hounsfield unit (HU). The pixel is a 2-D representation of a corresponding tissue volume

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Continued The diameter of image reconstruction is called the field of view (FOV). When FOV is increased for a matrix size, the size of each pixel is increased proportionately. The tissue volume is known as a voxel (volume element). It is determined by multiplying the pixel size by the thickness of the CT imaging slice.

Image Reconstruction:

Image Reconstruction The projections acquired by each detector during CT are stored in computer memory. The image is reconstructed from these projections by a process called filtered back projection. The term filter refers to a mathematical function.

Multiplanar Reformation:

Multiplanar Reformation Multislice spiral CT excels in three-dimensional (3D) Multiplanar Reformation (MPR). Three 3-D MPR algorithms are used most frequently: maximum intensity projection (MIP), shaded surface display (SSD), and shaded volume display (SVD). MIP reconstructs an image by selecting the highest value pixels.

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Continued MIP is the simplest form 3-D imaging. SSD is a computer-aided technique that has been borrowed from computer-aided design and manufacturing applications. It was initially applied to bone imaging and now is used regularly for virtual colonoscopy.

Image Quality:

Image Quality CT images consists of discrete pixel values, image quality is somewhat easier to characterize and quantitative. A number of methods are available for measuring CT image quality, and five principal characteristics are numerically assigned. These include spatial resolution, contrast resolution, noise, linearity and uniformity.

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