RAD 216 ADVANCED IMAGING MODALITIES

Transcription

1 RAD 216 ADVANCED IMAGING MODALITIES Introduction to Computed Tomography COMPUTED TOMOGRAPHY Also called CT, was marketed in the 1970 s by EMI, a British corporation. CT s inventor, electrical engineer Dr. Godfrey Hounsfield, was awarded a Nobel Prize (1979) for his work in producing the first CT images in FIRST PROTOTYPE CT SCANNER The first CAT scanners were limited to the head. 1

2 CT SCANNERS The scanner consists of the housing (gantry) that contains the x-ray tube and detector elements. A scan table is situated so that the patient can be positioned within the gantry s opening. CT SCANNERS A minicomputer is used to process data using an array processor. Rather than a single microprocessor (CPU), array processors use several. This enables image processing to be divided among the microprocessors in order to shorten scan time. 2

3 CT SCAN DEVELOPMENT: FIRST GENERATION Early scanners were limited to the head. An x-ray tube and single detector was used. During the scan, the tube and detector moved in a series of sweeps and rotations called translate-rotate. CT SCAN DEVELOPMENT: FIRST GENERATION The single detector system required long scan times, a single slice requiring 5 minutes to complete. 1 st GENERATION CT 3

4 CT SCAN DEVELOPMENT: SECOND GENERATION By adding more detectors, the beam could be directed to cover a broader area, called a fan beam. The beam was thin enough to define thin slices. The scan still utilized a translaterotate pattern, but scan time was reduced. 2 nd GENERATION CT CT SCAN DEVELOPMENT: THIRD GENERATION A curved detector array was introduced with an increased number of gas-filled detectors. The x-ray tube and detector array rotate during the scan (a departure from the translate-rotate mode. 4

5 CT SCAN DEVELOPMENT: THIRD GENERATION Scan times, although considerably shorter than in previous generation scanners, could take 20 seconds to complete (per slice). 3 rd GENERATION CT CT SCAN DEVELOPMENT: FOURTH GENERATION The major innovation was the use of a fixed detector array system. Only the x-ray tube rotates during the scan. The x-ray tube moves along a track, called a slip ring. 5

6 CT SCAN DEVELOPMENT: FOURTH GENERATION By using a slip ring, the x-ray tube can rotate continuously without snagging power cables. Depending on design, the high voltage transformer rotates along with the x- ray tube. FOURTH GENERATION CT CT SCAN DEVELOPMENT: FOURTH GENERATION Short scan times (1 slice per second) and continuous tube rotation makes helical or spiral CT possible. 6

7 HELICAL (SPIRAL) CT Helical CT involves the rotation of the x-ray tube with continuous movement of the scan table. The process results in a scanning pattern similar to a coil. HELICAL (SPIRAL) CT Because the slices are connected in a coil-like pattern, individual slices must be created. Part of the image reconstruction process, called interpolation, is used. HELICAL (SPIRAL) CT 7

8 HELICAL (SPIRAL) CT MULTIDETECTOR CT (MDCT) Instead of using a single ring of detectors, MDCT adds several rows, permitting acquisition of several slices in one sweep of the x-ray tube. Scan times are very short, permitting entire body scanning in just a matter of seconds. The number of detector rows and detector configurations vary with each system and manufacturer. HELICAL (SPIRAL) CT: ADVANTAGES ENTIRE ORGANS CAN BE SCANNED IN A SHORT PERIOD LESS LIKELIHOOD OF MISREGISTRATION ARTIFACTS MULTIPLANAR AND 3D RECONSTRUCTION USING IMAGE DATA IS POSSIBLE 8

9 5 TH GENERATION CT Electron beam computed tomography (EBCT), also called ultra fast CT, was a major innovation in scanner design. Rather than using a rotating x-ray tube, the scanner itself acts as an x- ray tube. ELECTRON BEAM CT An electron gun in the back of the An electron gun in the back of the scanner directs a fine stream of electrons to a set of target rings in the gantry. The electron-target collision produces several x-ray beams which pass through the patient. There are no moving parts other than the table. 9

10 ELECTRON BEAM CT EBCT is useful in producing highresolution cardiac images with scan times of between 50 and 100 ms. Cine and flow studies are possible CT DETECTOR ARRAYS Third generation scanners used gas- Third generation scanners used gas filled detectors. Gas detectors use compressed xenon (Xe). Each detector is only 1 mm wide. Ionization caused by the capture of x-ray photons is converted into electrical signals which are sent to an array processor. 10

11 GAS-FILLED DETECTORS CT DETECTOR ARRAYS Fourth generation scanners use solidstate or gas detectors. Solid-state detectors detect radiation by interaction with photoemissive crystals. The light given off these crystals stimulate photodiodes, converting light into electrical impulses that are relayed to an array processor. SOLID-STATE DETECTOR 11

12 CONTROL OF SCATTER RADIATION Scatter radiation is controlled by means of proper selection of kvp and the use of pre- and post-patient collimation. PREPATIENT COLLIMATION Prepatient collimation controls the thickness of each scanned slice. Scatter production is reduced when using thinner slices POSTPATIENT COLLIMATION Collimators positioned in front of the detector array keeps scatter that emerges from the patient from reaching the detectors. This helps increase signal-to-noise ratio. 12

13 IMAGE RECONSTRUCTION During a CT scan, an x-ray beam passes through the patient at various angles. The attenuated beam that emerges at each angle (called a projection) is captured by the detector array. IMAGE RECONSTRUCTION The attenuation data (called raw data) is digitized and manipulated by the array processor using a mathematical technique called filtered back projection. 13

14 IMAGE RECONSTRUCTION Back projection is a summation technique that is used to add and average attenuation data. Filtering is used to smooth image irregularities resulting from the scanning process. Unless these irregularities are corrected, the resulting image will have rough edges and streak artifacts. IMAGE DISPLAY CT images are arrayed on a matrix. Each of the pixels within the matrix has a shade of gray that correlates with a numerical value representative of a tissue, fluid or air. These numbers are called CT numbers. CT NUMBERS CT numbers depend on the kvp which CT numbers depend on the kvp which affects the attenuation coefficient of the tissues. Attenuation coefficient (cm -1 ) is the amount of attenuation experienced by x-rays as they go through each linear centimeter of material. 14

15 CT NUMBERS μ t μ w μ w 1000 CT REFERENCE NUMBERS WATER = 0 AIR = FAT = -80 MOST SOFT TISSUES = +30 TO +80 CALCIUM > +100 SOLID BONE = DISPLAYED IMAGE & FIELD OF VIEW The technologist determines at the time of the scan how large or small the displayed image should look. This is called the reconstruction field of view (FOV). 15

16 DISPLAYED IMAGE & FIELD OF VIEW A small field of view means that the reconstructed image will depict a close-up of the anatomy in question. A large field of view will depict more of the anatomy making details appear smaller on the monitor. CT NECK CT ABDOMEN 16

17 FIELD OF VIEW & PIXEL SIZE Pixel Size = Field of View Matrix Size FIELD OF VIEW & PIXEL SIZE Given a field of view (FOV) of 20 cm and a 512 x 512 matrix size, what is the size of each pixel? FIELD OF VIEW & PIXEL SIZE Answer: 20 cm x 10 = 200 mm 200 mm 512 = 0.39 mm 17

18 MEASURES OF CT IMAGE QUALITY SPATIAL RESOLUTION LOW CONTRAST RESOLUTION SPATIAL FREQUENCY MODULATION TRANSFER FUNCTION NOISE LINEARITY SPATIAL UNIFORMITY SPATIAL RESOLUTION The ability to clearly distinguish the borders between tissues. Abrupt changes in tissue composition tend to create loss of detail on CT images. LOW CONTRAST RESOLUTION The ability to distinguish subtle differences in tissue composition. At a 0.5% difference in tissue composition, CT is capable of distinguishing between objects that are 4 mm in size or larger. 18

19 SPATIAL FREQUENCY The ability of an imaging system to detect small details as measured with a test pattern. For example, if a scanner can resolve a test pattern with a spatial frequency of 12 line pairs per cm (12 lp/cm), what is the smallest size object that the scanner can see clearly? SPATIAL FREQUENCY SPATIAL FREQUENCY Answer: 12 lp/cm = 24 bars and spaces/cm 1 cm = 10 mm 1/24 x 10 mm = 0.42 mm 19

20 MODULAR TRANSFER FUNCTION A measure of how well a scanner is able to faithfully represent the anatomy scanned (image fidelity). Generally, the smaller the objects scanned, the less faithful the representation of the image NOISE Any number of factors that will diminish the amount of useful information (signal). Slice thickness, kvp, ma and matrix size affect noise. It manifests itself as variation in CT numbers, leading to incorrect information. QUALITY CONTROL WATER BATH PHANTOM (STANDARD DEVIATION TEST) WATER BATH PHANTOM (SPATIAL UNIFORMITY HISTOGRAM) AAPM TEST (LINEARITY) 20

21 STANDARD DEVIATION A statistical measure of variation. The degree to which individual measurements stray from the average measurement is called the standard deviation STANDARD DEVIATION Std. Dev. = ( x-x ) n 1 2 SPATIAL UNIFORMITY Similar to the test of standard deviation, the results are depicted in a bar chart generated by the CT computer. 21

22 LINEARITY A test to determine the degree of accuracy of a CT system by identifying objects of varying attenuation coefficients. When the system is functioning correctly, the graphical plot of the CT numbers will form a straight line. FACTORS THAT INFLUENCE PATIENT DOSE IN CT VOLUME OF COVERAGE MA AND KVP SLICE THICKNESS SCAN PITCH FILTERING (CONVOLUTION) 2D vs. 3D IMAGING MULTIPLE SCAN AVERAGE DOSE (MSAD) CONCEPT Calculating patient dose in CT is a fairly complex exercise when compared to diagnostic radiography. This is due to the many parameters and imaging techniques used in CT and the fact that many image slices are taken during scanning. 22

23 MULTIPLE SCAN AVERAGE DOSE (MSAD) CONCEPT In the 1980 s the FDA (Center for Devices and Radiologic i Health) devised d a method to determine dose from CT scanning. The dose taken at the center of a slice is called the CT Dose Index (CTDI). The average dose from multiple slices within a volume of tissue is called the Multiple Scan Average dose (MSAD). MULTIPLE SCAN AVERAGE DOSE (MSAD) CONCEPT When a scan is taken with a pitch of 1 (meaning that t all slices are taken contiguously) then the MSAD is equal to the CTDI. As scan pitch increases, MSAD decreases consistent with the formula: MSAD = CTDI (INVERSE OF SCAN PITCH) PROBLEM Assume a CTDI of 1000 mrem. If spiral CT images are obtained using a table increment (BI) of 10mm per tube rotation and a slice thickness (SW) of 2.5 mm, what is the MSAD? 23

24 ANSWER CTDI = 1000 MREM PITCH = 10 mm 2.5 mm = 4 MSAD = CTDI x INVERSE OF PITCH MSAD = 1000 x (1/4) = 250 mrem 24