RAD 216 ADVANCED IMAGING MODALITIES THOMAS A. EDISON EARLY FLUOROSCOPY IMAGE INTENSIFICATION AND TV IMAGING SYSTEMS

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1 RAD 216 ADVANCED IMAGING MODALITIES IMAGE INTENSIFICATION AND TV IMAGING SYSTEMS THOMAS A. EDISON is credited with the development of fluoroscopy. His work enabled physicians to see DYNAMIC or moving images of internal structures of the body. EARLY FLUOROSCOPY 1

2 FLUOROSCOPY CIRCA 1920 s FLUOROSCOPY CIRCA EARLY FLUOROSCOPY relied on the radiologist s ability to DARK-ADAPT the eyes in order to take advantage of SCOTOPIC (ROD)VISION. This was achieved by wearing red goggles for at least 15 minutes. 2

3 EARLY FLUOROSCOPY took place in a darkened room. The patient stood behind a leaded glass plate coated with a fluorescent material. The radiologist could see the internal anatomy by staring very closely at the glass plate. IMAGE-INTENSIFIED FLUOROSCOPY came into common use in the 1960s as a by-product technology used during the Vietnam conflict (nightvision equipment). As used in radiology, it is the ability to convert a few photons of x-ray into many photons of light. THE IMAGE INTENSIFIER TUBE is the heart of the image intensifier system. It consists of a large vacuum tube approximately 50 cm (20 in.) in length. Many are 25 cm (10 in.) in diameter at the largest end and 2.5 cm (1 in.) at the other end. 3

4 IMAGE INTENSIFIER TUBE 25 cm 2.5 cm 50 cm IMAGE INTENSIFIER COMPONENTS INPUT PHOSPHOR PHOTOCATHODE FOCUSING LENSES ANODE OUTPUT PHOSPHOR INPUT PHOSPHOR consists of CESIUM IODIDE crystals. They convert the x-rays emerging from the patient into light photons. 4

5 PHOTOCATHODE is bonded directly to the input phosphor. It is PHOTOEMISSIVE. In other words, it releases electrons when stimulated by light. It is made of CESIUM and ANTIMONY COMPOUNDS. ELECTROSTATIC FOCUSING LENSES direct the electrons toward the anode end of the image intensifying tube. This is accomplished by applying an electric charge to the plates. 5

6 THE ANODE attracts the electrons from the cathode end of the image intensifier tube. However, the electrons actually pass through a hole in the anode to collide with the... OUTPUT PHOSPHOR which is made of ZINC CADMIUM SULFIDE. The output phosphor converts the electrons into visible light. The conversion from x-rays to light is many times brighter than simply using a large intensifying screen. IMAGE INTENSIFIER TUBE 6

7 MULTIFIELD IMAGE INTENSIFIER TUBES are tubes designed to provide magnification capability. This is done by decreasing the field of view (FOV). The voltage applied to the electrostatic focusing lenses is increased to refocus the electrons being directed to the output phosphor. MULTIFIELD IMAGE INTENSIFIER (normal mode) MULTIFOCUS IMAGE INTENSIFIER (mag. mode) 7

8 MAGNIFICATION FACTOR The amount of magnification an image intensifier tube is capable of can be calculated as the normal diameter (nd)/magnification diameter (md) VIGNETTING is the loss of detail and brightness seen on the periphery of the fluoroscopic image due to the curve of the input phosphor. Because only the central portion of the input phosphor is used in magnification mode, the image is sharper and less distorted. 8

9 VIGNETTING VIGNETTING PINCUSHION AND S-DISTORTION Two type of fluoroscopic image distortion that results from the shape of the input phosphor (pincushion) or severe magnetic irregularities affecting the way that electron travel toward the output phosphor (sdistortion). 9

10 PINCUSHION ARTIFACT S-DISTORTION MAGNIFICATION vs IMAGE BRIGHTNESS When an image intensifier tube is switched to magnification mode, less of the input phosphor is utilized (smaller FOV). The result is fewer electrons striking the output phosphor, resulting in a darker image (all other factors unchanged). 10

11 MAGNIFICATION vs IMAGE BRIGHTNESS To correct for this loss of brightness, the x-ray generator increases the ma, permitting more electrons to be generated. The tradeoff, however, is increased patient and occupational dose until the system is returned to the normal viewing mode. MEASURES OF IMAGE INTENSIFICATION EFFICIENCY There are two measures which determine an image intensifier s ability to convert x-rays into light. These are MINIFICATION GAIN and FLUX GAIN. MINIFICATION GAIN is defined as the ratio of the square diameter of the input phosphor to the square diameter of the output phosphor: (d i /d o ) 2 11

12 EXAMPLE The diameter of the input phosphor is 9 inches. The diameter of the output phosphor is 1 inch. Therefore, the minification gain is 9 2 /1 2 = 81 FLUX GAIN The ratio of the number of light photons created at the output phosphor to the number of x-ray photons striking the input phosphor. This can vary by as much as 3000:1 BRIGHTNESS GAIN is the product of minification gain and flux gain. Therefore, if an image intensifier tube has a minification gain of 81 and a flux gain of 3000, the brightness gain is 243,

13 VIEWING THE FLUOROSCOPIC IMAGE Early image-intensified fluoroscopy relied on a MIRROR SYSTEM to view the image. Unfortunately, this method permitted only the fluoroscopist to see the image because of the mirror s small size. Present day image-intensified systems use CLOSED-CIRCUIT TV. A FEW WORDS ABOUT IMAGE INTENSIFICATION Because of its high efficiency, the fluoroscopic current on image-intensified systems is between 1.5 and 5 ma. This permits a lower patient dose per unit time. However, prolonged use of fluoroscopy during procedures actually increases patient dose when compared to routine overhead imaging. A FEW WORDS ABOUT IMAGE INTENSIFICATION QUANTUM MOTTLE is almost always unavoidable when using high kvp techniques during image-intensified fluoroscopy. It is the result of fewer X- ray photons being generated as a consequence of the very low fluoroscopic ma being used. 13

14 QUANTUM MOTTLE The resulting fluoroscopic image appears grainy. To offset the grainy appearance, fluoro ma could be manually increased, but at a cost of higher patient and occupational dose. QUANTUM MOTTLE If the problem cannot be corrected by increasing fluro ma, then the problem is system noise which may be due to faulty settings on the vidicon tube (to be discussed in another lecture). There will always be system noise, but should not be so severe as to prevent visibility. A FEW WORDS ABOUT IMAGE INTENSIFICATION Image densities are reversed on fluoroscopic images compared to radiographic images. For example, barium appears dark and air appears brighter during fluoroscopic examinations of the colon. 14

15 RADIOGRAPHIC vs. FLUOROSCOPIC FLUOROSCOPY VS. RADIOGRAPHY 15