DIGITAL FLUOROSCOPY Flashcards by Unknown Unknown

is a digital x-ray imaging system that produces dynamic images obtained with an area x-ray beam
during ?

Digital fluoroscopy (DF)

Early 1970s

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Advantages of DF over conventional fluoroscopy ○

Speed of image acquisition and postprocessing to enhance image contrast

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DF, the spatial resolution is determined both by the ? and by the ?

● Spatial resolution is limited by ?

image matrix

size of the image intensifier

pixel size

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poxel size=

image intesifier/matrix

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DIGITAL FLUOROSCOPY IMAGING SYSTEM

● Much the same as a conventional fluoroscopic study with an addition of a computer

● X-ray tube operates in the ? mode

● Tube current is measured in hundreds of mA instead of less than ? mA, as in image-intensifying fluoroscopy

● Continuously energizing the tube will result if failure because of thermal overloading, and the patient radiation dose would be exceedingly high

radiographic

5 mA

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Images are obtained by pulsing the x-ray beam
○ (?Fluoroscopy)

Pulse-progressive

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Conventional fluoroscopy
○? ms is required to produce a single video frame

● Image acquisition rates of 1 per second to? 10 per second

● X-ray exposures longer than? ms can result in unnecessary patient radiation doses

● This is a theoretical limit, however, and longer exposures may be necessary to ensure low noise and good image quality

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○ The time required for the x-ray tube to be switched on and reach selected levels of kilovolt peak (kVp) and mA ○ Less than 1 ms

●
○ The time required for the x-ray tube to be switched off ○ Less than 1 ms

Interrogation time

Extinction time

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○ The fraction of time that the x-ray tube is energized

Duty cycle

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● A major change from conventional fluoroscopy to DF is the use of a ? instead of a TV camera tube

Military applications (night vision scopes) ○ Currently: digital camera, commercial television, security surveillance, and astronomy

charge-coupled device (CCD)

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charge-coupled device (CCD) developed in ?

The sensitive component of a CCD is a layer of ?

1970’s

crystalline Silicon

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The CCD is mounted on the output phosphor of the image-intensifier tube and is coupled through ? or ?

● With a lens-coupled CCD, a sample of light is measured and is used to drive the ?

● When the CCD is directly coupled to the image intensifier, the entire CCD signal is sampled and drives the ABS system

fiberoptics or a lens system

automatic brightness stabilization (ABS) system.

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● The CCD is mounted on the output phosphor of the image-intensifier tube and is coupled through ? or ?

● With a lens-coupled CCD, a sample of light is measured and is used to drive the ?

● When the CCD is directly coupled to the image intensifier, the entire CCD signal is sampled and drives the ABS system

fiberoptics or a lens system

automatic brightness stabilization (ABS) system.

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The principal advantage of CCDs in most applications, such as a digital camera, is their ? & ?

small size and ruggedness.

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Spatial resolution of a CCD:
○ Determined by its ? & ?

A 1024 matrix can produce images with ? lp/mm spatial resolution

physical size and pixel count

10 lp/mm

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Television camera tubes: “?” or “?” artifact

■ No such distortion occurs with a CCD

pin cushion” or “barrel” artifact

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The linear response feature is particularly helpful for digital subtraction angiography (DSA) and results in improved ? and better ? compared with conventional fluoroscopy

dynamic range and better contrast resolution

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Flat Panel Image Receptor (FPIR) much smaller and lighter and is manipulated more easily than an image intensifier

Is composed of ? or ?, as described in digital radiography

cesium iodide (CsI)/amorphous silicon (a-Si) pixels

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If a ? is the fluoroscopic image receptor instead of an II tube, x-ray exposure time can be continuously varied for even greater patient radiation dose reduction

● Each time the ?l is exposed, it is read immediately and the image projected until the next image is acquired

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flat panel

Advantages of Flat Panel Image Receptors Over Charge-Coupled Device Image Intensifiers in Digital Fluoroscopy

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• Distortion-free images
• Constant image quality over the entire image
• Improved contrast resolution over the entire image
• High DQE (see Chapter 16) at all radiation dose levels
• Rectangular image area coupled to similar image monitor
• Unaffected by external magnetic fields

Advantages of Charge-Coupled
Devices for Medical Imaging

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• High spatial resolution
• High SNR
• High DQE
No warm-up required
• No lag or blooming
• No spatial distortion
• No maintenance
• Unlimited life
• Unaffected by magnetic fields
• Linear response
• Lower patient radiation dose

Image intensifier

○ Limited by nonuniform ? & ?from the center to the periphery of the circular image
○ ? & ? distortion increase with age

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spatial resolution and contrast resolution

Veiling glare and pincushion

○ Response is uniform over the entire receptor and does not degrade with age ○ Insensitive to external magnetic fields ■ This has made possible a new area of interventional radiography: image-guided catheter navigation

FPIR

IMAGE DISPLAY: Video System ● Conventional fluoroscopy:?-line system ● Two limitations 1. The ?mode of reading the target of the television camera tube can significantly degrade a digital image. 2. Conventional television camera tubes are relatively noisy. ■ They have an SNR of about? an SNR of ? is necessar

525 interlaced 200 : 1; 1000 : 1

? mode ● Two fields of 262 ½ lines were read individually in 1/60 s (17 ms) to form a 525-line video frame in 1/30 s (33 ms)

Interlaced

? mode ● Used in DF (TV camera tube) ● Electron beam of the TV camera tube sweeps the target assembly continuously from top to bottom in 33 ms ● The video image is formed similarly on the television monitor. ● No interlace of one field with another occurs. This produces a sharper image with less flicker

Progressive

All ? electronic devices are inherently noisy ○ Because of ? and ?, a very small electric current always is flowing in any circuit ○ This is called background ?

analog heated filaments and voltage differences electronic noise

Conventional television camera tubes ○ ? ; maximum output signal will be ? times greater than the background electronic noise ■ Not sufficient for DF ○ An SNR of? is minimally visible ○ In DF lower signals become even more lost in the noise ○ This is especially true when ? techniques are used ○ Image contrast resolution is severely degraded by a system with a low SNR

200 : 1 5 : 1 subtraction

Dynamic range ○ 200 : 1 = less than? ○ 1000 : 1 =? The tube with a 1000 : 1 SNR provides ? times the useful information and is more compatible with computer-assisted image enhancement

256 1024 five

Important characteristics of a DF System

1. Image matrix size 2. System dynamic range 3. Image acquisition rate

The output signal from the image-intensified digital image receptor is transmitted to an ? ○ it accepts the continuously varying signal—the analog signal—and digitizes it

analog-to-digital converter (ADC)

An 8-bit ADC would convert the analog signal into values between ?& ? ● A 10-bit ADC would be more precise, with an ADC range from ? to ? or ? to ?

0 and 255 0 to 1024 or 0 to 1023

The signal from an ? is already digital The output of the ADC is then transferred to ?and is manipulated so that a digital image in matrix form is stored

FPIR main memory

The ?(3)determine the speed with which the image can be acquired, processed, and transferred to an output device

dynamic range of each pixel, the number of pixels, and the method of storage

f image storage occurs in primary memory, which is usually the case, then data acquisition and transfer can be as rapid as ? images per second ● In general, if the image matrix is doubled (e.g., from 512 to 1024), the image acquisition time will be increased by?

30 four

512 matrix = ? images per second 1024 matrix = ? images per second

30 8

The principal advantages of DF examinations are the image ? techniques that are possible and the enhanced visualization of vasculature that results from venous injection of contrast material ● DF provides better ? hrough postprocessing of image subtraction

subtraction contrast resolution t

○ Refers to a number of computer-assisted techniques whereby an image obtained at one time is subtracted from an image obtained at a later time.

Temporal Subtraction

TWO TEMPORAL SUBTRACTION METHODS

Two methods are commonly used: the a.) mask mode and the b.) time-interval difference (TID) mode

○ Occurs if patient motion occurs between the mask image and a subsequent image ○ Same anatomy is not registered in the same pixel of the image matrix k ■ Done by shifting the mask by one or more pixels so that superimposition of images is again obtained

Misregistration artifact

○ Uses two different x-ray beams alternately ○ Images are results of differences in photoelectric interaction ○ The basis for this technique is similar to that described in for rare Earth screens ○ Based on the abrupt change in photoelectric absorption at the K edge of contrast media compared with that for soft tissue and bone

Energy Subtraction

energy subtraction Probability of x-ray interaction with iodine, bone, and muscle as a function of x-ray energy ● The probability of photoelectric absorption in all three decreases with increasing ? At an energy of 33 keV, an abrupt ? in absorption is noted in iodine and a modest ? in soft tissue and bone

x-ray energy increase, decrease

? ● This energy corresponds to the binding energy of the two K-shell electrons of iodine ● When the incident x-ray energy is sufficient to overcome the K-shell electron binding energy of iodine, an abrupt and large increase in absorption occurs ● Graphically, this increase is known as the ?

Energy Subtraction K absorption edge

energy subtraction If monoenergetic x-ray beams of ? & ? could be used alternately, the difference in absorption of iodine would be enormous, and resultant subtraction images would have very high contrast

32 and 34 keV

other energy subtraction methods

(1) Alternately pulsing the x-ray beam at 70 kVp and then 90 kVp and ○ (2) Introducing dissimilar metal filters into the x-ray beam alternately on a flywheel

?subtraction techniques are used most frequently because of high-voltage generator limitations associated with the energy subtraction mode ● Two techniques are combined the process is called? subtraction

Temporal hybrid

○ Combining temporal and energy subtraction techniques ○ Image acquisition follows the mask-mode procedure however, the mask and each subsequent image are formed by an energy subtraction technique ○ If patient motion can be controlled, hybrid imaging theoretically can produce the highest-quality DF images

Hybrid Subtraction

DIGITAL FLUOROSCOPY Flashcards

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