Fluoroscopy Flashcards

Question

How is a frame transfer CCD made?

Answer 1

Frame transfer CCD chips comprise two conjoined sub-arrays of elements of equal area as shown in the diagram: One sub-array contains the active light-sensitive elements and is used to capture the initial image which is subsequently transferred to -> The second sub-array, which is shielded from light and provides temporary charge storage of the preceding image frame during readout Frame transfer CCDs have large charge capacity, low noise, fast temporal response and wide dynamic range and therefore are ideal for dynamic x-ray imaging.

Answer 2

Each pixel in the CCD array is addressed by an associated electrode. Application of a positive bias voltage to an electrode forms a 'potential well' in the region of the silicon substrate below. Each light photon (from the II output) absorbed in the silicon substrate of the CCD gives rise to an electron-hole pair (where the electron is negatively charged and the hole is positively charged). The holes drain away while signal electrons accumulate in the potential well. The quantity of electronic charge which accumulates at each pixel is directly proportional to the intensity of the incident light and the frame integration time. Charge packages build up in the light-sensitive array before being transferred to the charge storage section. The data output register then reads out the image signal from the storage section line-by-line. Meanwhile the next image frame is being acquired in the active area of the CCD. The sequence of charge packets are converted to an analogue video or (using an ADC) a digital signal as appropriate.

Answer 3

Small, inexpensive and compact with low-power consumption Self-scanning image readout (no large electromagnetic deflection coils required) Negligible lag (temporal unsharpness) Resilience against burned-in signals at high-lights Geometrical precision and spatial uniformity Excellent thermal, electrical and magnetic stability Excellent serviceability and long life-time Compatibility with digital x-ray imaging modalities

Answer 4

A. False. Light is absorbed in the photoconductive target, not the signal plate. B. False. The photoconductor is exposed to light, not x-rays. C. True. D. True.

Answer 5

A. Incorrect. CCD sensors are more compact than camera tubes but they do not have electromagnetic deflection coils. B. Correct. C. Correct. D. Incorrect. The output from a CCD sensor is an analogue signal.

Answer 6

A. Incorrect. An intensifying screen is used in a screen film radiography system, not an IITV fluoroscopy system. B. Correct. A tandem lens pair is used to couple the TV camera to the output screen of the II. C. Correct. A photoconductive TV camera tube may be used to capture the image from the output screen of the II (in more modern equipment, a CCD sensor may be used instead). D. Incorrect. An array of detector elements would be found in a modern fluoroscopy system using a flat panel detector, not in an IITV fluoroscopy system.

Answer 7

II entrance dose rate ( The dose delivered to the image intensifier entrance plane per unit time)

Answer 8

The design of the II (e.g. II diameter and conversion factor) Calibration of the optical iris diaphragm Design of television systems (e.g. TV sensor type) The specific needs of the clinical examination (reflecting the trade-off between dose rate and image quality)

Answer 9

Effect of zoom field on dose rate - In principle, if the same TV signal is to remain constant the fall in gain will invoke a proportionate increase in II entrance dose rate. In a modern IITV channel however, the TV signal is stabilised partly by increasing II entrance dose rate and partly by opening the iris diaphragm to increase light transmission. In this way, the dose penalty from selecting an II zoom field is moderated but without affecting the TV signal. Fluoroscopy systems often provide two or three automatic brightness control (ABC) dose rate selections, and correspondingly these settings result in differing levels of II entrance dose rate.

Answer 10

pincushion and S-distortion

Answer 11

Pincushion distortion This manifests itself as a non-linear increase in magnification toward the periphery of the image field. This arises from the convex shape of the input phosphor screen and the diverging geometry of the x-ray beam.

Answer 12

This arises if the stream of electrons crossing the II interact with the Earth’s magnetic field and/or a stray magnetic field from an adjacent piece of electrical equipment. Movement of the electrons through a magnetic field results in a lateral displacement of the electron trajectories which increases in magnitude toward the periphery of the field. As a result linear structures in the image field, such as the x-ray beam collimators, exhibit a distinctive S-shaped curvature (hence the name). The degree of S-distortion will vary with the orientation of the imaging chain with respect to the Earth’s magnetic field. S-distortion is minimised by shielding the II tube housing with anti-magnetic alloy (such as Mu-metal).

Answer 13

Associated with the pincushion distortion, is a radial variation (decrease) in image brightness toward the periphery of the image field. The degree of vignetting increases for large field II tubes. Vignetting can reflect a 20 to 30% reduction in signal intensity from the centre to the peripheral rim of the displayed image field.

Answer 14

Together these effects cause a veiling glare over the image field which results in a reduction in displayed contrast. The effect of veiling glare adds to the effect of x-ray scatter in the patient but obviously is not moderated by the presence of an anti-scatter grid.

Answer 15

Real II systems typically produce a contrast ratio in the range 10 to 20. Veiling glare reduces when a zoom field is selected and this is reflected in an improvement (increase) in contrast ratio.

Answer 16

Lag is the term used to describe the temporal unsharpness of an x-ray IITV fluoroscopy system. Presence of lag in fluoroscopy can seriously degrade image quality producing blur, artefacts and smearing of moving structures in the image.

Answer 17

The predominant source of lag is the TV sensor. TV lag arises from the persistence of the photosensitive material and temporal integration in the TV readout mechanism, which lead to the carry-over of the recorded signal into subsequent TV frames.

Answer 18

the unsharpness (or blur) of X-ray image intensifier - defined by the: Unsharpness of the input phosphor screen, Focusing of the electron optics and Unsharpness of the output phosphor screen TV channel - which depends upon depends on the Properties of the photo-sensor layer, TV scan format (e.g. standard 625-line vs. high definition 1250-line) which essentially defines the vertical resolution and the Bandwidth of the camera electronics which essentially defines the horizontal resolution Display

Answer 19

The limiting spatial resolution is the finest periodic test pattern that can be discriminated visually. It corresponds (approximately) to the high frequency limit of the modulation transfer function (MTF). The typical limiting spatial resolution of a large field (say 35 cm) II is ~2.5 lp mm-1.

Answer 20

The spatial resolution of an IITV fluoroscopy system varies across the image field (allied to the vignetting). Spatial resolution is normally maximum at the centre of the image field but falls towards the periphery.

Answer 21

X-ray quantum noise arises from the random fluctuations in the fluence (number per unit area) of x-ray photons which form the image. These random fluctuations are known as dynamic noise. The fluctuations in photon fluence are described by Poisson statistics (counting statistics).

Answer 22

Dynamic x-ray quantum noise (the primary factor) II structure (or fixed pattern) noise, due to the random micro-structure of the II input and output screens TV noise arising in the camera electronics which is added to the video signal

Answer 23

Nx (the fluence of x-ray photons in the II entrance plane) x η (the detective quantum efficiency (DQE) of the II) x Τ (the temporal integration time of the IITV process (including the lag of the TV camera))

Answer 24

SNR can be improved by increasing the II entrance fluence (dose); however this will come at a cost of increased patient dose The improvement in SNR with dose is slow because of the half power law: A four-fold increase in dose only results in a two-fold (square root of 4) improvement in SNR While a 100-fold increase in dose only results in a 10-fold (square root of 100) improvement in SNR SNR is better improved by using an II with a greater DQE as this will not incur a dose penalty to the patient. This can be achieved by using an II with an input window of higher x-ray transmission and/or one with an input screen of greater x-ray absorption efficiency

Answer 25

The overall image quality of an x-ray IITV fluoroscopy system can be evaluated via the threshold contrast detail detection (TCDD) test. TCDD uses a specialised test object which comprises an array of disc-shaped details of varying (and calibrated) subject contrast and diameter. Trained observers view the fluoroscopic images and determine the minimum (or threshold) contrast which is visible for details of each size.

Answer 26

reduce in proportion to the square root

Answer 27

A. Incorrect. All parts of the II input screen are (almost) the same distance from the source of x-rays so the inverse square law has an imperceptible effect on image uniformity. B. Incorrect. The conversion factor is constant across the II input screen. C. Correct. The anode heel effect causes the intensity of the x-ray beam to decrease towards the anode side of the x-ray tube and thus contributes to the non-uniformity of the image. D. Correct. Vignetting causes a decrease in brightness toward the periphery of the image field.

Answer 28

A. False. The spatial resolution of an IITV based imaging system is typically around 1 lp/mm. B. True. The use of a zoom field magnifies the image seen on the TV screen and therefore decreases the degradation of spatial resolution caused by the TV system. C. False. The spatial resolution of an IITV based imaging system (around 1 lp/mm) is inferior to that of a SFR system (around 5 to 10 lp/mm). D. False. A contrast detail test object is used to assess overall image quality by determining the threshold contrast for details of different sizes. This will depend on a combination of factors including not only spatial resolution but also contrast and noise.

Answer 29

A. Incorrect. A lower II entrance dose rate is associated with a decrease in SNR. B. Correct. A thicker layer of CsI will have greater absorption efficiency. C. Incorrect. Selecting a zoom field will not, of itself, change the photon fluence and therefore will not affect the SNR. However, in reality the situation is more complicated because most systems automatically increase the II entrance dose rate when using a zoom field. D. Correct. Replacing the CCD sensor (which has negligible lag) with a Vidicon camera tube (which has large lag) will increase the SNR.

Answer 30

A. Correct. The II entrance dose rate determines the level of noise in the image, and this affects the visibility of a low contrast detail. B. Incorrect. The II conversion factor is the light output per unit entrance exposure rate. It does not have a direct effect on image quality. C. Correct. The contrast ratio is a measure of the contrast loss in an II, and this affects the visibility of a low contrast detail. D. Incorrect. Geometrical distortion affects the appearance of the image (for example, straight lines appear curved) but it does not affect the visibility of a low contrast detail.

Answer 31

Continuous mode fluoroscopy- In continuous mode fluoroscopy x-rays are delivered continuously to the patient albeit at a comparatively low dose rate. The TV system operates asynchronously with the x-ray source. In the UK, continuous mode normally operates at 25 frames per second and has a signal integration time of 40 milliseconds. Pulsed mode fluoroscopy - In pulsed mode fluoroscopy x-rays are delivered as a series of pulses of short duration and comparatively high intensity; these pulses are synchronised with the TV readout process. Pulsed fluoroscopy systems typically operate at 30 frames per second and each x-ray pulse is ~ 3 to 10 milliseconds in duration. Lower pulse rates, such as 15 or even 7.5 frames per second, can be selected if clinically appropriate to reduce dose. Pulsed mode fluoroscopy offers much better spatio-temporal resolution than continuous mode fluoroscopy. This is seen when imaging rapidly moving anatomy, such as coronary vessels in cardiac angiography or structures within the young infant.

Answer 32

Grid-controlled x-ray tubes Such an x-ray tube incorporates a third (fine-mesh) electrode, mounted between the cathode filament and the anode target. When the grid is biased close to earth (zero) potential the current will flow across the tube undisturbed producing x-rays. However, when a large negative bias voltage is applied to the grid this instantaneously arrests tube current flow and therefore the x-ray exposure. This makes it possible to generate a rapid sequence of very short x-ray pulses

Answer 33

The feedback signal required to control the x-ray generator is derived by measuring either the II light output with a photo-sensor or by electronically sampling the corresponding video signal. In its most basic form an ABC operates by regulating the mA while the kV is set manually. A modern ABC more commonly uses a combination of mA and kV regulation. Two such algorithms include: One which favours keeping the kV low in order to maximise subject x-ray contrast, while allowing mA and therefore patient dose to rise Another algorithm favours a higher kV setting and a lower mA, which leads to lower patient entrance skin dose but at a price of reduced subject x-ray contrast

Answer 34

Patient entrance skin dose rates in IITV fluoroscopy typically lie in the range: 3 to 10 mGy per minute for the average patient 10 to 30 mGy per minute for the larger patient In the UK the maximum entrance skin dose rate limit for a standard patient is 100 mGy per minute (for all II fields). In practice in the UK, a lower maximum dose rate limit of 50 mGy per minute is commonly adopted.

Answer 35

Using a patient table manufactured from a low x-ray absorption material, such as carbon fibre minimising the fluoroscopy x-ray beam on time Tightly collimate the x-ray beam onto the clinical region of interest Select the ABC mode with the lowest dose rate which achieves the required diagnostic information Only use an II zoom field when justified by an improvement in diagnostic information When available, always use pulsed fluoroscopy and select minimum acceptable pulse rate Use the appropriate x-ray beam spectral filter to minimise patient entrance skin dose rate Increase the distance between the x-ray source and patient entrance surface to reduce divergence of the x-ray beam entering the patient The gap between the patient and the II entrance should be minimised Periodically shift the projection angle to help prevent over-irradiation of the same area of skin The x-ray anti-scatter grid should be removed if possible Use the last-image-hold facility to digitally capture and store a positional reference image (e.g. when positioning a catheter during angiography) Take note of on-board dose measurement system which typically displays values of instantaneous dose area product (DAP) rate, cumulative DAP etc. Implement radiation safety education/training for staff Ensure compliance with Ionising Radiation (Medical Exposure) Regulations 2000

Answer 36

Leakage of x-rays from the tube housing - typical tube leakage 5 μGy per hour at 1 m distance X-rays scattered from the patient (the dominant cause of staff irradiation) Secondary scatter from structures in the room

Answer 37

Fluoroscopic equipment must be designed, selected (prior to purchase) and operated with staff radiation dose safety in mind Staff must always wear a lead apron and where relevant other personal radiation shields to protect eyes, thyroids, gonads, hands Individual staff doses must be routinely monitored Lead-rubber drapes and movable lead glass shields should be available for added protection Operators should be aware of x-ray beam on warning lights and audible warnings Staff must try to maintain a maximum distance from the patient Generally, minimising patient dose minimises staff dose

Answer 38

A. False. The ABC uses the output of the image intensifier during fluoroscopy. It is not relevant when the equipment is operated in fluorographic mode using film. B. False. The ABC maintains the II entrance dose rate. The patient entrance dose rate must vary to achieve this, with larger patients requiring a higher entrance dose rate. C. True. The ABC uses a combination of mA and kV regulation to maintain the II entrance dose rate and thus keep the brightness of the displayed image constant. D. False. II entrance dose rate (and therefore patient skin dose rate) normally increases when an II zoom field is selected.

Answer 39

A. Incorrect. The gap between the patient and the II entrance should be minimised. Introducing an air gap between the patient and the II requires an increase in the patient entrance surface dose rate in order to maintain the II entrance dose rate. B. Correct. An appropriate x-ray beam spectral filter is used to reduce patient entrance skin dose rate. C. Correct. Reducing the pulse rate from 30 to 15 frames per second will halve the average patient entrance surface dose rate. D. Incorrect. Patient entrance surface dose rate normally increases when an II zoom field is selected.

Answer 40

A. Correct. Minimising the x-ray beam on time reduces staff dose. It also reduces patient dose. B. Incorrect. Staff dose is higher using an overcouch x-ray source rather than an undercouch x-ray source. C. Incorrect. Lead aprons should be worn by staff. D. Correct. Staff must try to maintain a maximum distance from the patient.

Answer 41

The introduction of digital IITV systems made it possible to acquire high quality dynamic x-ray images at comparatively low patient dose and view them in a flexible and ergonomically efficient way. Digital IITV images can be computer enhanced to improve the presented information. Image results can be archived digitally and/or on film via laser hardcopy as preferred. As a result of these advances, digital IITV systems have broadened the range of clinical imaging applications which can be supported in the fluoroscopy suite

Answer 42

Automatic brightness control and automatic exposure control systems are used in fluoroscopy and fluorography respectively. Fluoroscopy low current (0.5-5 mA), continuous or near-continuous x-ray exposures relatively low signal to noise ratio (SNR) prioritises temporal resolution for procedures 512 x 512 pixel matrix with 8-bit greyscale real-time imaging viewed on a display monitor in the clinical room Fluorography relatively intense (50-1000 mA), pulsed x-ray exposure (pulses are of short duration and applied at 1-12 pulses/second) relatively high SNR prioritises spatial resolution for diagnostic purposes 1024 x 1024 pixel matrix with 10-bit greyscale images usually viewed after acquisition

Answer 43

Greyscale range compression, typically using a non-linear response curve, is often used to suppress or highlight intensities and improve the contrast balance of the image - this can be implemented via an analogue (video) circuit or a digital look-up-table (LUT) Contrast and brightness adjustment is used to improve the presented contrast of the viewed image or to optimise hardcopy image quality

Answer 44

similar to the contour or edge enhancement used to enhance computed radiography (CR) and digital radiography (DR) images) is employed to improve the displayed spatial resolution of the images acquired with a digital IITV system

Answer 45

this technique uses a sliding average of the current frame with a set of [N -1] preceding frames. The noise content of the final displayed image reduces with the square root of the number of frames averaged This technique uses a digitally generated lag (temporal unsharpness) to smooth the noise fluctuations, and therefore it works best for quasi-static image content.

Answer 46

Road mapping is a subtractive image processing technique which is widely used during cardiovascular diagnosis and intervention. Here, a digital fluorographic image is first acquired depicting the contrast medium enhanced vessel of interest. This reference image is then digitally subtracted from the fluoroscopy image in real time removing the common background anatomy. This leaves a reference (or road) map, against which the live catheter or guide-wire is more clearly visible. Road mapping is used to reduce the time required to route the catheter through the vessel and therefore the radiation dose delivered to the patient.

Answer 47

A. Incorrect. In fluorography (digital spot imaging) the receptor dose per frame is typically less than 5 μGy, the exact amount depending on the quality required. This is comparable to the dose to a typical screen film combination used for radiography. B. Incorrect. The spatial resolution of a digital fluorography system is lower than that of a screen film radiography system. C. Incorrect. Fluorographic film was used in old analogue systems but is not used in a digital fluorography system. D. Correct. Selected image frames can be hard copied on film for distribution to clinical colleagues using a laser printer.

Answer 48

A. False. The last-image-hold facility is used to capture and temporarily store an image during fluoroscopy. This provides the radiologist with an on-going image on the reference display monitor without the need to expose the patient to further radiation unnecessarily. B. True. Road mapping is a subtractive image processing technique which is widely used during cardiovascular diagnosis and intervention. A digital fluorographic image is first acquired depicting the contrast medium enhanced vessel of interest. This reference image is then digitally subtracted from the fluoroscopy image in real time removing the common background anatomy. This leaves a road map against which the live catheter or guide-wire is more clearly visible. C. True. The DQE of dynamic flat panel detectors is 10 to 20% higher than that of a modern IITV, so a lower dose per frame can be used. D. False. As a fixed matrix array is used, the spatial resolution of a flat panel detector does not improve when a zoom field is selected (unlike IITV).