Physics questions 6
Diagnostic Imaging Physics
MPHY-501
Bushberg Chapters 9
Fluoroscopy
During the next two years you will learn the basics of Diagnostic Imaging. Ultimately after this period you will enroll in a Medical Physics Residency Program and finally take the Board examination that, once you pass, will certify you as a medical physicist.
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Fluoroscopy systems
Projection x-ray images
Real-time x-ray
High temporal-resolution
Imaging guidance for interventional procedures
Image intensifiers (IIs) coupled to digital video systems
or flat panel digital detectors as image receptors.
Improvements in fluoroscopic systems
increased x-ray beam filtration,
higher x-ray detection efficiency phosphors,
lower acquisition frame rates,
Dose-monitoring on all interventional systems since June 10, 2006.
Fluoroscopy the early years
Functionality
“Real-time” imaging: considered to be 30 frames per second (FPS)
Modern fluoroscopy systems: pulsed x-ray beam with digital image acquisition, variable frame rates ranging from 3 to 30 FPS.
Cardiac systems, Imaging heart at rates of 15 to 60 FPS
can be played back as a movie loop
For angiographic procedures after catheter is in place, image sequence is recorded:
using high frame rate pulsed fluorography
as x-ray contrast material is injected through the catheter into the vessels or body cavity
Gastrointestinal fluoroscopy, much of the diagnosis is formulated during the unrecorded fluoroscopy sequence; however, fluorographic images are acquired for
documenting the study and
illustrating important diagnostic findings
Fluoroscopic Imaging-Chain Components
Added filtration in fluoroscopy systems has increased in recent years, as pulsed systems with high mA capabilities have replaced older continuous fluoroscopy systems.
Copper filtration combined with lower kV values allows angiography systems to provide lower dose operation while still delivering high image contrast for angiographic applications.
Collimators on fluoroscopy
circular diaphragms for II-based systems and
rectangular collimation for flat panel fluoroscopy systems.
The collimators on fluoroscopy systems are motorized, which allows the collimated x-ray beam to change size in response to adjustment of the source-to-image distance (SID) or when the FOV is electronically changed
operator adjustable attenuating wedges provide additional attenuation at specified locations in the x-ray field,
pulmonary space between the heart and chest wall,
The use of wedges can reduce the glare from these areas, equalize the x-ray beam incident on the image receptor, and lower radiation dose to the patient.
Fluoroscopy as Distinguished from Radiography
Ability to produce:
Real-time x-ray images
High frame rates
Low-dose per image.
Illustrated of Fluoroscopic Imaging Chain
Why is x-ray tube under table?
What is KAP meter?
What kind of filters are used?
Five configurations of fluoroscopy systems:
Gastrointestinal/Genitourinary (GI/GU) (x-ray tube is under the patient table)
Remote fluoroscopy (over-table x-ray tube)
Interventional vascular (vascular surgery, radiology)
Cardiology and interventional electrophysiology,
Mobile C-arm configurations (surgery, pain medicine).
All have same basic components.
Mobile C-ARM
X-Ray Bi-Plane
With
MRI
MRI uses
no ionizing radiation
X-Ray Bi-Plane
With
DSA
Table Top
X-Ray tube is under table:
In many systems the tube can move toward and away from the table
The chest bucky (on the left) indicates that this is a rad/fluoro room
Meaning that there is another x-ray tube that is not in the picture…
Mini C-Arm
Equalization Filters:
mounted at collimator
controls adjust the blade location and Hence conforming to patient regions of low attenuation.
a) lead-rubber blades
b) Lead-acrylic blades
IIs & fluoroscopic flat panel detectors
Several thousand times more sensitive than a standard radiographic detector
Produce images using several thousand times less radiation.
Standard fluoroscopy uses about 9 to 17 nGy
Computed radiography system use about 5 to 9 μGy
Types of Fluoroscopy Acquisitions
| Dose per technique used | |||
| Spot Images are the highest dose of all techniques! | |||
| One Fluoroscopy Frame | Cine (per 10 frames) | Digital Subtraction Angiography | One Spot Image |
| ~ 1 nanoGy | 10 nanoGy | 100 nanoGy | 1000 nanoGy |
Fluoroscopic Detector Systems
The Image Intensifier, Based Fluoroscopy Systems
(skip, read for homework)
Flat Panel Detector Based Digital Fluoroscopy Systems
Flat Panel Fluoroscopy Detectors
Thin film transistor (TFT) arrays (individual detector elements (dexels) packaged in square or rectangular areas).
Indirect and direct x-ray conversion
Each detector element (DEL) has:
a capacitor, which accumulates and stores the signal as an electrical charge,
a transistor which serves as a switch.
For indirect detection TFT systems, absorbed x-rays are initially converted to light in the adjacent layered phosphor,
each dexel has a transistor and a capacitor, in addition to a photodiode, which converts the x-ray induced light from the phosphor into a corresponding charge.
For direct detection fluoroscopy detectors,
semiconductor (selenium) produces x-ray induced charge directly, which is collected under a voltage to ensure that the signal is captured within the same dexel as the x-ray absorption event
For fluoroscopic applications, the flat panel image receptor replaces the II, lenses, and camera system, and directly records the real-time fluoroscopic image sequence. The size of a detector element (dexel) in fluoroscopy is usually larger thanP.291
in radiography, and some flat panel systems have the ability to adjust the dexel size by binning four dexels into one larger dexel.
For example, four 100-μm detector elements are grouped together (“binned”) electronically to form an effective 200 μm × 200 μm dexel.
dual-use systems have dexels small enough for high-resolution radiography (e.g., 100 to 150 μm), but the dexels can be binned to provide a detector useful for fluoroscopy (e.g., 200 to 300 μm).
Binning of Dexel and Pixel
Slower flat panel <- This section is rapidly becoming OUT DATED!
Larger dexels (less spatial resolution) are necessary as data transfer rate (“bandwidth”) limitations restrict the amount of image data that can be displayed at 30 FPS.
2 × 2 binning reduces data (factor of 4), makes real-time display feasible. lower per frame “dose” results in, significant quantum noise & poor statistical integrity (low SNR), limiting small objects visibility.
Other flat panel detectors:
Detector is read at full resolution
number of pixels in the digital image may exceed those on display monitor,
real-time video processors interpolate and average information as displayed to the fluoroscopist.
Averaging on video monitor, results are similar to binning method, but in image playback mode the full image resolution can be viewed with image zoom.
Automatic Exposure Rate Control
to keep the signal-to-noise ratio (SNR) of the image constant when possible.
By regulating the x-ray exposure rate incident on flat panel detector.
When the system is panned from a region of low attenuation to one of greater attenuation (e.g., thin to a thick region of soft tissue) of the patient, fewer x-rays strike the detector. An AERC sensor (Fig. 9-1) measures the x-ray intensity and the AERC sends a signal to the x-ray generator to increase the x-ray exposure rate.
The AERC circuitry strives to keep the detected photon fluence used for each fluoroscopic frame at a constant level, keeping the SNR of the image approximately constant regardless of the thickness of the patient.
It also increases the exposure rate when magnification modes are used, so that the detected photon fluence per pixel is approximately constant as the effective pixel area on the displayed image becomes larger.
The AERC circuitry in the x-ray generator is capable of changing the mA and the kV in continuous fluoroscopy mode. For pulsed fluoroscopy systems, the AERC circuitry may regulate the pulse width (the time duration) or pulse height (the mA) of the x-ray pulses as well as kV (to increase or decrease penetrability of the beam).
Regulatory Limits:
x-ray exposure rates reach a maximum:
87 mGy/min (10 R/min) maximum tabletop exposure rate
Boost (175 mGy/min [20 R/min] tabletop exposure)
utilized when absolutely necessary (very thick patients).
!!!!! Spread Sheet !!!!!
AERC
x-ray generator changes the mA and the kV in a predetermined manner;
varies with different models of generators
selectable on a given fluoroscopic system.
How mA and kV change as a function of patient influence patient dose and image quality.
When the fluoroscopist pans to a thicker region of the patient,
AERC circuit requests more x-rays from the generator.
increasing the kV, the subject contrast decreases,
dose to the patient is lower because more x-rays penetrate the patient at higher kV.
When contrast is crucial (e.g., angiography)
Generator can increase the mA instead of the kV (preserving contrast but higher dose)
In practice, the mA and kV are increased together, but the curve that describes this (sometimes called color or flavor) can be designed to aggressively preserve subject contrast, or favor a lower dose examination
Some systems have “low dose,” “conventional”, or “high contrast” selections on the console, which select different mA-kV curves
Fluoroscopy Modes of Operation
Continuous Fluoroscopy
Variable Frame Rate Pulsed Fluoroscopy
Pulsed fluoroscopy selectable frame rates
Typically 30, 15, 7.5 and 3.75 FPS
Field of View and Magnification Modes
Image Intensifiers (IIs)
Common II sizes: 23-, 30-, 35-, and 40-cm diameter FOV.
Large IIs for GI/GU work, can image entire abdomen.
Cardiac imaging: 23-cm II is adequate
IIs have several magnification modes, which yield higher spatial resolution with a smaller field of view.
Fixed Geometry systems (C-arms): Increasing magnification results in smaller area on input of II
When magnification mode is engaged, beam collimator automatically adjusts to match the x-ray beam dimensions to the smaller FOV.
(1mm Aluminum)
(0.5mm Aluminum)
(Act like light pipes)
(X-ray to visible light conversion step)
Thin layer:
Little scatter
Minimal resolution loss
Example of Mag-mode and exposure
A 30-cm II with 23- and 17-cm magnification modes.
Switching from the 30-cm mode to the 23-cm mode will increase the x-ray exposure rate by a factor of (30/23)2 = 1.7
Going from the 30-cm FOV to the 17-cm FOV will increase the exposure rate by a factor of (30/17)2 = 3.1.
To minimise patient dose, use the least magnification and smallest collimation area.
Frame Averaging
Fluoroscopy images are relatively noisy
It may be beneficial to (reduce) temporal resolution for lower quantum noise.
This can be accomplished by averaging a series of images,
Appreciable frame averaging can cause noticeable image lag with reduced temporal resolution.
frame averaging can provide lower dose imaging in many circumstances.
Mobile fluoroscopy have limited output hence temporal frame averaging is a common feature.
Recursive Filtering:
Last-Frame-Hold, Last-Sequence-Display
When the fluoroscopist steps off the pedal and deactivates fluoroscopy, rather than seeing a blank monitor, last-frame-hold continuously displays the last acquired image or set of images on the fluoroscopy monitor.
Rather than orienting oneself with patient anatomy while the fluoroscopy pedal is depressed and x-rays are on, last-frame-hold allows the fluoroscopist to examine the image on the monitor for as long as necessary, using no additional radiation.
A playback of the last sequence of images, programmable over several seconds, is available on many advanced fluoro systems with larger video buffer memories, and can be useful for complicated procedures.
Last-frame-hold is a necessary feature for dose reduction, especially at training institutions.
Road Mapping
Two monitors
One monitor has contrast injected image that shows veins and arteries
Contrast washes out rapidly so this frame is held and does not change
Second monitor is real-time fluoroscopy showing where the catheter is located
Another approach to road mapping
capture a contrast injection image or subtracted image,
use this as an overlay onto the live fluoroscopy monitor.
angiographer has the vascular “road map” superimposed
C-Arm CT
Some modern C-arms have computed tomography (CT) acquisition modes
capable of motorized rotation of about 220 degrees around the patient,
which allows two-dimensional projection images to be acquired at many different angles around the patient.
provides tomographic capability during interventional
Images from such a cone-beam CT acquisition have poorer image quality compared to a conventional CT scanner
fidelity of the images is not as good as with whole body CT scanners
rotational axis on the C-arm system is not as rigid,
cone beam geometry results in detection of much higher scatter levels than whole body CT systems.
Image Quality in Fluoroscopy
Resolution
Resolution
Intrinsic resolution of fluoroscopy cannot exceed the Nyquist frequency, FN = 1/2 Δ, where Δ is the size of a detector element.
A 30-cm FOV and 1,024 × 1,024 imaging matrix,
Δ = 300 mm/ 1,024 = 0.29 mm; FN = 1/(0.29×2) = 1.7 line pairs/mm.
FLUOROSCOPY: LIMITING SPATIAL RESOLUTION
| FOV cm (inches) | 525 LINE VIDEO LINE PAIRS/mm | 1023 LINE VIDEO LINE PAIRS/mm | FLAT PANEL 0.157 mm LINE PAIRS/mm |
| 14 (5.5) | 2.7 | 1.4 | 3.2 |
| 20 (7.9) | 2.0 | 1.0 | 2.8 |
| 27 (10.6) | 1.6 | 0.7 | 2.5 |
| 40 (15.7) | 1.3 | 0.5 | 1.8 |
Contrast Resolution
Step wedge: Holes of different depths drilled into each step.
Thicker step attenuates more x-rays, resulting in a noisier image.
Noise differences between steps (in this example) were amplified for demonstration.
Grid ratios in fluoroscopy are 6:1 to 12:1 and short
(100 cm or less) focal distances are used in fluoroscopic examinations
Grid can be moved out of the beam, particularly in the case of smaller-size pediatric patients
Temporal Resolution
Radiation Dose
Patient Dose Rate
MAX = 87.3 mGy/m or (10 R/m)
Boots = 175 mGy/m (20 R/min)
Typical entrance exposure rates for fluoroscopic imaging are about:
8.7 to 17 mGy/m (1 to 2 R/min) for thin body parts and
26 to 44 mGy/m (3 to 5 R/min) for the average patient.
Dose rate may be much larger for oblique and lateral projections and for obese patients.
Skin Entrance exposure rates substantially exceed these values if patient is much closer to the x-ray source than the dose reference point (1/r2).
Neither FDA nor states provide dose limits for image recording.
Dose Rates Evaluation
Dose rates limits are assessed at specified positions
x-ray tube is below the patient table, the measurement position is 1 cm above the table;
C-arm fluoroscopes, this position is 30 cm from the image receptor toward the x-ray source along the central axis of the beam.
Tissue-equivalent phantom made of a material such as polymethyl methacrylate (PMMA, also known as Lucite or Perspex) placed in the field,
ionization chamber positioned in front of it to measure the entrance skin dose
Dose rates are measured in all typical modes (e.g., magnification modes, low dose/high contrast, cine, DSA, etc), with the antiscatter grid in.
Dose rates also are measured using different thicknesses of PMMA to determine the entrance dose as a function of “patient” thickness, for example, using 10, 20, and 30 cm of PMMA.
To determine the maximum exposure rate, a sheet of lead is placed between the PMMA and the detector system to attenuate the x-ray beam and drive the AERC system to its maximum output.
This “geometry” (set-up) shown includes measurement of backscatter because ionization chamber is very close to the PMMA attenuator.
KAP = KERMA area product.
KAP does not change with distance from source!
1 cm
II
X-ray
30 cm
Wen measuring dose and dose rate there are two different reference points to consider.
In systems with variable SID the FDA reference point is used.
With fixed systems it may be necessary to use the IEC reference point.
Remember:
x-ray tube is below the patient table, the measurement position is 1 cm above the table;
C-arm fluoroscopes, this position is 30 cm from the image receptor toward the x-ray source along the central axis of the beam.
Cumulative Air Kerma Reporting Requirement
Fluoroscopy systems manufactured since June 10, 2006 are required to display the cumulative air kerma (commonly in the units of mGy) at a reference point.
For a C-arm fluoroscope, this reference point is on the beam axis, 15 cm from the isocenter, toward the x-ray tube. The cumulative air kerma provides an actual dose value that will have more meaning to most users.
To avoid the additional cost of active dose monitors, it is also possible (with proper calibration) to compute the cumulative air kerma from the mA, time, kV, and SID used during the fluoroscopic procedure—no physical dose monitoring is necessary.
The cumulative air kerma may significantly over or underestimate the actual peak skin dose for three reasons:
(1) The cumulative air kerma does not account for backscattered radiation. Due to back scatter, the actual skin dose will exceed the air kerma at that location by a factor of about 1.25 to 1.5, depending upon the kV, beam filtration, and x-ray field size.
(2) The x-ray beam may be incident upon a single area on the skin, or may be incident on multiple areas. If these areas do not overlap, the dose is spread over the multiple areas.
(3) The skin may closer or farther from the x-ray tube than the reference point at which the cumulative air kerma is specified. If the skin is much closer to the x-ray tube, the skin dose may greatly exceed the cumulative air kerma.
Reporting and Regulations
Radiation Skin Burn
May result from acute radiation dose > 15 Gy
Use optimal techniques to minimize dose
If erythema does not appear approximately 10 to 15 days after the procedure or if moist desquamation does not appear within 8 weeks, further follow-up is likely unnecessary
| Reportable Radiation Doses | |
| Dose (Gy) | Reporting |
| Radiation Safety Officer - x5075 Medical Physicist - x6153 | |
| > 2 Gy Air Kerma > 60 minutes Fluoroscopy Time > 500 Gy*cm2 Dose Area Product | |
| Anything over 3Gy: Radiation Safety Officer/Medical Physicist reports to Local Radiation Protection Authority within 1 business day Provide written QMP analysis of Medical Event within 15 business days Provide clinical summary within 15 business days to prescribing physician & patient Maintain record of Medical Event in patients permanent record (http://www.fda.gov/cdrh/radhealth.html) Anything >15 Gy: reports go to both Local Radiation Protection Authority and Joint Commission |
Dose Reduction, Equipment and Complications
Personnel Shield % Attenuation:
Lead Aprons >90%
Thyroid Shields >90%
Leaded Eye wear >85%
Lead apron shielding of 0.5 mm lead equivalence is required by state regulatory agencies
Part 2: Dose Reduction, Equipment and Complications
Pulldown shields:
Fluoroscopy suites may have ceiling mounted leaded acrylic or leaded glass shields
Use these shields when possible to reduce radiation dose to you and to staff behind you
Gundersen Health System
Dose Reduction
Keep source at distance (Use low magnification)
Keep source under table if possible
Use less Fluoroscopy time
Use fewer frames
Use more filtration
Use pulsed low frame rate
Try not to use spot films and keep CINE and DSA at a minimum
Dose Reduction, Equipment and Complications
ALARA Techniques:
Reduce fluoroscopy time:
Short taps on the foot pedal
Low pulse mode
Low frame rate
Increase distance from the radiation source:
Keep the tube under the table if possible
Only stand near the tube when necessary
Keep body parts out of the x-ray beam
Optimize shielding:
Use lead apron & thyroid shield, shielded goggles, mobile walls, lead curtains
Stand behind others
Other techniques to minimize dose
Use tight collimation
Maximize x-ray tube to patient distance
Maximize use of last image hold
Avoid magnification – decreasing the FOV by a factor of 2 increases the dose by a factor of 4
Minimize patient & image receptor distance
Minimize the use of spot images
Minimize field overlap
Minimize oblique projection use (increased dose)
Minimize cine frame rate (15 fps, 7.5 fps or 3 fps)
Dose Reduction, Equipment and Complications
Reporting and Regulations
Fluoroscopy operators must be monitored for occupational radiation exposure
A radiation dosimeter is used for this purpose
Depending on the type and volume of your work, you may be issued either one or two dosimeters
The dosimeters are exchanged monthly or quarterly, depending on the type and volume of your work
Dosimetry
Wear your dosimeter outside your lead apron on your thorax
If you have two dosimeters, wear the “collar dosimeter” as shown
Wear the “body dosimeter” at waist level underneath your lead apron
Environmental Health & Safety, Weill Cornell Medicine
Reporting and Regulations
Pregnant staff may be issued a fetal dosimeter to be worn at waist level underneath the lead apron
Estimated & Deep Dose Equivalent (EDE and DDE)
EDE 1: (Two dosimeters). One dosimeter is worn at the waist level under a lead apron and one dosimeter is worn at the collar level outside the lead apron. 1.5 (Waist DDE) + 0.04 (Collar DDE) = Assigned Deep Dose Equivalent
EDE 2: (One dosimeter). One dosimeter is worn outside the lead apron. 0.30 (Collar DDE) = Assigned Deep Dose Equivalent
Quiz
True
False
This image demonstrates optimal positioning of the x-ray tube and image intensifier.
Why are the x-rays coming from underneath the table?
Why is the detector so close to the table?
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