Méthodes de réduction de la dose en angiographie

Dose and Radiation Risk in Angiography

Several dose parameters are specific to angiography: the detector dose, the patient entrance dose, the dose rate and the DAP (dose area product), which will be covered in the following sections.

In the past, when angiography was performed using traditional photographic film technology, the general rule was the higher the dose, the better the image quality.

With today’s improvements in imaging technology, is there still a trade-off between improving quality and saving dose?

Yes

In general, low dose goes hand in hand with less visibility, while higher image quality requires, among other factors, a higher dose. To obtain a specific image quality, it is necessary to choose the “right” dose for the tissue being penetrated.

Therefore, the solution is to make best use of dose and equipment.

Good image quality can be expressed in terms of detectability. Rose found:¹

Modern detector systems make it easy to obtain high-quality results simply by selecting the required image quality by choosing an appropriate protocol; the requested dose at the detector entrance will automatically be kept constant (as much as possible) by adjusting the tube output dose. This automatic dose control compensates for patients of different body sizes.

The dose is highest at the point where the beam enters the patient. The absorbed dose at this beam entrance is an important measure: It signifies the accumulated patient entrance dose over the length of the procedure, measured in mGy (1,000 mGy = 1 Gy). This accumulated patient entrance dose is relevant for determining the skin burn damage resulting from the intervention.

Detector systems display and report only an estimate of the patient entrance dose at the interventional reference point (IRP). Values at the actual exposure entry location can be different depending on the patient’s body shape and other geometric measures, such as table and C-arm position.

The IRP is the measuring point for:

Dose area product (DAP) in μGy m²

Dose in mGy

Dose rate in mGy/min

Note: The IRP does not change with table height.

As a general rule, the closer the beam entrance to the tube, the higher the real patient entrance
dose and vice versa.

The Siemens Artis system has two built-in safety regulations for fluoroscopy:

1. By default, the dose rate (air kerma rate) at a specific location (30 cm in front of the detector) is limited to a certain level (e.g., 10 R/min = 87 mGy/min for the U.S. and European countries). It is assumed that this point is identical to the patient’s skin
2. entry point. It is possible to increase the dose rate by using the “Fluoro +” button (high-contrast button). An audible warning occurs.
3. After every five minutes of fluoroscopy, a message pops up on the display and a sound is emitted to remind the user of the applied dose. If the operator does not acknowledge this signal, radiation exposure stops after the next five minutes of fluoroscopy.

Mean patient entrance dose has been reported for interventions in the brain. Although head size varies little among individuals, body size varies greatly. An increase in patient thickness of about 3 cm results in twice the entrance dose for a constant detector entrance dose (Figure 4). This rule of thumb is based on the assumption that tissue absorbs radiation in a similar way to water and that a certain quality of beam is applied.

In air, X-rays travel in a straight line. Their intensity decreases as the distance from the X-ray tube focus increases along with the surface area of the beam. The dose D at the distance d from the focal spot F drops to 1/4 D at the distance 2d and to 1/9 of D at three times the distance (Figure 6). The inverse square law for radiation dose shows that at twice the distance from the focus, the dose D is reduced by a factor of four with respect to the quadrupled surface – it is spread across a four-fold area.

The DAP of a certain exposed area of constant dose is defined as dose times area and is independent from the distance to the source.

An example for distances d₁ = d and d₂ = 2d and for associated doses D₁ = D and D₂ = 1/4 D and the irradiated
areas a₁ = a and a₂ = a d₂²/d₁² = 4a proves:

DAP₁ = D₁ · a₁ = D · a

DAP₂ = D2 · a₂ = 1/4 D · 4a = D · a = DAP₁

This means that the Dose Area Product remains constant at different distances.

Inverse square law:
The inverse square law for radiation dose shows that at twice the distance from the focus, the dose D is reduced by a factor of four with respect to the quadrupled surface – it is spread across a four-fold area.

Determining the effective dose in angiography depends on several factors, primarily on the variability in organ sensitivity to radiation. Recall that bone marrow is far more sensitive to radiation than the liver (refer back to the section on “Equivalent and Effective Dose”). The degree to which organs are affected by radiation also depends on the angle of the beams. Because dose distribution in angiography is not “homogeneous” as it is for CT, these factors must be considered when estimating the damage caused by irradiation.

Converting patient entrance dose and DAP to effective dose is reliable only if the X-ray parameters and the location of the beam running through the body are known. In modern angiography, the role of the effective dose is not as significant as it is, for instance, in CT.

Remember: The effective dose includes the sensitivity to radiation of the different organs. It is the sum of the equivalent doses in all irradiated organs multiplied by the respective tissue weighting factors.

Several parameters affect dose in angiography.

• The footswitch on time controls how long the beam is on the body and thus how long the body is irradiated; less time means less radiation.
• High frame rates are used to visualize fast motion without stroboscopic effects. However, the higher the frame rate, the more radiation. Therefore it is best to keep the frame rate as low as possible.
• SID: according to the quadratic law and a constant requested dose at the detector,a greater distance between the source and the imager increases the patient entrance dose. Raising SID from 105 cm (=SID 1) to 120 cm (=SID 2) increases patient entrance dose (i.e. the dose at the IRP) by approximately 30%.¹

¹If C-arm angles, table position, patient, and requested dose at the detector do not change.

Dose Reduction Advances in Angiography

This section focuses on various technologies that Siemens has implemented or developed to reduce, monitor, and report the radiation dose applied during interventional procedures.

Siemens strives to implement all dose-saving, monitoring, and reduction methods available in the interventional market today. As a leader in the field of dose reduction, we consistently develop our own solutions. As such, we were the first to implement several features that save and monitor the radiation dose in the interventional routine. In addition, we are a leading vendor to offer these cutting-edge solutions for a large number of features.

Our products clearly follow the ALARA principle (As Low As Reasonably Achievable) to reduce radiation dose to the lowest possible level. This desire for as little radiation exposure as possible is at the heart of the CARE (Combined Applications to Reduce Exposure) research and development philosophy.To maintain our leading position, and to improve health care for patients, we cooperate closely with experts at universities, and at public and private radiology centers all over the world – to convert research developments into practical components of everyday clinical routine.

In addition to implementing the newest technology, dose reduction efforts in angiography require training to become familiar with reduction methods and factors. We therefore attempt to make our dose-saving products as transparent as possible. We also provide a broad range of dose-monitoring products to interventionalists and technologists, and offer an ongoing selection of seminars and resources on dose reduction.

Figure 1 outlines our dose-saving, monitoring, and reporting products and tools for angiography, which are all available with all Artis systems.

Reducing the dose during interventional procedures is not only critical for the patient, but also for the cardiologist and staff in the examination room. By integrating a broad range of dose-saving features into the Artis zee, Siemens minimizes the dose to both patients and the interventional staff.

CAREvision provides variable fluoro pulse rates. The pulsing frequency of the Artis systems can be adapted according to the clinical need: from 30 pulses per second (p/s) in various steps, down to 0.5 p/s. This is the easiest way to reduce exposure to the patient. A reduction to half pulse rate reduces the dose by about half. The reduction from 30 p/s to 7.5 p/s results in a dose saving of 75%² (Figure 2).

CAREfilter achieves skin dose reduction by allowing adjustment of the filter thickness. Additional copper filters reduce the skin dose through beam hardening. The variable filtration, 0.2 to 0.9 mm during fluoroscopy and 0.0 to 0.9 mm during digital acquisition, is adjusted automatically according to the absorption of the patient entrance dose along the path of the X-ray beam through the patient. This automatic filter insertion always maintains the lowest skin dose possible without degrading image quality. The filter selection is shown on the data display section of the monitor. Increasing prefiltering from 0.2 to 0.9 mm at 70 kV results in a dose saving of approximately 50% (Figure 3).

Using the last image hold (LIH) as a reference, CAREprofile allows radiation-free collimation and semitransparent filter position setting to precisely target the region of interest.

CAREposition provides radiation-free object positioning. Graphic display of the outline of the upcoming image allows panning the table without fluoroscopic radiation exposure (Figure 5).

CAREprofile and CAREposition can reduce the total fluoroscopy time by 0.5 to 3 minutes. This can results in a dose saving for normal fluoro at a SID of 100 cm and a phantom and table equivalent thickness of 207 mm at 70 kV of 12 to 70 mGy.¹

The operator can easily reduce the radiation exposure by changing the fluoroscopy protocol – i.e. by switching tableside ECC/TSC¹ from “Fluoro med” to “Fluoro low” during fluoroscopy using the touch screen; or by increasing the exposure to “Fluoro high” because of a thick object or a steep angulation (Figure 6).

Switching between the three different fluoroscopy modes can be done tableside in the Examination tab card (see Figure 6) or in the control room in the Examination Set (see Figure 6). “Fluoro low” typically means half dose compared to “Fluoro med.”

For especially dose-sensitive patients, it is possible to generate a special low-dose acquisition protocol. An acquisition pedal of the footswitch can be configured as a low-dose acquisition alternative to the ECC/TSC (Figure 7).

A dose saving of 67%¹ can be achieved by using an acquisition dose of 80 nGy/f instead of 240 nGy/f for interventional cardiology and an acquisition dose of 0.8 μGy/f instead of 2.4 μGy/f for interventional radiology.

The low-dose syngo DynaCT protocol achieves acceptable image quality at lowest possible dose values in a lot of cases (Figure 66). This protocol is for radiosensitive patients, such as pediatric patients, and provides adequate diagnostic image quality. In clinical practice, the balance between image quality and dose has to be considered. For the prerequisites mentioned above, a five-second high-contrast DR rotational 3D run applying 0.36 μGy/f can be reduced to 0.1 μGy/f. Switching from 0.36 μGy/f to 0.1 μGy/f results in a dose saving of up to 72%.¹ Low-dose syngo DynaCT can be achieved with an effective dose of 0.1 mSv.

In combination with syngo InSpace3D/3D Fusion, low-dose syngo DynaCT results can be fused with diagnostic preinterventional CT, MR, or PET•CT results. These fused data sets provide an excellent basis for planning and guidance during interventional procedures.

Slab mode allows you to collimate the image from top to bottom before doing the 3D rotational run (Figure 9). The benefits are lower dose, because of the small exposed area, with better image quality, because of less scattered radiation.

The Artis systems can store the last 1024 images from fluoroscopy to hard disk (Figure 10). This feature can be used for documentation, and can make additional digital acquisitions unnecessary. The operator only has to press one button at the ECC/TSC console.

For specific cardiac protocols, using fluoroscopic recording instead of digital acquisition results in a dose saving by a factor of 8 to 10 per minute at 15 fps.¹

¹Nickoloff et al., Cardiovasc Intervent Radiol (2007) 30:168-176.

A simple way to reduce the dose in pediatric examinations, especially for babies or very thin patients, when scatter radiation can be expected to be negligible, is to remove the scatter grid in the flat detector housing (Figure 11).

The grid factor (i.e. the absorption of primary radiation due to the anti-scatter grid compared to free air) is 1.35, which translates into a dose saving of 26% when removing the grid.

Monitoring the patient dose is another element in controlling radiation exposure. To keep the burden of this task off the interventionalist, the Artis systems are equipped to monitor patient dose in various ways. This allows for more transparency during and after the procedure as to how much radiation was applied. The following sections discuss how to monitor dose with the Artis systems.

CAREwatch displays the dose values during the patient examination on the image monitors in the examination room and also in the control room (Figure 12).

1. When radiation is off, the dose area product and the accumulated dose at the IRP (Figure 12) are displayed.
   
2. When radiation is on, the dose area product and dose rate at the IRP are displayed.

CAREguard provides an effective way to control patient entrance dose (i.e. air kerma at the patient entrance reference point PERP¹). Three dose threshold values (low, medium, and high) can be individually defined. If the accumulated patient entrance dose exceeds one of the defined thresholds:

1. An audible warning sound is given
2. A patient entrance dose indicator on the live display flashes
3. A warning popup is displayed on the ECC/touch screen

¹PERP = Patient entrance reference point = IRP.

More and more countries and authorities require the reporting of patient exposure following an intervention. To meet current and future regulations, Artis systems allow effective reporting of dose exposure and thus enable enhanced in-house dose reporting and analysis.

After the patient has been examined, an examination or patient protocol is stored together with acquired images. The complete information for each run is stored. At the end of the protocol, the dose information is listed: number of exposures, total fluoro time, total dose area product, and total dose at IRP. These values are separated per plane in the lines below (Figure 16).

CAREreport, the DICOM structured dose report, contains all patient demographics, procedure and dose information. Using commercially available programs or in-house software, this information can be filtered for further processing, such as dose analysis (Figure 17).
CAREreport provides consistent dose reporting and prepares for future potential legal requirements.

The GIGALIX X-ray tube has been designed around a unique flat emitter technology that generates powerful short pulses. Compared to filament technology, the higher maximum current of the flat emitter enables CLEARpulse and enhances image quality in challenging situations such as with obese patients or in steep angulations. The small square focal spots of the GIGALIX result in higher spatial resolution for all clinical applications and help to better visualize small devices and vessels.

Together with the higher contrast resolution, this results in up to 70% better visibility of small devices².

With CLEARpulse, the pulse length can be shortened. This allows visualizing moving objects such as coronary vessels more sharply.

CLEARpulse also optimizes the X-ray spectrum by lowering the required tube voltage and allowing for additional filtration. Together with small focal spots, this generates equal image quality with up to 60% less dose².

The GIGALIX X-ray tube in the Artis Q product line scores a double win: enhanced image quality at a significantly lower dose for both patients and staff.

The active matrix of the Artis Q.zen detector allows the signal to be amplified directly where it is generated at each pixel of the matrix. This on-pixel amplification enhances the signal to electronic noise ratio compared to amorphous silicon detectors significantly and for the first time enables imaging with very low radiation, down to only 6 nGy per pulse.

We call this new acquisition mode “ultra-low-dose imaging”.

The image guidance of EP catheters can now be done with ultra-low-dose imaging. This reduces radiation to the patient and personnel in the room, which is especially important for complex, longlasting procedures such as pulmonary vein isolations. The detector delivers clear image quality even when using other systems in the room, such as mapping systems, without additional shielding.

When treating babies and children, reducing radiation is of particular importance. Especially for complex interventional procedures in pediatric cardiology and radiology, ultra-low-dose imaging might help to reduce the radiation significantly. The ultra-fast readout technology of the new crystalline silicon detector allows for higher frame rates in 3D imaging, up to 99 f/s. In addition, the crystalline silicon detector provides more coverage compared to small cardiology detectors, allowing views of the entire heart.

Radiation Protection for Medical Staff in Angiography

In addition to protecting patients from excessive radiation exposure, physicians, technicians, and other medical staff should be protected from unnecessary (i.e. scattered) radiation as well. Scattered radiation does not come directly from the X-ray tube, but rather is scattered by the patient, table, or other devices within the path of the X-ray beam (Figure 1). Usually most of the scattered radiation is generated where the X-ray beam hits the patient.

Scattered radiation is roughly proportional to the dose area product (DAP). If the area of the irradiated field is reduced by half, scattered radiation is similarly reduced by 50%.

Radiation protection reduces medical staff exposure to scattered radiation by 99% – a highly effective method.

RaySafe i2 dosimeter system is indispensable in creating a successful radiation safety culture.
Once in place, both healthcare workers and management benefit from the radiation insight gained. Moreover, focus is returned to treating patients, versus worrying about unnecessary radiation exposure.

i2 Dosimeter
An active dosimeter that measures and records radiation every second. Data is transferred wirelessly to the i2 real-time display.
It is maintenance-free, easy to wear and can be personalized with different colors and names.

A comprehensive overview on dose issues and image quality can be found in the following article: Stephen Balter et al. ACCF/AHA/HRS/SCAI Clinical Competence Statement on Physician Safety and Image Quality in Fluoroscopically Guided Invasive Cardiovascular Procedures, Journal of the American College of Cardiology, Vol. 44, No. 11, 2004.