**3. Imaging techniques in the hybrid operating room**

The imaging capabilities of modern, fixed C-arms have dramatically changed in the last five years. Traditionally, fixed C-arms have been used either for simple 2D fluoroscopy or 3D rotational angiography. Nowadays, C-arms, which are able to acquire CT-like 3D images, are used for image-based guidance and even provide intra-operative functional imaging, like flow analysis.

### **3.1 Fluoroscopy**

Traditional fluoroscopy provides real-time, high resolution, low-contrast images in two dimensions through the use of an image intensifier. With ultrasound and endoscopy it is the main imaging modality to guide devices in real time through the body (see Fig. 2a). Brilliant image quality is needed to depict fine anatomic structures and devices. In particular, in cardiac interventions, imaging the moving heart requires a high frame rate (30f/s, 50Hz) and high power output (at least 80kW). Thus, the image quality needed for cardiac applications can only be achieved by high powered fixed angiography systems. In modern fluoroscopy devices image intensifiers have been replaced with digital flat panel detectors which enabled fluoroscopy to transition into three dimensions, producing CT-like images (see below). Fluoroscopy is performed with continuous X-ray to guard the progression of a catheter or other devices within the body in live images. To minimize the doses for the patient and the surgeon, dose saving measurements are essential in modern fluoroscopy devices (see section 3.4).

Fig. 2a. 2D fluoroscopic image

### **3.2 Data acquisition**

Angiographic systems provide a so-called *acquisition mode,* which stores the acquired images automatically on the system to be uploaded into an image archive later. While standard fluoroscopy is predominantly used to guide devices and to re-position the field of view, data acquisition is applied for reporting or diagnostic purposes. In particular, when contrast media is injected, a data acquisition is mandatory, because the stored sequences can be replayed as often as required without re-injection of contrast media. To achieve a sufficient image quality for diagnoses and reporting, the angiographic system uses up to 10 times

electrophysiologists, neuroradiologists, and pediatric cardiologists. Their specific needs

The imaging capabilities of modern, fixed C-arms have dramatically changed in the last five years. Traditionally, fixed C-arms have been used either for simple 2D fluoroscopy or 3D rotational angiography. Nowadays, C-arms, which are able to acquire CT-like 3D images, are used for image-based guidance and even provide intra-operative functional imaging,

Traditional fluoroscopy provides real-time, high resolution, low-contrast images in two dimensions through the use of an image intensifier. With ultrasound and endoscopy it is the main imaging modality to guide devices in real time through the body (see Fig. 2a). Brilliant image quality is needed to depict fine anatomic structures and devices. In particular, in cardiac interventions, imaging the moving heart requires a high frame rate (30f/s, 50Hz) and high power output (at least 80kW). Thus, the image quality needed for cardiac applications can only be achieved by high powered fixed angiography systems. In modern fluoroscopy devices image intensifiers have been replaced with digital flat panel detectors which enabled fluoroscopy to transition into three dimensions, producing CT-like images (see below). Fluoroscopy is performed with continuous X-ray to guard the progression of a catheter or other devices within the body in live images. To minimize the doses for the patient and the surgeon, dose saving

Angiographic systems provide a so-called *acquisition mode,* which stores the acquired images automatically on the system to be uploaded into an image archive later. While standard fluoroscopy is predominantly used to guide devices and to re-position the field of view, data acquisition is applied for reporting or diagnostic purposes. In particular, when contrast media is injected, a data acquisition is mandatory, because the stored sequences can be replayed as often as required without re-injection of contrast media. To achieve a sufficient image quality for diagnoses and reporting, the angiographic system uses up to 10 times

measurements are essential in modern fluoroscopy devices (see section 3.4).

have to be carefully considered and weighted when planning the hybrid theatre.

**3. Imaging techniques in the hybrid operating room** 

like flow analysis.

**3.1 Fluoroscopy** 

Fig. 2a. 2D fluoroscopic image

**3.2 Data acquisition** 

higher x-ray doses than standard fluoroscopy. Thus, data acquisition is not recommended as long as fluoroscopy is sufficient or the images do not need to be stored.

Data acquisition can be combined with specific imaging protocols, for example, to enhance blood vessels while removing background structures (see section 3.3) or to acquire 3D images (see section 3.5).

### **3.3 Digital subtraction angiography**

Over the past three decades, digital subtraction angiography (DSA) has become a wellestablished 2D imaging technique for the visualization of blood vessels in the human body (Katzen, 1995). With this technique, a sequence of 2D digital X-ray projection images is acquired to show the passage of an injected contrast agent through the vessels of interest. Background structures are largely removed by subtracting an image acquired prior to injection (usually called the mask image) from the live images (often referred to as contrast images). It is obvious that in the resulting subtraction images, background structures are completely removed only if these structures are exactly aligned and have equal grey-level distributions (see Fig. 2b). Therefore, various motion correction algorithms are applied to reduce such artifacts in the image.

Fig. 2b. 2D digital subtraction angiography shows the difference between an initial fluoroscopic acquisition and a fluoroscopic acquisition after injecting contrast agent. Thus, the vessels are clearly depicted in these images. Other remaining structures (white next to black structures) caused by motion, are considered artefacts, and can be partly compensated by modern angiography devices.

DSA is clinically used for diagnostic and therapeutic applications of vessel visualization throughout the entire body. During complex interventional procedures, DSA is often combined with so-called *road mapping*. In this mode, a DSA sequence is performed and the frame with maximum vessel opacification is identified, which becomes the road map mask. The road map mask is subtracted from subsequent live fluoroscopic images to produce realtime subtracted fluoroscopic images overlaid on a static image of the vasculature. Road mapping is useful for the placement of catheters and wires in complex and small vasculature, because fluoroscopy alone may not adequately show the vessels and may not visualize small wires in the distracting underlying tissue. It is also possible to combine the road mapping feature with a feature called image fade, which allows the user to manually adjust the brightness of the static vessel road-map overlay.

The Hybrid Operating Room 81

3. *Source-Image- Distance (SID)*: according to the quadratic law and a requested constant dose at the detector, a greater distance between the source and the imager increases the skin dose. Rising SID from 105 cm (=SID 1) to 120 cm (=SID 2) increases skin dose (i.e. the dose at the IRP) by approximately 30%, if C-arm angles, table position, patient, and requested dose at the detector do not change. Fig. 3 illustrates the setup including the

lower (SID = 105 cm) and the upper (SID = 120 cm) position of the detector.

reduction from 30 p/s to 7.5 p/s results in a dose saving of 75%.

Fig. 3. C-arm, two different SIDs, constant table height, location of the IRP

without degrading image quality and can result in a dose saving of up to 50%.

may result in a dose reduction of 20 to 120 mGy.

house dose reporting and analysis.

Additionally, modern angiographic systems provide some inherent features to reduce dose. For example, variable copper filters reduce the skin dose by filtering the low-energy photons out of the X-ray, called *beam hardening*. Some systems adjust the thickness of such filters 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 maintains low skin dose

Other measurements include radiation-free collimation or radiation-free object positioning. Using the last image hold (LIH) as a reference, the system allows radiation-free collimation and semitransparent filter parameter setting to precisely target the region of interest (see Fig. 4). A similar approach is implemented for optimal patient positioning for imaging: graphic display of the outline of the upcoming image allows translation of the table without fluoroscopic radiation exposure and provides an indication of which anatomy is in the fieldof-view of the detector. For specific cardiac interventions, such measurements can reduce the overall fluoroscopy time by 0.5 to 3 minutes. Under typical fluoroscopy conditions, this

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

angiographic systems can adjust the frame rate downward in various steps, from 60 pulses per second (p/s) used in pediatric cardiology to 0.5 p/s in some systems for slowly moving objects. A reduction to half pulse rate reduces dose by about half. The

### **3.4 Radiation dose and dose reduction**

Ionizing radiation may, depending on the dose, cause damage to organic tissue. The mechanisms by which radiation damages the human body are two-fold: (1) radiation directly destroys the DNA of the cells by ionizing atoms in its molecular structure and, (2) radiation creates free radicals, which are atoms, molecules, or ions with unpaired electrons. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions that eventually change or harm the DNA of the cells.

The human body can repair damaged cells to a certain extent, but if exposed to a high amount of radiation beyond a given threshold in a short period of time, "deterministic" damage will occur. Deterministic radiation damage includes changes of the blood count, hair loss, tissue necrosis or cataract. Exposure levels of typical medical diagnostic imaging procedures are far below the threshold for deterministic radiation damage. However, deterministic effects are an important consideration in external radiation therapy and radionuclide therapy.

In order to assess the risk of radiation exposure, quantitative measurements of dose were introduced:


Determining the effective dose in angiography depends on several factors, primarily on the variability in organ sensitivity to radiation. For instance, bone marrow is far more sensitive to radiation than the liver. The degree to which organs are affected by radiation also depends on the angle of the beams. Because dose distribution in angiography is not as "homogeneous" as it is for CT, these factors must be considered when estimating the damage caused by irradiation. 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.

Effective dose provides a good comparison with natural background radiation, which is on average about 2.4 mSv per year. Typically, during a cardiac diagnostic intervention with 15 p/s, the effective dose per minute is 0.6 mSv (Cusma et al., 1999).

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.

Because guidance of endovascular devices requires continuous X-ray, modern angiographic systems include several measures for dose reduction (Balter et al., 2010). There are three parameters which can be adapted by the user to reduce the radiation exposure:


Ionizing radiation may, depending on the dose, cause damage to organic tissue. The mechanisms by which radiation damages the human body are two-fold: (1) radiation directly destroys the DNA of the cells by ionizing atoms in its molecular structure and, (2) radiation creates free radicals, which are atoms, molecules, or ions with unpaired electrons. These unpaired electrons are usually highly reactive, so radicals are likely to take part in

The human body can repair damaged cells to a certain extent, but if exposed to a high amount of radiation beyond a given threshold in a short period of time, "deterministic" damage will occur. Deterministic radiation damage includes changes of the blood count, hair loss, tissue necrosis or cataract. Exposure levels of typical medical diagnostic imaging procedures are far below the threshold for deterministic radiation damage. However, deterministic effects are an important consideration in external radiation therapy and

In order to assess the risk of radiation exposure, quantitative measurements of dose were




Determining the effective dose in angiography depends on several factors, primarily on the variability in organ sensitivity to radiation. For instance, bone marrow is far more sensitive to radiation than the liver. The degree to which organs are affected by radiation also depends on the angle of the beams. Because dose distribution in angiography is not as "homogeneous" as it is for CT, these factors must be considered when estimating the damage caused by irradiation. 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

Effective dose provides a good comparison with natural background radiation, which is on average about 2.4 mSv per year. Typically, during a cardiac diagnostic intervention with 15

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

Because guidance of endovascular devices requires continuous X-ray, modern angiographic systems include several measures for dose reduction (Balter et al., 2010). There are three

1. *Footswitch on-time:* footswitch on-time controls how long the body is exposed to X-ray

2. *Frame rate:* 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, according to the clinical need. Modern

required to deposit 1 Joule (J) of energy in 1 kilogram of any kind of matter.

characteristic for the particular type of radiation. For X-ray, H = D.

multiplied by the respective tissue weighting-factors.

p/s, the effective dose per minute is 0.6 mSv (Cusma et al., 1999).

necessary to choose the "right" dose for the tissue being penetrated.

parameters which can be adapted by the user to reduce the radiation exposure:

beams, thus how long the body is irradiated: less time means less radiation.

the respective tissue weighting-factors.

chemical reactions that eventually change or harm the DNA of the cells.

**3.4 Radiation dose and dose reduction** 

radionuclide therapy.

introduced:

angiographic systems can adjust the frame rate downward in various steps, from 60 pulses per second (p/s) used in pediatric cardiology to 0.5 p/s in some systems for slowly moving objects. A reduction to half pulse rate reduces dose by about half. The reduction from 30 p/s to 7.5 p/s results in a dose saving of 75%.

3. *Source-Image- Distance (SID)*: according to the quadratic law and a requested constant dose at the detector, a greater distance between the source and the imager increases the skin dose. Rising SID from 105 cm (=SID 1) to 120 cm (=SID 2) increases skin dose (i.e. the dose at the IRP) by approximately 30%, if C-arm angles, table position, patient, and requested dose at the detector do not change. Fig. 3 illustrates the setup including the lower (SID = 105 cm) and the upper (SID = 120 cm) position of the detector.

Fig. 3. C-arm, two different SIDs, constant table height, location of the IRP

Additionally, modern angiographic systems provide some inherent features to reduce dose. For example, variable copper filters reduce the skin dose by filtering the low-energy photons out of the X-ray, called *beam hardening*. Some systems adjust the thickness of such filters 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 maintains low skin dose without degrading image quality and can result in a dose saving of up to 50%.

Other measurements include radiation-free collimation or radiation-free object positioning. Using the last image hold (LIH) as a reference, the system allows radiation-free collimation and semitransparent filter parameter setting to precisely target the region of interest (see Fig. 4). A similar approach is implemented for optimal patient positioning for imaging: graphic display of the outline of the upcoming image allows translation of the table without fluoroscopic radiation exposure and provides an indication of which anatomy is in the fieldof-view of the detector. For specific cardiac interventions, such measurements can reduce the overall fluoroscopy time by 0.5 to 3 minutes. Under typical fluoroscopy conditions, this may result in a dose reduction of 20 to 120 mGy.

More and more countries and authorities require the reporting of patient exposure to radiation following an intervention. To meet current and future regulations, modern angiographic systems allow effective reporting of dose exposure, thus enable enhanced inhouse dose reporting and analysis.

The Hybrid Operating Room 83

Fig. 5. Cardiac DynaCT image: the C-arm CT results were obtained with syngo DynaCT running on a syngo X-workplace (Siemens AG, Healthcare Sector, Forchheim, Germany)

Recent post-processing algorithms analyse an entire digital subtraction angiography (DSA) sequence at once and represent the sequence in one single colour-coded image. In order to obtain a colour-coded image, the algorithm takes the time to maximum opacification of each individual pixel, starting with the injection and subsequently visualising the distribution of the contrast medium through the vessels. These time measurements are then represented by a colour, allowing visualisation of the complete vessel tree in one image. Thus, the colours represent the contrast agent from its initial entry into the blood vessels to its flow

Such dynamic flow evaluations provide a greater understanding of the contrast flow within the pathology, greater ease in visualizing the success of a procedure, and they assist the

**3.6 Advanced visualization** 

throughout the anatomy of interest in one image.

Fig. 4. Radiation-free collimation: The collimator position is indicated on the last image hold (LIH) by a white frame.

### **3.5 3D DynaCT imaging**

Three-dimensional (3D) C-arm computed tomography (DynaCT) is a new and innovative imaging technique. It uses two-dimensional (2D) X-ray projections acquired with a flatpanel detector C-arm angiography system to generate CT-like images (Kalender & Kyriakou, 2007). The C-arm sweeps around the patient acquiring up to several hundred 2D views serving as input for 3D cone-beam reconstruction. Usually, a minimum angular scan range of 180 degrees, plus the so-called fan-angle, is required. For typical C-arm CT devices, this results in an angular scan range requirement of at least 200 degrees. Resulting voxel data sets can be visualized either as cross-sectional images or as 3D data sets using different volume rendering techniques.

Thanks to a detector optimized for high-resolution 2D fluoroscopic and radiographic imaging, the spatial resolution provided by DynaCT can be very high. For example, a common FD for large-plate C-arm systems, such as the 30 cm × 40 cm Pixium 4700 flat-panel detector (Trixell, Moirans, France) offers a native pixel pitch of 154 µm in a 2480 ×1920 matrix. Due to read-out bandwidth limitations, such detectors are operated in 2 × 2 binning mode during DynaCT, which means the smallest high contrast object that can be resolved has a size of about 0.2 mm (Strobel et al., 2009).

 Initially targeted at neuroendovascular imaging of contrast enhanced vascular structures, 3D C-arm imaging has been continuously improved over the years. It is now possible to acquire CT-like soft-tissue images directly in the hybrid OR (see Fig. 5). Beyond their use for trans-arterial catheter procedures, these 3D data sets are also valuable for guidance and optimization of percutaneous treatments. In combination with 2D fluoroscopic or radiographic imaging, information provided by DynaCT can be very valuable for therapy planning, guidance, and outcome control – in particular for complicated interventions (Doelken et al., 2008).

There are low-dose DynaCT protocols that achieve acceptable image quality for radiosensitive patients, such as pediatric patients, and provide 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 resulting, in a dose saving of 72%. Low-dose DynaCT can be achieved with an effective dose of 0.3 mSv.

Fig. 4. Radiation-free collimation: The collimator position is indicated on the last image hold

Three-dimensional (3D) C-arm computed tomography (DynaCT) is a new and innovative imaging technique. It uses two-dimensional (2D) X-ray projections acquired with a flatpanel detector C-arm angiography system to generate CT-like images (Kalender & Kyriakou, 2007). The C-arm sweeps around the patient acquiring up to several hundred 2D views serving as input for 3D cone-beam reconstruction. Usually, a minimum angular scan range of 180 degrees, plus the so-called fan-angle, is required. For typical C-arm CT devices, this results in an angular scan range requirement of at least 200 degrees. Resulting voxel data sets can be visualized either as cross-sectional images or as 3D data sets using different

Thanks to a detector optimized for high-resolution 2D fluoroscopic and radiographic imaging, the spatial resolution provided by DynaCT can be very high. For example, a common FD for large-plate C-arm systems, such as the 30 cm × 40 cm Pixium 4700 flat-panel detector (Trixell, Moirans, France) offers a native pixel pitch of 154 µm in a 2480 ×1920 matrix. Due to read-out bandwidth limitations, such detectors are operated in 2 × 2 binning mode during DynaCT, which means the smallest high contrast object that can be resolved

 Initially targeted at neuroendovascular imaging of contrast enhanced vascular structures, 3D C-arm imaging has been continuously improved over the years. It is now possible to acquire CT-like soft-tissue images directly in the hybrid OR (see Fig. 5). Beyond their use for trans-arterial catheter procedures, these 3D data sets are also valuable for guidance and optimization of percutaneous treatments. In combination with 2D fluoroscopic or radiographic imaging, information provided by DynaCT can be very valuable for therapy planning, guidance, and outcome control – in particular for complicated interventions

There are low-dose DynaCT protocols that achieve acceptable image quality for radiosensitive patients, such as pediatric patients, and provide 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 resulting, in a dose

saving of 72%. Low-dose DynaCT can be achieved with an effective dose of 0.3 mSv.

(LIH) by a white frame.

**3.5 3D DynaCT imaging** 

volume rendering techniques.

(Doelken et al., 2008).

has a size of about 0.2 mm (Strobel et al., 2009).

Fig. 5. Cardiac DynaCT image: the C-arm CT results were obtained with syngo DynaCT running on a syngo X-workplace (Siemens AG, Healthcare Sector, Forchheim, Germany)
