1. Introduction to C-arm technology
Published: August 2013
AOTrauma topic list:
Basic principles (Imaging); Image-guided surgery (Techniques)
Key words: C-arm imaging, radiation exposure
Related AOTrauma lectures:
‘Intraoperative use of the C-arm in orthopaedic trauma care’
‘Radiation hazard in orthopaedic trauma care’
Other useful reference:
Bott OJ, Wagner M, Duwenkamp C et al (2009) Improving education on C-arm operation and radiation protection with a computer-based training and simulation system. Int J Cars; 4:
Florian Gebhard, DE
AOT Basic Principles Course

2. Learning outcomes Identify all C-arm components and related equipment
Describe configuration and constraints of C-arm
Describe capabilities and clinical application of C-arm for pre-, intra-, and postoperative imaging
Explain optimal positioning of patient, healthcare staff, and equipment, while minimizing radiation exposure
It is important to know how a C-arm works and how it can be used and positioned optimally to acquire appropriate images, with the least possible radiation exposure to the patient and staff.
The C-arm can be used to acquire high-quality images to assist during many pre-, intra-, and postoperative procedures. The images can provide useful information about preoperative planning (if performed in the OR), intraoperatively how the reduction and fixation can be guided, and postoperatively, the quality of fixation.

3. Outline What is a C-arm? Image acquisition: collimation
Image intensifier
C-arm movement
Use of C-arm: pre-, intra-, and postoperatively
C-arm attitude:
Protective clothing
Technical contributions to radiation dose reduction
How much radiation is safe?
A C-arm consists of an x-ray source, an image intensifier, and a display screen. The C-arm itself is mobile in all planes. Understanding the capabilities and limitations of each element allows optimal visualization during clinical procedures.
Optimal positioning of the patient, C-arm, and operating room staff, results in the minimum irradiation dose for all, while achieving high-quality imaging. C-arm 'attitude' and operator knowledge is the key to successful use of this technology.

4. Image courtesy of Siemens
What is a C-arm?
Semi-circular ‘C’-shaped arm
X-ray source fixed to one end
Image intensifier fixed to other end
Display screen shows live image feed
The C-arm is an imaging device named because of its semi-circular ‘C’-shaped arm.
An x-ray tube is fixed to one end and an image intensifier to the other.
The x-ray tube produces x-rays using high-voltage electricity in a vacuum (40kV- 110kV).
The image intensifier amplifies low-intensity x-rays, thereby reducing the amount of radiation used. Images are projected in real time on to a display screen.
Image intensification is the process whereby a reduced amount of radiation is used to produce the image.
The C-arm was introduced in the 1960s, and design changes since then have resulted in clearer images, less irradiation, and smaller and more maneuverable machines.
C-arms may be fixed or mobile. Only mobile units are covered in this lecture.
Image intensifier requires reduced amount of radiation to produce image
Image courtesy of Siemens 4

5. Image acquisition: collimation
X-rays pass out of vacuum tube through a window sealed onto vacuum envelope of x-ray tube
Size of window can be controlled (collimation)
The smaller the window, the sharper the x-ray and the smaller the dose of radiation
The collimator is a device that narrows the beam in order to avoid unnecessary radiation dose and to increase the image quality.
The smaller the size of the focal spot, the sharper the image will be. 5

6. Image intensifier
X-rays absorbed by image intensifier, and thereby fluoresce
Image intensifier allows low-intensity x-rays to be amplified
Magnifies intensity produced in output image
Result: less radiation emitted
The image intensifier component of the C-arm consists of a vacuum tube that increases the intensity of light, and an imaging plate. Elements on this plate emit photons and fluoresce when exposed to radiation. The system magnifies the image produced allowing lower doses of radiation to be used. 6

7. C-arm movement Mobile C-arms limited in degree of movement Horizontal
Swivel
Vertical
Although C-arms are capable of considerable movement in all three planes, they cannot move completely freely. The most common problem occurs when the C-arm is used in the lateral position. The image intensifier part of the C-arm cannot rotate beyond the horizontal position.
Rotation 7

8. C-arm orbital rotation
Orbital rotation around AP and lateral views ~132°
C-arm cannot rotate more than the horizontal position
X-ray source on far side of body part being imaged
Orbital rotation
Image intensifier
X-ray tube
The C-arm can rotate around the AP and lateral views, but most cannot rotate more than the horizontal position.
Therefore the x-ray beam cannot be made parallel with the axis of a hole in intramedullary nail merely by rotating the C-arm This therefore requires another solution for intramedullary nailing.
The x-ray source should be placed on the far side of the body part being imaged, and on the far side of the surgeon.
The lower limb normally lies in slight external rotation so the table must be tilted to obtain the correct view. 8

9. Image acquisition using the C-arm
High-quality images
Beam of x-ray travels perpendicular to limb/bone
Image intensifier as close to patient as possible
The quality of the image acquired is dependent on the relationship of the beam direction and the target to be visualized. For high-quality images, the beam of the x-ray should travel perpendicular to the bone, with the image intensifier as close to the patient as possible.
The x-ray source-to-patient distance must be at least 38 cm to minimize radiation hazard.

10. Surgical application of C-arm
Before surgical incision
Eg, confirm and grade degree of ligament damage
Intraoperatively
Eg, guide reduction and fixation
Postoperatively
Eg, check fixation

11. Surgical C-arm application: workflow example
Conventionally, the workflow is as follows:
Conventional x-rays are taken to show the injury.
A CT scan is carried out in complex injuries.
This information is then used to plan the surgery.
At the end of the operation, C-arm images are taken. If these show a problem the fixation is revised before wound closure.
If the C-arm image indicates that the fixation has been accurately achieved, the patient is returned to the ward. Subsequently x-rays and CT scans are taken. If these show a problem that was not apparent on the intraoperative C-arm images, the surgeon must decide whether to operate again or leave the fixation as is. Revision surgery at this time carries an increased risk of complications, eg, sepsis, thromboembolic disease, and anesthetic complications. There is therefore a tendency for surgeons to accept the suboptimal result in order to avoid potential complications.
There is also the problem of explaining to the patient why he/she needs a second operation and why the problem was not detected at the time of the first surgery.
Note to lecturer: this slide builds and requires one click to reveal the second part of the flowchart.

12. Preoperative application: traction films
Films taken in the emergency department are often imperfect and high-quality films may be impossible to obtain without causing the patient too much pain. Preoperative films taken under anesthesia in the OR provide valuable, accurate information. The C-arm is an excellent tool for obtaining this information.
Bone overlap prevents planning
Traction films facilitate planning 12

13. Preoperative application: screening for ligament instability in ski thumb
Instability is mainly assessed by clinical examination, however, pain in the acute phase may make this difficult. C-arm screening can be used preoperatively to confirm and to grade the degree of ligament damage.
This image shows a stress view confirming complete rupture of the ulnar collateral ligament of the metacarpophalangeal joint (ski thumb).
Stress view of complete rupture of ulnar collateral ligament of metacarpophalangeal joint of thumb (ski thumb) 13

14. Intraoperative application: dynamic hip screw (DHS)
This clinical example of a DHS (dynamic hip screw) shows correct positioning, using the laser to mark the body, and using pulsed acquisition for each important step of the procedure.
Pulse acquisition results in 12 seconds of x-ray exposure in the AP and axial position.
Note to lecturer: the slide on the right-hand side is layered, click through to view the four images.

15. Intraoperative application: distal radial pinning
Distal radial fractures are the most common indication for the intraoperative use of the C-arm. Reduction and fixation can be guided by C-arm images. The wrist should not be placed directly on the C-arm even if it is draped, as it is difficult to maintain sterility. The wrist should be placed on a radiolucent arm board.
Do not place wrist directly on x-ray source during surgery
Intraarticular distal radius fracture treated with multiple K-wires 15

16. Postoperative application: assessment of reduction/fixation
At the end of a surgical procedure, the C-arm can be used to screen the quality of the reduction and fixation.
Examples shown are:
Schatzker II tibial plateau fracture fixed with a subchondral raft of lag screws and a buttress plate, and
a femoral shaft fracture where the correct rotational reduction has not been obtained. Note the mismatch in cortical thickness.
Assessment of femoral nailing (showing malrotation)
Assessment of Schatzker II tibial plateau fracture fixation 16

17. Radiation: protective clothing
Gloves
60–64% protection at 52–58 KV
Eye protection
0.15 mm lead-equivalent goggles provide 70% attenuation of radiographic beam
Thyroid collar 2.5-fold decrease in scattered radiation
It is very important to use protective clothing when operating the C-arm, to protect the hands, eyes, thyroid, and body from exposure to radiation.
The hands have the greatest exposure risk (during reduction and checking reduction). Radiation-protective gloves give a 60-64% decrease in exposure with 52–58 KV.
The eyes are the most sensitive area of the body to radiation, and the first determinant of workload (radiation cataracts). Goggles with 0.15 mm lead-equivalent attenuate radiographic beams by 70%.
A thyroid collar decreases the scattered radiation a further 2.5-fold. One study has shown that 85% of papillary carcinomas are radiation-induced. (Source: Devalla KL, Guha A, Devadoss VG (2004) The need to protect the thyroid gland during image intensifier use in orthopaedic procedures. Acta Orthop Belg; 70 (5): )
An apron in the AP position decreases scattered radiation by 16-fold and in the lateral position by 4-fold.
Leaded apron AP: 16-fold decrease in scattered radiation
Lateral: 4-fold decrease in scattered radiation

18. C-arm 'attitude' and technical contributions to radiation dose reduction
Position x-ray tube under and close to patient
Mark C-arm on floor and beam position on patient
Use integrated lasers on x-ray tube and image intensifier for positioning
Use pulse acquisition, avoid screening
Collimate when possible
Virtual patient anatomy selection: correct dose for specified body area
Select dose rate in line with patient size
Maintain distance from patient
Protection from radiation exposure requires a change in attitude by surgeons and ORP.
Screening produces much more radiation than single, pulsed views
Reserve screening for dynamic studies, ie, testing joint stability with stress views
Insertion of guide wire in a DHS or proximal femoral nail can be safely carried out with multiple single shots rather than continuous screening
Single shots repeated as required give as much information as screening.
Maintain distance from the patient to minimize scattered radiation.

19. Reducing exposure to radiation
Minimize duration of exposure
Keep beam time to minimum
Trial screen after positioning patient
Rely on stored images when possible
Absorbed doses of radiation are related to duration of exposure, distance from source, and shielding, as well as quantity of dose.
Reducing radiation exposure is achieved by reducing radiation time. Screening time is often controlled by the surgeon rather than the radiographer. 19

20. Basic C-arm positioning
Place x-ray tube and image intensifier to minimize scatter
Patient as close to image intensifier and as far from x-ray tube as possible
Not all x-rays pass through the object on which they are focussed. Some are reflected or refracted as they penetrate through an object, resulting in scatter. This scatter is potentially hazardous to surgical staff.
Maximum scatter reflects from the side of the patient that is closest to the x-ray source.
Therefore, the x-ray beam should be directed in such a way that the scatter is directed towards the floor. In practice, this means placing the x-ray beam under the patient.
As the amount of scatter produced increases with the size of the area irradiated, it is desirable to restrict the field size to the area requiring imaging.
The image intensifier screen should be kept as close as possible to the patient (as is practicable). This reduces scatter, improves image quality, and reduces radiation dose for the patient. In distal locking of IM nails the surgeon must be able to position the drill and drill bit between the image intensifier and the patient, and in this situation scatter is inevitable. 20

21. X-ray tube position Staff exposed to increased radiation
It is important to know the effect of the x-ray tube position. The cornea is the most vulnerable part of the body to radiation exposure (radiation cataracts).
With a standard x-ray exposure, doubling the distance between the surgeon and the patient from 0.5 m to 1 m reduces exposure considerably.
If the x-ray tube is positioned above the patient at a distance of 1 m (left), the surgeon’s/ORP’s eyes receive a dose of 2.2 milisieverts per hour (mSv/h).
If the C-arm is turned and the x-ray tube is below the patient (right), the surgeon’s/ORP’s eyes will be exposed to just 55% of the scattered radiation, so 1.2mSv/h. Therefore, the x-ray tube position is of paramount importance.
Staff exposed to increased radiation
Staff exposed to reduced radiation

22. Absorption and scatter
For every 1000 photons reaching patient
~ 20 reach image detector
~ 100–200 scattered
remainder are absorbed by patient (radiation dose)
Scattered dose is higher at x-ray tube side
image intensifier
x-ray tube
The main source of radiation to the staff in the OR is scatter reflecting from the patient. Most of this is reflected back towards the x-ray source.
Therefore in the lateral position, staff should be positioned behind the image intensifier side and NOT on the side of the x-ray source. 22

23. Relative patient entrance dose mSv/h
Factors affecting patient doses
Intensifier diameter
Relative patient entrance dose mSv/h
12’ (32 cm)
Dose 100
9” (22 cm)
Dose 150
6” (16 cm)
Dose 200
The smaller the diameter of the image intensifier the greater the entrance dose of radiation. Laser aiming devices on modern image intensifiers enable accurate targeting and allow for smaller image intensifier diameters to be used.
4.5” (11 cm)
Dose 300
The smaller the image intensifier diameter, the greater the patient entrance dose

24. Example of dose-rate around the C-arm
For staff, the further from the patient the lower the dose of scattered radiation
This illustration shows the dose rate of scattered radiation around a C-arm.
The further away the surgeon/ORP is/are from the x-ray tube, image intensifier, and the patient, the lower the dose of scattered radiation.
The 'inverse square law' states that the dose is reduced by the power of 2 of the distance to the x-ray source, ie, when the distance between source and surgeon/ORP is doubled, the dose of radiation is reduced by a quarter. Distance from the source is the best radiation protection.
When the C-arm is in the lateral position, image intensification should occur for the shortest possible time, as scattered radiation is higher in this position than when the C-arm is in the AP position. The emission of scattered radiation when the C-arm is in the lateral position is at its maximum diagonally and laterally in the direction of the x-ray tube.

25. How much radiation is safe?
20 mSv per year, average over defined periods of 5 years
How do you know how much radiation you have received?
Radiation dosimeter
(monitor)
The maximum doses of radiation to which staff should be exposed is defined as 20mSv per year based on a period of five years. Radiographers, radiologists and staff working in the operating room should all wear personal monitors which should be checked on a regular basis.
The best policy is the ALARA principle - As Low As Reasonably Achievable. 25

26. Take-home messages
Main components of C-arm: x-ray source, image intensifier, image display screen
C-arm used to improve outcomes during pre-, intra, and postoperative procedures
To reduce radiation exposure:
Position image intensifier close to patient
Surgeon and staff wear protective clothing and stand at safe distance from C-arm
Pulse acquisition used as often as possible (avoid screening unless necessary)