Equipment design for CV fluoroscopy has to take in the following considerations:
- Types of procedures
- Radiation dose to patients (protection)
- Patient skin entrance doses
- Screening times
- Radiation dose to staff (protection)
- Image quality
- Manoeuvrability of C arm and table for patient positioning
- Sterile environment
- Workstations and imaging acquisition and processing
Procedures
Cardiovascular fluoroscopy equipment design takes into consideration the procedures involved in angiography. Equipment is highly specialised for the imaging of arteries, veins and the heart, so has to take into consideration the low contrast of the structures, screening times and radiation doses and imaging quality.
The most common types of procedures performed in cardiovascular fluoroscopy include angiography and angioplasty, stent insertion, pacemaker insertion and repair of valves and atrial septal defects. (Bushong, 2004). The basic principals of CV fluoroscopy involve the use of catheters to gain arterial access, guidewires to allow the introduction and positioning of the catheter and injecting radiopaque contrast to use in conjunction with the fluoroscopy equipment, to show the imaging of vessels involved in the procedure. (Bushong, 2004).
Catheters must be introduced to the patient for imaging through a vessel. The Most common percutaneous technique used today is the Seldinger technique. (Bontrager & Lampignano, 2005). One of three vessels are considered for this technique; the femoral, the brachial or the axillary, the femoral being the most popular, (Bontrager & Lampignano, 2005).
The steps for this technique are as follows:
- Insertion of needle / cannula
- Placement of needle into lumen of vessel by removal of inner cannula
- Insertion of guide wire
- Removal of needle after positioning of guide wire
- Catheter inserted over the guide wire and forwarded to the area of interest
- Removal of guide wire once the catheter is in place
(Bontrager & Lampignano, 2005).
Percutaneous access is performed under sterile conditions. Contrast is introduced via the introduced catheter. Contrast media used is water soluble non-ionic, used for its low osmolality and lower incident of patient reaction. (Bontrager & Lampignano, 2005). Other types of advanced imaging equipment also use contrast medium for imaging. General fluoroscopy is enhanced by the use of water soluble contrast, as well as procedures that require contrast mediums such as barium. (Bontrager & Lampignano, 2005). Computerised tomography imaging is also supported by the use of contrast medium delivered by a timed pump in conjunction with the screening. (Bontrager & Lampignano, 2005).
Radiation Protection
Design of equipment in CV fluoroscopy has to consider minimising patient doses, as all types of fluoroscopy equipment and procedures involve much higher doses than conventional radiography. This is due to the requirement of penetration of contrast medium and the screening time needed to map and manipulate its journey. (Bontrager & Lampignano, 2005). Patient doses are measures in 2 ways; by skin entrance exposure, significant in CV fluoroscopy, and effective dose to the internal organs, with the risk of further genetic damage. (Bontrager & Lampignano, 2005). With CV fluoroscopy having the largest doses out of any other diagnostic procedures, there are no limits set to patient doses per procedure, (Bushong, 2004), as iInterventional procedures are viewed as too dependant on individual patient situations to be managed by reference levels. (Medicine on-line, 2006). The Ionising Radiation (Medical Exposure) Regulations 2000 document and the Medical and Dental Guidance Notes in conjunction with ALARP make it clear that not only should procedures be justified, with the risks outweighing the benefits, but that radiation doses should be kept ‘as low as reasonably practicable’. (IR(ME)R, 2000),( ALARP, 2009), (Ipe, 2002). Short term risk to patients mainly involves skin damage, or erythema. Long term risks include potential cancer, with significant increase of risk with interventional fluoroscopy, dependant on age and gender. (National Cancer Institute, 2005).
Risks to operators are also to be considered. Cases of skin changes and injury to the lens of the eyes are increasing, and although cancer is uncommon, this kind of exposure may induce leukaemia and breast cancer. (National Cancer Institute, 2005).
The following table from the National Cancer Institute outlines strategies to manage radiation dose in interventional fluoroscopy procedures.
Patient dose reduction features incorporated in interventional equipment include copper filtration, low dose selection, removable anti scatter grids, virtual collimation, automatic wedge filters, real time dose display and tantalum filters. (MHRA, 2005).
Equipment Components
The main components that make up cardiovascular equipment are as follows:
- the x ray tube
- C arm (single or biplane)
- Table
- Generator
- Image intensifier / flat panel detector
- Display system
- Operator controls
- Processing and storage facilities
Room size, layout and manoeuvrability need to also be taken into consideration within design, with space for emergency equipment, storage, sterile preparation, cleaning areas and large teams of staff. (Bontrager & Lampignano, 2005).
Tube
The tube has an anode angled at between 11 and 15 degrees, compared with 23 degrees in mammography. This supports a focal spot size of 0.4 to 0.8mm² to minimise geometric unsharpness required for small vessel magnification (Bushong, 2004), compared with the large amounts of soft tissue imaging in mammograms. The anode used is usually large in diameter, 15cm rather than the usual 10cm used in general fluoroscopy, rotating at 7500 rpm compared to 10,800 rpm in general fluoro and a static anode in dental radiography. Heat dissipation is crucial in CV fluoroscopy due to high mA, frame rates and multiple acquisition, which sets it apart from other equipment. (Bontrager & Lampignano, 2005). A higher heat capacity composition anode disk of and larger size accommodates the extra heat load, accompanied with rapid cooling. (Bushong, 2004). Anode composite of a mix of rhenium tungsten molybdenum can withstand temperatures of up to 3400°. (Bushong, 2004).
In pulsed fluoroscopy, X-rays are not delivered continuously but delivered in pulses in rapid succession. This reduces actual screening time, and radiation dose, In line with ALARP. The gaps between pulses are filled with the last stored digital image until a more current image is available. The short X-ray pulses also result in image definition being increased. Lower pulse rates, although lower in dose can produce jerky images. (GE Healthcare, 2009). Pulse rates on more recently designed equipment can be controlled from 10 pulses per second to 30 pulses per second. (K care, 2006). Pulsed fluoroscopy is suited to cardiovascular procedures, du to the rapidly moving structure of the heart. (Radiographics, 2000).
C-arm
Most c-arms on recent equipment are floor mounted and motorised, with image intensifier or flat panel detector above the table and the x-ray tube below to reduce scatter, (Bushong, 2004) with auto positioning . CV equipment can have either single plane or biplane c-arms. Single plane units are usually used for adult procedures, providing easy access to the patient. Biplane units are able to obtain 2 views, frontal and lateral at once, and are used for paediatric procedures, where a single injection of contrast can be viewed in two planes, reducing the amount of contrast injected. (Radiographics, 2000).
Table
The table is composed of carbon fibre and is radiolucent, for minimal attenuation. It comes with accessories such as arm rests and supports, and is designed to withstand maximum loads of 200kg. Most tables today are floating or moveable, with minimal or no need for a tilt capability, as does general fluoroscopy for barium studies. (Bontrager & Lampignano, 2005). The table can be timed in conjunction with injected contrast, to image the transport of the media through the vessels. (Bushong, 2004). Tables for cardiovascular work are generally narrower than those used for general fluoroscopy. (Kcare, 2006).
Pads for support should also be made of low attenuation material. Foam is a common material used, with lower attenuation than gel pads. (Radiographics, 2000).
Generator
A high voltage generator with three phase 12 pulse power capability of at least 100 kW, with low voltage ripple is required for CV fluoroscopy with an output of up to 125 kVp due to its high power requirements and pulsing. Compare this with mammography which only requires low kilo-voltage of between 20 to 40 kVp. (Bontrager & Lampignano, 2005).
The generator output must reach at least 100 kW, with an additional filter of 0.1 mm copper as well as an appropriate system of collimators for the radiation area. This should include an iris diaphragm, and rectangular and semi-transparent collimators. (GE Healthcare, 2009).
The generator will also control ABC (automatic brightness control) which keeps image brightness at a constant level, regardless of the area of interest, attenuation and thickness. Automatic adjustment of the kVp and mA regulates the amount of brightness seen on the display. (Radiographics, 2000).
Filters
Filters are of paramount importance in interventional equipment, due to the high dose of the procedures. Filters are made of copper rather than aluminium, for improved low energy x-ray filtering. (Radiographics, 2000). Equalization filters, or wedge filters are seen in interventional equipment but are not relevant in general fluoroscopy. They are partially radiolucent blades for further collimation, made from lead-rubber or lead-acrylic, with either straight or shaped edges. Wedge filters help in automatic brightness control, as glare is reduced from unattenuated radiation. (Radiographics, 2000).
Flat panel detector
Newer systems have digital flat panel detectors, with a detector size of around 18 to 20 cms. Digital acquisition with the flat detector does away with the video camera and digital conversion system used with the old image intensifier. (Bontrager & Lampignano, 2005). Flat panel detectors have a dynamic range 10 times greater than image intensifiers, better image quality and better temporal and spatial resolution. (Cowan, Davies, Sivananthan, 2008). Copper grids are incorporated within the detector, and can be removable or fixed. The fixed grids can be troublesome when imaging paediatrics, as doses to the patient are increased. Similarly, with a removable grid, it is easy to start imaging without it in place, causing unnecessary scatter. (Kcare, 2006).
Display System
Images during procedures are displayed on 2 to 3 ceiling mounted LCD television screens. One captures immediate imaging whilst the others hold the previous images. Images connect with the main work station where post processing takes place.
Operator controls
The units c-arm and table positioning, collimation, filters, magnification, manipulation of image and pps (pulses per second) rates can be controlled by the health professional from the console. C-arm and table positions can be stored here as well. (Kcare, 2006). Images can be flipped and rotated, have black and white inversion applied as well as adjust contrast and brightness.
Acquisition and review
Reviewing and storage of images is based at the systems workstation, comprising of 1 or more computers with system software, mouse and keyboard. Acquisition and storage is compatible with PACS, DICOM and RIS.
Conclusion
Cardiovascular and interventional fluoroscopy is increasing in popularity due to its capability of supporting non evasive procedures, which would have once been evasive surgical procedures. With infection control currently a priority for hospitals, this can only be an advantage in reducing infections and costs to the NHS. With constantly improving technology, and in line with ALARP and IR(ME)R, patient and operator doses are decreasing, without detriment to image quality.