Digital Subtraction Angiography: The Essentials of Image Quality

What is Angiography? The display of blood vessels Arteries Veins Performed with digital imaging techniques    By the end of this course, you will be able to: Reduce motion Position patients properly Use collimation and filtration Select the correct measuring field Set the optimum injector flow rate and pressure Why is CM necessary? Blood is not visible in x-ray images Dynamic blood flow processes can only be displayed if contrast medium (CM) is injected only visible in magnetic resonance imaging (MRI) scans without injections Recent internal bleeding is readily detectable in computed tomography (CT) scans Normal blood flow is not readily detectable in CT scans             Without CM With CM What is CM? A substance which attenuates x-rays either more or less than normal tissue does. Which CM are used for angiography? Normal contrast media (positive contrast) containing iodine Gaseous contrast media (negative contrast) carbon dioxide can be used under certain conditions          Why iodine? Iodine is a chemical element which has a high atomic number, i.e. a high density. Iodine causes a stronger attenuation (absorption) of x-rays than blood Iodine is discharged through the kidneys Why carbon dioxide? CO2 has a very low density. CO2 attenuates the x-rays only slightly. CO2 is a "natural" substance for the body. Native imaging Displays not only the desired vascular structures, but also the bones, ribs and organs to the extent that they attenuate x-rays Subtraction images Only the vessels are displayed The procedure is based on photographic subtraction, first described in 1934 by the Dutch radiologist Dr. Ziedses des Plantes X-ray film exposure without CM Inverse copy of exposure without CM = mask (precontrast) image X-ray film exposure with CM = filled (CM-filled) image The mask and filled images are exactly superimposed A copy of the two superimposed films is then prepared = subtraction image Digital exposures are acquired After two seconds, the system will regulate and select a mask CM is injected with a two-second time delay so that at least one of the exposures contains no CM The images are immediately subtracted by the processor The subtraction scene is displayed on a TV monitor (on-line) in real time MASK IMAGE - FILLING IMAGE = SUBTRACTED IMAGE     Mask Image                                   Fill Image Subtracted image   Advantages Lower amount of CM required Minimal overlapping of vessels Display of even the finest vascular structures High detail recognition Disadvantages Increased preparation necessary Immobilization of patient Immobilization of part Breathing instructions Motion Collimation Filtration Patient Positioning Selection of Measuring Field Injector settings Why is reduced motion important? The system creates a mask that is a reverse copy of the native image The combination of these two images creates the subtracted image If the patient moves after the mask is created, the mask and the fill image will not line up Post-processing, such as pixel-shifting and moving or replacing the mask, will be necessary to realign these two images Image results due to patient motion   Pixel shifting improved the image     Patient Motion       Pixel Shifted    Tips to help reduce motion artifacts Patient immobilization Tape Sponges Special head holders Instruct the patient to: Hold his or her breath Hold still     Tape Positioning Sponges Special Head Holder Collimate to the area of interest Lack of proper collimation can result in burnout Scatter radiation may result in reduced image quality   Burnout due to density difference in the image Result: decreased image quality Used to even out the density in an image Used to make the image more homogeneous Placement is critical Position filters over areas of least density air space between legs lung fields in a coronary study Improper placement may result in decreased image quality     Graphical representation of semi-transparent filter blades     Vascular/Angio systems Have two outer filters Can be positioned from each side of the image Finger filter Can be positioned anywhere on the image Cardiac systems Have a single filter Can be rotated and is used to even out the density difference between the diaphragm and the lungs Good positioning for center filter The two outer filters should have been used also Center the region of interest (ROI) Make sure the part is in the center of the image Immobilize patient Use tape to secure the head Use tape or gauze to secure the legs and feet Direct the patient to hold still General Remove dental prosthesis and jewelry Remove hair clips Positioning Supine Use tape to immobilize the head Make sure no ECG and other cables are in the field of view Instructions to patient No breathing No swallowing Patient must be supine with head in a neutral position Use tape to secure patient's head Angulation Straight PA ( 0°) Lateral view ( 90°) Collimation Collimate to the head Filtration Semitransparent filters for density compensation in the neck area Proper positioning of the common carotid artery Carotid arteries branching off the aortic arch Region of interest is centered in the image   Patient's head is positioned too high No semi-transparent compensation material used to even out the density caused by the air Angulation C-arm 0° = Carotid Frontal C-arm 30°to 45° RAO/LAO = Carotid oblique Collimation Collimate to the neck Filtration Density compensation using semi-transparent filter If necessary, position finger transversely between the chin and the aortic arch, above the cervical spine  Patient instruction Don‘t talk, swallow, or move Carotid artery is not centered Semi-transparent compensation is not used to even out the density caused by air Used to overcome density differences for lower peripheral vascular procedures Helps immobilize the patient as well as even the density of the legs and the airspace between Improves image quality   Suitable compensating material for lower peripheral studies   Secure feet with tape Make sure legs are close together Knees together, toes pointed inward Remove unnecessary clothing Supine position Use peri-bolus kit between legs Use fixation for feet         . Angulation I.I. overtable position (pa) Maximum source to image distance (SID) Allows table to be raised as high as possible Table as high as possible Decreases the geometric magnification of the part to ensure that all anatomy is included in the image Especially important for bilateral leg runoffs Collimation Collimate to ROI Use the optimal dominant Filtration Use semi-transparent filters or finger filter for density compensation            Top images: Cables, wires, and tubings are visible.  Bottom images: Density compensation materials would have evened out the density between the legs.  Legs should be positioned closer together. Collimation or filtration should be utilized.  Decreased image quality due to excessive direct radiation. General Remove unnecessary clothing Positioning Supine Immobilize the forearm Tape the hand Patient instruction Instruct the patient to remain still Angulation C-arm 0° Collimation Collimate to ROI Filtration Use semitransparent filters Immobilizing the patient improves image quality   Selecting the measuring field The injection Timing of the injection Set the right pressure Pressure and flow rate Select the proper measuring field (dominant) for the best image quality For peripheral work or cervical carotid work, use the center rectangular dominant For head work, use the round center dominant             Larger vessels require higher flow rates Arterial flow rates will be higher than venous flow rates Selective vessels require smaller flow rates Timing of the Injection Remember it is critical to allow the system time to make the mask prior to injecting The system may take up to two seconds to regulate Always set your injector to a two-second x-ray delay to allow for this dose regulation and mask selection Pressure set at the injector represents the upper limit of what the injector will use The injector may or may not use the pressure set in order to achieve the set flow rate The set flow rate may not be achieved if the pressure is reached before the desired flow rate Remember to check the status on the injector! Consider the patient’s medical condition Pressure and Flow Rate Pressure was set too low on the injector for the catheter and vessel Desired flow rate and volume were not reached before pressure limit was reached Proper positioning for the bi-plane carotid artery injection The region of interest, the carotid artery is centered Now that you have completed this course, you should be able to: Reduce patient motion Position patients properly Use collimation and filtration Select the correct measuring field Set the optimum injector flow rate and pressure