Computed Tomography 02 PDF

Computed Tomography 02 By Dr. Ammar Oglat Reference  Chapter #1 Contrast Agents in CT -To distinguish adjacent tissues on a CT image, the tissues must have different densities (attenuation). These varying densities will result in distinct attenuation coefficients, which p...

Computed Tomography 02 By Dr. Ammar Oglat Reference  Chapter #1 Contrast Agents in CT -To distinguish adjacent tissues on a CT image, the tissues must have different densities (attenuation). These varying densities will result in distinct attenuation coefficients, which produce an image that clearly displays the different tissues. -In some parts of the body, such as the chest, subject contrast is inherently high. Why? Bcz The pulmonary vessels and ribs have significantly different densities from the adjacent aerated lung, which allows easy identification on the image. Unfortunately, not all areas of the body possess this level of inherent tissue contrast. -Contrast agents fill a structure with a material that has a different density than that of the structure. When the agent is of a higher density than the structure it is referred to as a positive agent. When the agent is of a lower density than the surrounding structure it is referred to as a negative agent. -Methods of contrast administration vary widely, but in CT, enhancement falls into the two main categories of intravascular and gastrointestinal. Less commonly, contrast agents can be administered intrathecally (into the subarachnoid space surrounding the spinal cord) or intraarticularly (directly into a joint space).  Contrast Media (CM) Uses in CT Applications Contrast agents are often used in CT exams to enhance certain details of anatomy 1. Iodine based IV contrast (Omnipaque 300) Also known as Iohexol. 2. Oral/Rectal Barium sulfate contrast. Note: Before take CM the patient should sign consent form. Properties of Iodinated Agents 1. Iodinated agents are universally used for a variety of radiology examinations because they are water soluble, easy to administer intravascularly, and have a high safety index. 2. Because of their relatively high atomic number of 53, the iodine atoms in the contrast material are responsible for this increase in attenuation. How? Adding an iodinated agent to the bloodstream will temporarily increase the beam-attenuating ability of the blood; structures with an adequate blood supply show an increase in attenuation, which is displayed as a change from darker to lighter on the image. CM Factors: 1. Concentration: concentration of iodine in the solution. 2. Osmolality 3. Viscosity  Osmolality -Osmolality is a property of intravascular contrast media that refers to the number of particles in solution, per unit liquid, as compared with blood. -High-osmolality contrast media may have as much as seven times the osmolality of blood. -Low-osmolality contrast media has lower the osmolality of blood. -Contrast media that is iso-osmolar has the same osmolality as blood.  Viscosity -Viscosity is a physical property of intravascular contrast media. It can be described as the resistance of the fluid as it flows. -The temperature, and concentration of the contrast affect its viscosity. -The viscosity of the contrast material can be significantly decreased by heating the liquid to body temperature for injection. There are Several Factors Affecting Contrast Enhancement, such as: 1. Pharmacokinetic factors (Concentration, osmolality, and viscosity) 2. Patient factors (These include the patient’s age, sex, weight, height, cardiovascular status, renal function, and the presence of other disease.) 3. Equipment factors  Patient Preparation for CT Contrast Media 1. Laboratory Tests: Patients who are expected to receive a dose of IV contrast media should do the following laboratory tests to get information on the Kidneys’ function: Serum Creatinine (Cr) [Normal values 0.6-1.4 mg/ dL Blood Urea Nitrogen (BUN) [Normal values 7-25 mg/ dL 2. Fasting: Some CT exams require the patient to fast for 6 8 hours. Note: Before injecting contrast media ensure the patency of the line by using a small syringe to flush the line with 2 to 5 mL of saline.  Main Phases of Tissue Enhancement during CM Injection A. Bolus phase B. Nonequilibrium phase C. Equilibrium phase The difference among phases is usually determined by: 1. The injection rate 2. The scan delay time: time between contrast medium injection and scanning.  Use the Arteriovenous Iodine Difference (AVID) to compare between these phases. A. HU of the Aorta Vs. HU of the IVC B. Higher HU indicates higher attenuation which indicates an increased iodine concentration  1.The Bolus Phase The bolus phase is that which A. immediately follows an IV bolus injection. B. It is characterized by an attenuation difference (AVID) of 30 or more Hounsfield units between the aorta (AO) and the inferior vena cava (IVC). (Fig. 13-6). C. In the bolus phase of contrast enhancement, the arterial structures are filled with contrast medium and brightly displayed on the image. Hence, this phase is also commonly called the arterial phase. D. Contrast media has not yet filled the venous structures. Note: CT angiography images are taken while contrast is in the bolus phase.  2. Nonequilibrium Phase The second phase is the nonequilibrium phase. It follows the bolus phase and is characterized by A. (AVID) a difference of 10 to 30 HU AVID (Fig. 13-7). B. The contrast agent is still much brighter in the arteries than in the parenchyma of organs, but now the venous structures are also opacified. Hence, it is also called the venous phase. C. This phase begins approximately 1 minute after the start of the bolus injection and lasts only a short time, approximately 1 minute. Note: This window can be manipulated to some degree by varying conditions such as the volume and flow rate of the injected contrast medium. Note: Most routine (nonangiographic) body images are acquired while contrast is in the nonequilibrium phase.  3. Equilibrium Phase The last phase of tissue enhancement after the IV injection of contrast media is known as the equilibrium (or delayed) phase. A. It can begin as early as 2 minutes after the bolus phase or after a drip infusion. B. In this phase contrast media is largely emptied from the arteries, is greatly diluted in the veins. C. In this phase, intravascular structures and interstitial concentrations of contrast material equilibrate and decline at an equal rate. D. It is characterized by an attenuation difference (AVID) between the aorta and the inferior vena cava of less than 10 HU (Fig. 13-8). Note: The equilibrium phase is the worst phase for acquiring scans of the body, particularly the liver. Note: Historically, CT scan acquisition was much slower, making it impossible to obtain all scans through the liver before contrast media reached equilibrium. In these situations liver lesions were often difficult, or impossible, to detect with only the contrast- enhanced study. For this reason patients were often first scanned without contrast enhancement through the liver, then the entire abdomen was scanned after the injection of contrast media. New (Modern) scanners are much faster and all scans can easily be obtained before the equilibrium phase. As a result, pre-contrast scans are now seldom needed for routine abdomen studies. The exact timing of the start and end of each of the three phases are affected by many factors, including injection parameters and the condition of the patient, particularly the patient’s cardiac output. Typical contrast arrival times for various organ systems are shown in Table 13-2 and Figure 13-10. (Route of IV Contrast Media) 1. Drip Infusion. In the drip infusion technique, an IV line is initiated and contrast medium is allowed to drip in during a period of several minutes.. Scanning begins after most, or all, of the contrast agent is administered (roughly 2 to 3 minutes).. Because this method relies on gravity the actual flow rate delivered is quite variable and affected by many factors (e.g., bottle height, contrast volume, tubing length, IV catheter size, contrast media viscosity). Disadvantages of The drip infusion method 1. This method is not recommended for scans of the neck, chest, abdomen, or pelvis because all of the scans acquired with this technique are taken in the non- equilibrium phase. 2. The drip infusion method cannot produce peak enhancement of sufficient magnitude for CT angiography. 2. Bolus Techniques The bolus technique of contrast enhancement is: uses scanning after a rapid injection of contrast material.. A volume of contrast of 50 to 200 mL is injected at a rate (or combination of rates) between 1 and 6 mL/s; scanning begins after a short delay.. The interval between the initiation of the injection and the start of scanning (the scan delay) is critical.. The contrast bolus can be delivered by hand (using syringes) or by a mechanical injection system. Extravasation is the leakage of fluid from a vein into the surrounding tissue during IV administration. Effect of extravasations: 1. Most extravasations involve small volumes and are not clinically significant. Slight swelling and erythema may develop and usually subside without complication. 2. However, severe tissue necrosis and ulceration may occur. Infants, young children, and unconscious patients are at higher risk of contrast media extravasation Following these guidelines will substantially reduce the risk of contrast extravasation: 1. Use an indwelling catheter set with a flexible plastic cannula; 18 to 20 gauge is preferred. 2. The use of metal needles should be avoided. 3. Monitor the injection site, preferably a medially directed antecubital vein, during the initial moments of injection. Swelling at the site of injection indicates extravasation, and the injection should be stopped immediately. 4. Warm the contrast medium to body temperature. Prewarming of the contrast material can help to reduce the viscosity so that it can flow more easily through indwelling IV catheters. 5. Use LOCM.  Bolus shaping Bolus shaping is a manipulating in the flow rate during an injection which can improve the likelihood of scanning during optimal vascular enhancement and change the characteristics of the time-density curves.