Rather than provide a power point illustration of the application CT diagnostics of a specific disease, this month we chose to discuss the basic principals of how our Helical CT at CCVS obtains it's spectacular images very quickly.
PRINCIPLES OF CT IMAGING
CT imaging offers superior diagnostic possibilities over radiography because of two primary advantages: tomographic nature and increased contrast resolution. As opposed to radiographs that represent flat projections of tridimensional (3D) structures, tissues are examined with CT in thin slices thereby eliminating superimposition. Organs and other structures can then be identified more easily and differentiated. Moreover, CT volume datasets can be reformatted in any imaging plane, or as 3D projections, allowing better representations of structural anatomic relationships.

Each CT image represents a thin section, or slice, of the body, composed of a matrix filled with small cubical sections, known as voxels, which is displayed on a flat monitor as a matrix of pixels. (Fig. 1). For CT imaging, each voxel of the matrix is associated with an electronic current that is spatially localized in body and processed by a specialized computer system. Variations in electronic current intensity are expressed as variable pixel brightness on the monitor. Thus, a voxel associated with greater tissue signal intensity on an x-ray attenuation on CT will be displayed as a brighter, or whiter, pixel, whereas lack of signal or x-ray attenuation such as with air, will appear dark. CT images are acquired along the axis of the gantry. Multiplanar reformat is the process of building new images along any plane that facilitates interpretation (Fig. 2).

A CT system mainly comprises a scanning unit, i.e. the gantry, with a rotating x-ray emitting tube and a detector system, a patient table, and a console equipped with a sophisticated computer that allows adjusting acquisition parameters and image reconstru ction (Fig. 3). The rotating x-ray tube is powerful enough to operate for long periods of time without overheating, thus permitting acquisition of large volumes of data. With helical CT scanning and particularly with multidetector units, an entire body region, or whole-body, can be imaged without interruption and at great speed. Helical scanning is possible because of concurrent table advancement into the gantry as the tube rotates around the patient, which then traces a helical path around the body and produces a data volume. Helical scanning reduces motion artifacts and facilitates angiographic procedures.
CT images are composed of pixels that represent the mean x-ray attenuation values attributed to the corresponding voxels. These values are expressed as Hounsfield units (HU), which range from approximately -1000 to +3095 HU, for a total of 4,096 shades of gray. This range of values cannot be resolved by the human eye (<90). To visualize and appreciate all tissues composed of variable HU, the window width (W) and level (L) of the image gray scale must be adjusted, according to the median and range of HU composing the area of interest. The DICOM viewing software allows adjustment of the extent (max-min) of gray shades (W) displayed, i.e. the image contrast, and the HU at the center of the window (L) (Fig.4). W/L must be adapted to the tissues evaluated and usually requires several adjustments during an examination for all tissues to be evaluated completely.

Interpretation of CT images necessitates an understanding of the range of HU found in different types of normal and abnormal tissues. While soft tissues share the same opacity on radiographs, mild differences can be detected with CT, thanks to its superior contrast resolution! (Fig. 5)

The use of iodinated contrast medium has become standard for several CT procedures in veterinary medicine. After intravenous bolus injection, the distribution of this hyperattenuating substance can be tracked throughout the body, providing information on the perfusion of tissues and integrity of natural barriers, such as the blood-brain barrier (BBB). Typically, for non- angiographic procedures, post- contrast scans are performed a few minutes following bolus injection, to allow contrast medium to pass from its original intravascular space into the extravascular space. Hence, the HU of a tissue will increase in proportion to the concentration of the contrast medium, the volume of its blood space and the permeability of its capillary bed. For instance, cerebral meningiomas are more easily detected with contrast medium (Fig. 6). Angiography represents another important application of CT in veterinary medicine. Such procedure requires helical scanning and is optimized with multi-row detector configurations and with the use of automated contrast medium injectors. Scans are performed rapidly after the injection of a contrast medium bolus, and timed according to the vascular phase examined and its enhancement peak. Precise acquisition timing is required to isolate a group of vessels, which is important when, for example, looking for portosystemic shunts (PSS) (Fig. 7). Principle applications of CT include detection of PSS and ectopic ureters, whole-body imaging for tumor staging, and evaluation of diseases affecting the head (including nose and middle ears), joint and bone, thorax, and abdomen.
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Tech Tip
This abstract of a paper presented at the ACVS Symposium Technician Program - Small Animal in 2011 is very helpful in understanding the cardiac effects of the commonly used anesthetic agents we use today.
CARDIAC EFFECTS OF ANESTHETIC AGENTS
Katy W. Waddell, RVT, VTS (ECC, Anesthesia) Texas A&M University
Autonomic nervous system controls involuntary functions i.e. regulation of the heart, blood vessels, smooth muscles and many glands. It is composed of two parts - sympathetic and parasympathetic.
Sympathetic activity is controlled by mediation or release and uptake of the hormones epinephrine, norepinephrine and related adrenergic hormones. (Fight or flight) The sympathetic nervous system often called the adrenergic nervous system. Direct-acting adrenergic receptor agonists (sympathomimetic agents) interact with and activate adrenergic receptors: Alpha adrenergic receptors- α1 stimulation causes the blood vessels to constrict Beta (β) adrenergic receptors- β2 stimulation causes the blood vessels to dilate Dopamine receptors
Anticholinergic agents - Atropine/glycopyrrolate: will cause an increase in heart rate, contractility, cardiac output and myocardial oxygen consumption. Often there will be no change in blood pressure and a decrease in right atrial pressure Atropine: Duration of action is 1-1 1/2 hours. Parasympatholytic agent, increases heart rate decreases salivary secretions, increases gastric pH. Glycopyrrolate: same mechanism as atropine, less elevation of heart rate compared to atropine effects last longer - up to 2 - 3 hours with a peak effect in 30-45 minutes given subq or IM.
Injectable analgesic and anesthetic agents: Alpha 2 agonists
Used in veterinary medicine to produce: sedation, analgesia and anxiolysis.
The addition of alpha 2 agonists may reduce requirements of inhalant anesthetics when used as a premedicant. Dexmedatomadine - Alpha 2 agonist- Produces profound sedation and analgesia but has significant cardiovascular effects. These include: vasoconstriction, bradycardia, decreased cardiac output. Reversal: Antisedan
Thiopental - Barbiturate
Reduction in blood pressure - peripheral vasodilation is the main action. Compensatory rise in heart rate - barorecptor response. Commonly associated with ventricular arrhythmias - bigeminy rhythm not uncommon Contraindications: Patient with known cardiac disease - particularly those with existing arrhythmias. Trauma patients - think of traumatic myocarditis!
Benzodiazepines
Midazolam and diazepam: Cause little or no direct myocardial depressant effects. May see increase in heart rate due to excitation with inadequate use of adjunctive agent, i.e. mu opioid.
Midazolam: Water soluble and is more useful for IM injections. When combined with an opioid, it will provide neuroleptanalgesia. NOT reliable as tranquilizers for dogs and cats when used as a sole agent!! Patients may lose inhibitions and become excitable. When combined with an opioid as a CRI can be utilitized to decrease MAC of inhalant agent. No analgesia provided.
Effects may be reversed with Flumazenil.
Hypnotics- Etomidate: no direct myocardial depression. Safe to use with cardiac, critical and septic patients. Cardiovascular stability may be better due to maintained barorecptor mediated responses. Will cause depression of adrenal function for 3-6 hours. Expense. Allows rapid induction/recovery. Non-cumulative. Propofol- Does cause direct myocardial depression as well as decrease in systemic vascular resistance. Decrease in contractility leads to increase in heart rate - will be transient - lasting several minutes. Profound bradycardia has been noted. Use cautiously in patients with heart disease and hypovolemia.
Mu opioids- Fentanyl is a pure mu agonist causes dose dependant bradycardia (increase in vagal tone). Bradycardia is responsive to anticholinergics - atropine/glycopyrrolate. Single dose IV is very short acting - up to 20 minute duration. Hydromorphone: morphine-like agonist, primary activity at the mu receptors. Cardiovascular effects: bradycardia due to central vagal stimulation, alpha-adrenergic depression causing peripheral vasodilaiton, decreased peripheral resistance and barorecptor inhibition. Oxymorphone- Similar effects to hydromorphone. Case management of side effects the same. Morphine- No direct myocardial effect. Dose dependent bradycardia - responsive to anticholinergics
Mixed agonist/antagonist agents= Buprenorphine: a partial mu agonist/antagonist. Slow onset of action, duration of 6-8 hours. Cardiovascular depression and respiratory depression not as profound as pure mu agonists. Butorphanol: partial agonist/antagonist. Similar to buprenorphine in cardiovascular/respiratory effects. Faster onset of action, shorter duration that buprenorphine. Recommended in multiple texts for premedication for cardiac patients due to sparing effects.
Dissociative Agents - Will indirectly stimulate the cardiovascular system by increasing sympathetic tone which may cause an increase in heart rate, cardiac output, mean arterial pressure, pulmonary arterial pressure and central venous pressure. Increase in rate causes an increase in myocardial work and oxygen demand/consumption. Ketamine- Does not produce a true anesthetic state - dissociation from the environment with analgesia and sensory loss. Heart rate and arterial pressure increase due to an increase in sympathetic tone (CNS derived). Peripheral vascular resistance is unchanged. Prior administration of benzodiazepine, acepromazine and/or inhalant agents may decrease or prevent cardiovascular effects. Telazol = Tiletamine and Zolazepam (benzodiazepine) - Clinical effects similar to ketamine. Prolonged or rough recovery when used as a sole agent.
Inhalant anesthetics. High cardiac output can delay anesthetic induction - blood flow through the lungs maintains the diffusion gradient between the alveoli and blood, i.e. - slower induction in excited patients vs. more rapid induction in decreased output patients - shock, hypovolemia, etc.
ALL inhalants cause dose dependant depression on the cardiovascular system. Some agents may sensitize the myocardium to catecholamine-induced arrhythmias.
MAC (minimal alveolar concentration) produces immobility in 50% of patients receiving noxious stimuli. Varies with agents and species. The lower the MAC, the higher potency of the anesthetic agent. Halothane- Acts to reduce cardiac output through direct depression of the myocardium. Heart rate changes may be minimal. Arterial blood pressure is decreased due to decreased output. Sensitizes the myocardium to catecholamine increasing the chances for arrhythmias. Isoflurane- Lesser degree of myocardial depression than halothane. Heart rate is seen to increase slightly. Decrease in arterial blood pressure - main cause is decreased vascular resistance vs. decreased cardiac output. Sevoflurane- Causes mild myocardial depression (decreased contractility). Mild systemic vascular resistance and arterial blood pressure depression. Less likely to see an increase in heart rate, vasodilation when compared to isoflurane.
Local anesthetics
Lidocaine w/o epinephrine - Acts directly on the heart to reduce conduction velocity and myocardial contractility. Low plasma concentrations - beneficial. Higher dosages decreased cardiac output, vasodilation and hypotension. CRI at 25-30 mcg/kg/min for analgesic adjunct CRI at 50-100 mcg/kg/min for ventricular arrhythmia control.
Bupivicaine More cardio toxic than lidocaine. Not used in CRIs for analgesia or arrhythmia control. Useful for local or regional blocks.
References:
- Lumb and Jones' Veterinary Anesthesia, 3rd edition, Lippincott, Williams & Wilkins, 1996
- Small Animal Anesthesia and Analgesia, G.L. Carroll, editor, Blackwell Publishing 2008
- Textbook of Medical Physiology, A. C. Guyton editor, 8th edition, W. B. Saunders, 1991
- Manual of Canine and Feline Cardiology, Tilley, Smith, Jr., Oyama, Sleeper, editors, 4th edition, W. B. Saunders, 2001
- Small Animal Clinical Pharmacology, Madison, Page, Church, editors, 2nd edition, W. B.Saunders, 2008
- Anesthesia workshop, Hanson, Waddell, proceedings, IVECCS, 2007
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