WK 1 Advanced Imaging and Contrast

Anatomical slices/sections

  • Sagittal (median/mid-sagittal, parasagittal): cuts into the right and left side

  • Coronal/frontal: separates front and back

  • Transverse/Axial/Horizontal: separates upper and lower part

  • Oblique/angled: between other planes

Viewing and orientation

  • Axial is looking from feet to head, anterior is superior in the image and posterior is inferior

  • Coronal: looking at patient face to face and anterior side is separated so you’re looking at posterior side

  • Sagittal: L to R or R to L. Left to right = seeing right side as left is cut away

Brain and skull anatomical viewing and orientation

  • Axial skull: feet from the top of the head - cutting away body and looking at the head from underneath

  • Skull is narrower and more pointed in the front, rounded at the back

  • patient lying their back so L and R is opposite from you

  • Sagittal is left or right side cut away so you’re looking at the other side

  • Coronal is the patient looking at us but the face is separated and we’re looking behind

Contrast

  • Definition: obvious difference between two or more things - light and dark areas on an image. Difference in densities

  • High contrast = few shades of grey between black and white - easy to differentiate

  • Low contrast = more shades of grey - able to see more detail

  • Good contrast = can see what is needed

Resolution and visual acuity

  • Resolution: measured of sharpness of an image with which a device can produce or record an image based on the number of pixels - how sharp an image can be demonstrated with pixels (high end vs low end MRI)

  • Visual acuity: measure of the ability of the eye to distinguish shapes and the details of objects even at a given distance

Contrast types

Innate/natural:

  • Tissues (density/atomic mass of tissues - more compact organ = higher density)

  • Gases (eg. gas in stomach)

  • Liquids: high in density

Administered (introduced)

  • Contrast media - adding something to increase/decrease the density

  • Chemicals (heavy metals) - iodine, barium and gadolinium which all increase density

  • Gasses introduced - decreases density

Positive contrast adds density/colour - increases signal

Negative density reduces density and colour - decreases signal

  • Contrast media administered when tissues have similar densities and look similar in imaging - fat and water

Contrast media

Routes and terms

  • Orally to see GIT lumen

  • Installation - rectally placed or into ducts/cavities

  • Injected into blood - intravenous (IV) or intraarterial (IA)

  • Injected - intrathecal (spinal canal), interosseous (into bone)

  • Retrograde (against flow - rectal enema) vs anterograde (with flow - barium swallow)

 

  • Contrast enhanced image

  • Barium and gas added

  • Lumen can be seen well

  • Double contrast as barium can close up the tube so gas is added afterwards to mimic the natural shape of the large intestines

Contrast media types

Barium based: x-ray, CT, fluro, angio, venography

  • Into the GIT

  • Not water soluble - cannot be excreted through kidneys so it needs to be expelled through faeces

  • barium must be used in a tube only

Iodine based: into GIT or into blood stream (IV/IA)

  • Water soluble - cleared through kidneys and urinary tract - also faeces

  • Health of kidney (eGFR) needs to be determined beforehand - if not healthy or diseased it can be cleared through faeces

Barium based contrast media

  • used to visualise GIT

  • Orally ingested for barium swallow (pharynx, oesophagus and stomach) and barium follow through (duodenum, jejunum and ileum) or rectally instilled for barium enema (large intestines, rectum and anus - find tumours and distension

  • Commonly added with gas to inflate the bowels - called double contrast

  • Contraindications: suspected perforation in GIT, vomiting, swallowing issues, toxic megacolon, pregnancy, ulcerative colitis

Iodinated contrast media

Categorised as;

  • non ionic (molecules stay together) or ionic (molecules can break up and become ions)

  • Ionic has higher rates of complications and HO but used for GI and lower rectal system when barium cannot be used

  • Non-ionic has lower rates of complications and LO, commonly used for IV, IA, intrathecal

  • High osmolarity (increases concentration by drawing in water) or low osmolarity (doesn’t draw in water or increase concentration)

Reactions to iodinated contrast

  • Very mild: nausea

  • Mid-range to severe: renal impairment/failure

  • Severe: anaphylactoid, angioedema, bronchospasm (airway constricting) and cardiac arrest (heart attack)

  • most severe reactions occur 20-30min after administration and in IV injections

  • Hives are common and itchiness

Anaphylactoid vs anaphylaxis

Anaphylactoid reactions:

  • not IgE related so the immune system doesn’t activate it

  • can be mild, moderate or severe

  • occurs with first time exposure

  • more likely to have a second reaction upon re-exposure - this is anaphylaxis

  • treated with adrenaline

Anaphylactic reactions

  • immune system initiates response (IgE)

  • requires first exposure - priming

  • likely to have follow up reactions with increasing severity after re-exposure

  • treated with adrenaline

  1. Injected idoniated contrast media causes water to enter the vessel due to increased concentration

  2. Water moving into the lumen from the vessel wall cells causes then to become stressed and release histamine

  3. Mast cells sitting in the tissues release large amounts of histamine

  4. Histamine causes vasodilation in the blood vessels which causes it to not function properly and reduce its volume

  5. This causes the patient to undergo a decrease in blood pressure and oedema

  6. Swelling (oedema) of the bronchial tree can restrict breathing - most severe anaphylactoid reaction

  7. when adrenaline is injected it causes the blood vessels to restrict and reverses all the processes