Computed Tomography (CT)

Introduction to Computed Tomography (CT)

Lesson Objectives

• Describe how CT images are obtained including basic physics, artifacts, and image processing techniques.
• Identify the different reconstructions used in CT (ie: coronal, sagittal, axial).
• Summarize when CT is the imaging modality of choice.
• Identify when the use of IV or oral contrast is indicated.
• Recognize and determine appropriate management of contrast reactions.

Basics of CT

CT uses x-rays to obtain images.

Much of the same principles that you learned about x-rays still apply.

• Ionizing radiation (x-ray photons) passes through patient.
• X-rays reach detector to form an image.

CT Scanner

• Unlike plain films, radiation source and multiple detectors rotate around the patient.
• Computer collects data as patient moves through the scanner.
• Density for each pixel can be calculated.

Axial Plane

• Data is typically obtained in the axial plane.
• Cross sectional "slice" through the patient.
• Viewed as if you are looking at the patient from the foot of the bed.

Sagittal plane

• Data already obtained in the axial plane can be reconstructed in the sagittal plane.
• Viewed as if you are looking at the patient from the side.

Coronal plane

• The coronal plane can also be reconstructed from the original axial data set.
• Slices viewed as if you are in front of the patient, looking at them standing upright.

3D imaging

• In addition to the 3 planes of imaging (axial, sagittal, and coronal), CT data can also be used to create 3D images.
• Helpful for visualization of vasculature and bony structures (complex fractures and surgical planning).

(Need pictures of 3D bone and 3D vessels*)

Density

• Dense material absorb x-rays and prevent them from reaching the detector.
• Air – does not absorb many x-rays – most reach detector – appears dark.
• Bone – absorbs more x-rays – few reach detector – appears light on image.
• In CT, we are able to measure the amount of x-rays that are transmitted (pass through) to the detector to get a value corresponding to the density of the material.
• Tissue density is measured in Hounsfield units (HU).

Hounsfield Units

Definition

• Water = 0 HU
• Air = -1000 HU
• Everything else = scaled based on density relative to air/water

Air < Fat < Fluid < Soft tissue < Bone < Metal

Fat

• Fat is less dense than water, negative HU
• Fat floats on water – easy way to remember this.

Soft Tissue and Blood

• Soft tissue is mostly intracellular water, but also contains additional substances (connective tissues) that raise the density slightly
• Positive HU, but not far from water (+25 to +60)

Lung

• Lung is a mixture of soft tissue and air – overall density of lung falls between these two values
• Approximately -500 HU

Bone

• Cortical bone – extremely dense – highly positive HU (+1000)
• Medullary bone – mix of dense bone and soft tissue (+300)

Windowing

• Focuses our gray scale to the tissues of interest – this is the "window width"
• Highlights subtle differences in tissues
• All densities below the specified values are black
• All densities above the specified values are white

Soft Tissue Window

• It is now very easy to tell the difference between fat (dark grey) and soft tissue (light grey)
• Notice that air (outside of patient) and lungs look exactly the same because they are outside of our specified window of HU values

Lung Window

• On the previous image we were unable to see the large pneumothorax
• Can now easily differentiate lung tissue from air in pneumothorax

Bone Window

• Can see differences in bony cortex and medulla
• Easier to detect fractures

Brain WIndow

• Window can be set narrowly enough that we can distinguish the very small differences in white matter and grey matter
• White matter is slightly less dense than grey matter due to presence of lipid-containing myelin

BRAIN IMAGE NEEDS TO BE REPLACED WITH ONE FROM PACS

CT Contrast

• CT contrast contains iodine, a relatively dense element that appears bright on CT.
• What actually "enhances"?
• Anything that receives blood flow, with brightness corresponding to amount of blood flow.
• Vessels
• Tumors – hypervascularity
• Inflammation/infection – increased blood flow and leaky capillaries.

Timing of contrast

• Arterial vs venous imaging is based on timing of image acquisition after contrast administration.
• Contrast is typically administered via a peripheral vein, flows to heart, then follows circulation throughout the rest of the body.
• Peripheral vein
• Right heart (RA/RV)
• Pulmonary arteries (image at this time point for PE study).
• Left heart (LA/LV)
• Aorta (image at this time point for CT angiogram).
• Peripheral arteries
• Capillaries
• Veins (image at this time point for evaluation of most abdominal organs).
• Back to heart

Renal excretion

• Each time contrast passes through the kidneys, some is removed via renal excretion, decreasing amount of contrast present in circulation over time.
• Contrast becomes concentrated in renal collecting system and bladder.
• Ureters and bladder fill on delayed images (image at this time point for CT urogram).

Contrast limitations

• Evaluation for intracranial hemorrhage – brightness from contrast will obscure bright blood, so we use a non-contrast head to rule out hemorrhage.
• Kidney stones – renal calculi are already dense on CT, so contrast is generally not needed/helpful.
• Renal impairment
• Iodinated contrast is cleared by the renal system.
• Generally avoid IV contrast if GFR < 30 mL/min due to higher risk of contrast induced nephropathy.
• Not an absolute contraindication – use clinical judgement to evaluate risks vs benefit.
• Hydration may reduce risk of AKI if low/borderline GFR.
• Oral hydration preferred.
• Normal saline if unable to drink fluids.
• Chronic dialysis – generally okay to give contrast as there is no residual renal function to preserve.
• No risk of nephropathy with oral contrast.