How does a CT work?
The Gantry, the large circular frame, holds the x-ray tube (generates x-rays) and detectors (captures x-rays)
Detectors are aligned directly opposite the x-ray tube
The x-ray tube and detectors move as one unit
CT scanners are often described as having a donut shape
The patient couch (bed) moves through the opening in the gantry (sometimes called the bore but is technically the gantry aperture), capturing images in each rotation of the gantry
Rotation times are between 0.25-3 seconds
The images are reconstructed by the computer unit
What are collimated x-rays?
Rays of light made accurately parallel, accurately aligned
X-ray beams are targeted in a specific area of interest in multiple angles and then thin cross-sectional slices are created by a computer
Does the x-ray tube and detectors rotate together around the patient during a scan?
Yes
Rotates 360 degrees
What is calculated by the computer?
The absorption of x-rays by the tissues (or the x-rays that pass through to the detectors)
Attenuation is summed from thousands of angles
The computer takes the raw data of the x-rays passing through the target and creates an image
Data is integrated by a digital computer to provide a cross-sectional image
Does the width of the x-ray beam determine the thickness of the slice?
Yes, for a single row of detectors
Thicker slice = higher noise in the image
What coloring is present in a CT?
Bone = white
Tissues = gray
Gas and liquid = black (all x-rays pass through)
Can data be reconstructed to form 3D images?
Yes
Is a CT scan made out of pixels with various brightness levels?
Yes
Vary based on x-ray absorption rates
The pixels themselves are organized into rectangular pieces called voxels
The 2D pixels correspond to an equal number of 3D voxels
The smaller the pixels, the better the image
Are there usually multiple rows of detectors rather than a single row?
Yes
This makes it so data can be collected for more than one slice at a time
Contributes to shorter scan times and reduced x-ray exposure
Do multidetector CT scanners acquire all raw data in the thinnest slice possible?
Yes
Combining raw data slices to create thicker slices (e.g., 1.0mm slices to create 2.0mm slice) improves the signal-to-noise ratio (improves clarity)
What is the signal to noise ratio and how does that contribute to clarity?
Signal- structure of interest
Noise- other structures, air, bone
Thin slices - crucial for small structures like the auditory-vestibular system
higher spatial resolution, clearer images (higher noise)
Thick slices - combining adjacent slices= more data; improved signal to noise ratio
Are high-resolution CT scans (HRCT) the gold standard for assessing the auditory-vestibular system?
Yes
Conductive, mixed and sensorineural pathology
Congenital malformation and structural anomalies
Soft tissue mass with bony erosion
Surgical planning
What are some characteristics of HRCT?
Enhanced image resolution - increased exposure to ionizing radiation
Narrow slice (0.67mm) - small structures in the temporal bone
The field of view is minimized to make each pixel smaller (think zooming in vs out on a camera)
Less artifact with narrow slices
Requires longer scan times (10-15 minutes)
What is helical/spiral CT?
X-ray tube moves continuously around the patient as the patient moves through the gantry
Advantage - very fast
Disadvantage - artifact
What is artifact?
A visual error or false appearance where the attenuation in Hounsfield Units do not correctly match the density of the anatomical structure being scanned
Shades of gray on the picture do not correctly match the true attenuation of the body part being scanned
Artifacts can obscure real pathology or mimic disease (potentially leading to misdiagnosis)
What causes artifact?
The scanning process itself
Movement of the object being scanned
Foreign body/metal
What are hounsfield units?
Standardized expression of CT attenuation measurements
Radiodensity has a wide range of attenuation
CT image values range between -1000 for air to approximately +2000 for dense bone and over +3000 for metal
How can the window of the hounsfield units affect an image?
Adjustment of the window provides contrast to an image
Different window options enhance the tissues targeted within that specified HU range
Setting the window optimizes a range
What can CT scans be useful in diagnosing?
Abnormalities of the auditory and vestibular systems (SSCD, EVA, cochlear abnormalities (mondini, common cavity))
Trauma - temporal bone fractures
Middle ear disease (mastoiditis, cholesteatoma, otosclerosis/tympanosclerosis)
Some neoplastic disease (tumors)
Pre-surgical planning and post-op complications
Are contrast agents commonly used?
Yes
To help delineate borders between tissues with similar radiodensity
Attenuate x-rays and therefore cause the area of concern to enhance
Iodine based compounds (small incidence of allergic reaction to new non-ionic isotonic contrast agents; reactions vary in severity)
Intravenous compounds are administered for vascular structures and highly vascularized organs, allowing visualization of a tumor or restriction of blood supply by a tumor.
IV contrast dye injection increases scan time by 10 to 15 minutes (total time patient is there)
When is non-contrast CT scanning used?
Default study for most ear and bone issues
Detect acute hemorrhage in the brain
Characterize bone fractures and other skeletal abnormalities
Best when suspected problem is structural, bony, or related to air/fluid spaces
Structural: Trauma, SSCD, otosclerosis
Conductive HL: Ossicular chain discontinuity, congenital malformations (SNHL too)
Masses and chronic infection: Cholesteatoma, OM/mastoiditis
When are IV contrast CT scans done?
Highlight blood vessels - assesses the vascular system and aneurysm (CTA) and vascularized tumors
Improves imaging of tumors, infection, Inflammation, and trauma in soft tissues
Best when suspected problem is a mass, tumor, or severe active infection with abscess
Mass/Tumor: Glomus/paraganglioma, Temporal bone cancer
Pulsatile Tinnitus: Arteriovenous Malformation (CTA)
Complicated/Severe infection: Malignant OE, abscess (pus filled pocket)
Can individuals with renal disease have contrast?
No
What are the best uses for CT?
Emergency - fast (usually 10 mins or less)
Low cost
Bony outline of the TB structures
High resolution - particularly important for temporal bone structures
