CT

What is CTDI?

Computed tomography dose index (CTDI): radiation dose descriptor calculated with normalized beam widths for 14 contiguous sections or slices

When should contrast be used for CT examinations?

To help distinguish normal anatomy from pathology and to make various disease processes more visible

Most scanners used today are variations of what scanner

3rd generation

What is the size range of a CT Gantry unit?

50-85 cm

What is iterative reconstruction?

Allows for a reduction in dose to the patient while maintaining image quality and reducing noise, which improves spatial resolution and low contrast detectability

What is the advantage to these procedures being performed in CT vs. The operating room?

Although these are invasive, they offer shorter recovery periods, no exposure to anesthesia, and less risk of infection

What interventional procedures are commonly performed in CT?

Biopsies, abscess drainage, tumors, Cath placement, orthopedic device.

What are the most common lab test to be performed prior to contrast administration?

Creatine levels and glomerular filtration rate (GFR) are most common lab values used to determine renal function

____ measure the transmitted x-ray values, covert them into an electric signal, and relay the signal to the computer.

Detectors

2nd generation scanners

Fan shaped beam that was measured by approx. 30 detectors placed closely together in a detector array Tube and detector were translate/rotate; gantry rotated 10 degrees between each translation Improved image quality and decreased scan time to 20 seconds for a single slice CT exam still took long

What size catheter must be used in CT while using the pressure injector?

For a non-CTA exam typically uses a 22-gauge Angiocath with an injection rate of 2 mL/s. A CTA study requires an 18-20-gauge angiocath with a injection rate of 4-7 mL/s

What are the three routes of contrast administration in CT?

IV, oral, rectally

Why is it so exceptional in CT vs. Radiography?

In CT the contrast resolution is better than in radiography because of the scatter radiation rejection of the pre-patient and pre-detector collimators.

What is the cause for streak artifacts?

Metallic objects such as dental fillings, pacemakers, and artificial joints

What factors affect image quality in CT?

Spatial resolution, contrast resolution, temporal resolution, noise, and artifacts

What is the reason for a ring artifact?

The ring artifact is for misalignment of the IR with the tube,

1st generation scanners

Worked by a process known as translate/rotate Tube produced a finely collimated beam, and 1-3 tubes were placed opposite the tube for radiation detection The linear tube movement (translation) was followed by a rotation of 1 degree Scan time was 3-5 minutes which required the patient to hold still for a long time Limited to neurologic examination

Pixel

a single square or picture element within the matrix

Matrix

array of numbers arranged in a grid of rows and columns

Field of view

determines the amount of data to be displayed on the monitor

What is window level?

determines the midpoint of the range of gray levels to be displayed on the monitor

What is window width?

determines the number of gray levels to be displayed in the image controlling contrast resolution. (Narrow WW = fewer shades of gray Wide WW = more shades of gray in the image)

What generation of CT scanners are ring artifacts associated with?

mostly associated with 3rd generation

Voxel

the slice thickness gives the pixel an added dimension

What is DLP?

· Dose-length product (DLP): commonly reported dose descriptor on CT scanners. This is the CTDIvolmultiplied by the length of the scan (cm).

3rd generation scanners

· Rotate/rotate movement: x-ray tube and detector array rotate simultaneously around the patient · More detectors with a curved arrangement improved image quality · Scan decreased to 0.35-1 second per slice, decreased motion artifact

What factors does the CT technologist control as it relates to image acquisition?

· Slice thickness · Reconstruction algorithm · Method of acquisition

What is contrast resolution?

· The ability to distinguish anatomical structures of similar subject contrast. · It is so exceptional because it helps differentiate anatomy that have similar contrast such as liver - spleen and gray matter- white matter.