Comprehensive CT and Angiography Imaging: Components, Artifacts, and Technical Factors

The prefix tomo means

to cut or section

Why do CT tubes seem to have a much shorter life span than the General x-ray tube?

they are used for multiple sequential exposures (heat and appt. times); can accumulate 10,000 exposures in a single month

Describe in detail what the gantry is and what components are housed inside the gantry.

The gantry is the movable frame that houses the CT tubes and detectors for the entire machine allowing it to rotate around the couch/table. It allows for less dose and angulation of patients.

What is translation

one sweep across the patient

What is a projection

what is formed from the sweep or an intensity profile (helical movement of the tube)

Why is the size of the aperture of the gantry and the table weight limit of significance to patient safety in CT?

it has a weight limit of 400-450 lbs and if that is exceeded, the table can break and hurt the patient; table would fail

What was the only body part that could be imaged in 1st generation CT scanners? Why was water important to this process?

The head. It eliminated air.

Why was the bowtie filter so integral to the 2nd generation of CT scanners?

It enhanced contrast resolution by reducing dose pre-patient and cleaned up scatter pre-detectors; reduced scatter as a result of adding the fan beam (think of it like a grid in XR)

The third generation of CT scanners came with multiple advancements in scan time. What was the major disadvantage?

Ring artifact

What are ring artifacts?

An artifact (3rd generation machines) that produces multiple rings within anatomy as a result of a single or a bank of detectors malfunctioning

List and describe the 3 factors that contribute to detector dose efficiency

Capture efficiency, absorption efficiency, and conversion efficiency

Capture Efficiency

how well the detectors receive primary photons from the patient; controlled by detector size and the distance between detectors

Absorption efficiency

How well detectors convert incoming x ray photons

Conversion efficiency

how well detector converts the absorbed photons to a digital signal for the computer

Describe the two collimation systems present in CT

Pre-patient usually consists of several sections, so a nearly parallel x-ray beam results (reduces dose). Pre-detector restricts the X-ray beam viewed by the detector array (reduces scatter)

What are Hounsfield units?

a relative comparison of XR attenuation of a voxel of tissue to an equal volume of water

What Hounsfield unit is assigned to water?

0

What Hounsfield unit is assigned to contrast?

+3000

What Hounsfield unit is assigned to air?

-1000

What Hounsfield unit is assigned to compact bone?

+150 to +1000

What is raw data?

The remnant radiation that is converted into electrical signal values; XR signal

What is CT's claim to fame?

Exceptional contrast resolution

Matrix

array of numbers arranged in a grid of rows and columns

Pixel

a single square or picture element within the matrix

Voxel

the basic unit of a three-dimensional digital representation of an image or object.

What is the reason that CT tubes have a ceramic backing as part of their design?

Dissipation of heat by decreasing tube weight

What is the primary difference in design between CT and X- Ray tubes?

Size

How long are CT tubes expected to last?

10 years with many scanners in multiple rooms, if only one room and high volumes, the tubes may only last up to 1 year.

CT tubes are ___________ the size of an XR tube

quadruple

What purpose do detectors serve in CT?

respond to signal (create attenuated image) without lag

What two detectors are currently used in CT?

Solid State current for all helical & Xenon Ionization for inexpensive CT Scanners (scintillator CsI or gas too)

What is multiplanar reconstruction?

the ability to reconstruct axial images into coronal, sagittal, or oblique body planes

What are the most common applications for CT examinations?

Head (1), abdomen (2), chest (3), and pelvis

Which CT exam should NOT be with and without contrast? Why?

abdomen; pt. does is too high

Why are chest exams the 3rd most common in CT

lung screenings

What is the advantage of CT angiography in comparison to conventional angio?

Image reconstruction without the use of more patient exposure to radiation or IV contrast; does not require arterial puncture, requiring only minimal post-procedure observation

What are three possible routes of contrast administration in CT?

IV, oral, and rectal

List 3 important factors that the technologist must obtain from a patient prior to injection of radiographic contrast agents.

Allergies, informed consent, and a time-out

What does GFR stand for?

Glomerular filtration rate.

What is the purpose of performing this prior to administration of contrast agents?

to make sure the patient doesn't go into renal failure.

Should I ever administer contrast without performing a GFR test?

No, we should always perform this test to avoid the patient going into renal failure.

What type of angiographic studies require a thicker contrast examination?

visceral angiography (35%)

Explain the advantages of bi-plane imaging

quicker procedure (can do AP and lateral images simultaneously... swift diagnosis), reduces contrast usage (good for kidney health), and has increased detail

Explain the disadvantage of bi-plane imaging

requires a lot of magnification, increasing patient dose

What post-processing is of great importance, in particular to digital radiography?

multiplanar reconstruction

What is the importance of interpolation and extrapolation?

For data near and away from source detectors

What can be used for the treatment of aneurysms

biplane imaging and endovascular coiling (the wire coils up inside to stop the blood flow and seals it off from the artery)

What is the quality factors selected in CT by the Radiologic Technologist?

Section thickness, Focal spot size, Display FOV, Technical Factors, Pitch, and Reconstruction. Quality factors related to dose and image quality, Factors that affect resolution, and noise considerations.

What is the #1 quality factor picked by CT techs

field of view

Where are the image technique factors located for CT

protocol book

What is the purpose of an array processor?

Array processors and computers receive and process the large amount of raw data received from the patient and construct the images that the operator sees on the operator's console. (raw data into high contrast images)

What type of contrast agents are more viscous and produce greater resistance in the catheter during injection?

Non-ionic agents

Why is it important to use a large gauge needle (18, 20 ) when administering contrast with a pressure injector?

To prevent loss of the catheter and a larger amount of pressure. It also prevents infiltration and extravasation.

What are two advantages of using a pressure injector?

A specific amount of contrast can be used. You can set specific times and a gradual increase in sequence to inject. Can also keep track of how much contrast is being used (helps prevent kidney failure)

What is a disadvantage to using a pressure injector

too much pressure = needle dislodged from vein, causing extravasation

What is French

outer diameter of catheter

What is Gauge

outer diameter of the needle

What is selective injection?

Only one selected vessel is injected.

What group of patients is it important to inject into a permanent catheter system or keep extreme observation directly on when injecting contrast with a pressure injector?

cancer patients, pediatric patients

Describe the syndrome that is synonymous with extravasations. What are possible results of this syndrome?

Compartment syndrome. Dissection, necrosis, amputations

What is the Seldinger Technique?

A six-step approach is designed to maintain consistent access to the vessels in angiography procedures.

Summarize the Seldinger Technique

An 18-gauge needle with a stylet is inserted in the femoral artery to get blood return, then the stylet is pulled out. A guidewire is then inserted into the catheter and then advanced. Once the catheter is in, remove the guidewire. A six-step approach is designed to maintain consistent access to the vessels in angiography procedures.

Why is digital post processing a vital component of diagnosis in angiography?

It optimizes the image appearance for better detecting pathology and amplification of image contrast.

Modern CT scanners are evolved from the ____ generation CT scanner

4th

What are the two ways to measure CT dose

CTDI / DLP

Iterative Reconstruction and Dose Modulation have decreased dose by as much as

50 %

If a patient if over the table weight limit, what other modality can they go to

sonography (ultrasound)

True/False: Iterative reconstruction is a program that aids in patient dose reduction

true

True/False: CT has exceptional Contrast Resolution

true

One sweep of the CT tube inside of the gantry is known as

translation

Processing of projections from tomographic sections from the CT scanner is known as

reconstruction

The program that changes the amount of radiation as the tube moves across the body in order to reduce patient dose is

modulation

True/False: Although not the most common route of administration, rectal contrast is still used in computed tomography

true

What is CT's claim to fame? What is the reason for this?

HIGH contrast without assisting substances

Which would be an acceptable needle gauge selection for use with a pressure injector?

18 or 20

True/False: In first-generation CT scanners, it took 14 minutes for a single projection to be visible on the monitor.

false

The computer program that is used that can reduce patient dose in CT between 30-50% is

iterative reconstruction

When a detector or bank of detectors malfunction, it is associated with a ________ artifact.

ring

What generation was known for the ring artifact?

3rd

True/False: In the absence of a clear protocol, when a decision needs to be made regarding patient dose, it is the CT technologist who is responsible

true

What generation of CT scanner was synonymous with only being able to scan the head?

1st

Instruments for diagnosis and intervention of the coronary arteries can be introduced through several access points, including the

radial, brachial, and, most commonly in IR, femoral arteries

The femoral artery is often favored for its

size, ease of insertion, and least tortuous path to the heart

Steps of Seldinger

(1) after shaving and sterilizing, infiltrate the lateral groin with local anesthesia, (2) locate the femoral artery insertion site, (3) insert Seldinger needle at a 30-45 degree angle, (4) insert guide wire through needle, (5) remove Seldinger needle, (6) insert catheter sheath with dilator along guide wire, (7) remove guide wire and dilator together

What is contrast resolution?

the ability to distinguish between small differences in tissue density within an image

Why is contrast resolution so exceptional in CT vs. Radiography?

CT can differentiate tissues with very similar densities, largely due to its ability to minimize scatter radiation through pre-patient and pre-detector collimation and its digital image processing capabilities

What interventional procedures are commonly performed in CT?

abscess drainage, tissue biopsy, cyst aspiration, radiofrequency ablation, and cryoablation of tumors.

What is the advantage of interventional procedures being performed in CT vs. the operating room?

CT-guided interventions are typically less invasive, require shorter recovery times, often avoid general anesthesia, and carry a lower risk of infection.

1st Generation Scanners

Used a translate/rotate motion with a pencil beam and 1-3 detectors. Scan times were very long (3-5 minutes per slice), limiting their use mainly to neurologic (head) imaging.

2nd Generation Scanners

Used a fan-shaped beam and about 30 detectors while still using translate/rotate motion. Image quality improved, and scan times decreased to about 20 seconds per slice.

3rd Generation Scanners

Introduced rotate/rotate motion, with the x-ray tube and detector array rotating together around the patient. With more than 750 detectors and scan times as short as 0.35-1 second per slice, image quality improved significantly while reducing motion artifacts and overall exam time.

____________ measure the transmitted X-ray values, convert them into an electric signal, and relay the signal to the computer

detectors

Describe FOV

The area of anatomy displayed on the monitor; can be adjusted to include the entire body section or a specific part of the patient's anatomy being scanned

Window width controls

the overall gray level and affects image contrast

Window level (center) controls

subtle gray images within a certain width range and ultimately affects the brightness and overall density of an image

__________ is the movement of the equipment, while a __________ is the data gathered from that position

translation; projection

Most CT gantries measure about

28 inches

To accommodate larger patients and for interventional applications, a ______ aperture is available

34-inch

Why is it important never to exceed table (couch) weight limits of CT units (answer = for image formation AND patient safety)

can cause inaccurate indexing, damage to the table motor, and even breakage of the tabletop, which could cause serious injury to the patient

When should contrast be used for CT examinations?

Contrast should be used during CT examinations when it is necessary to distinguish normal anatomy from pathology and to make disease processes more visible. Contrast enhancement improves tissue differentiation and helps radiologists better identify abnormalities.