Computed Tomography Midterms

temporal resolution

A third type of resolution called __________ refers to how rapidly data are required. From the word tempo meaning time.

low contrast detectability.

Ability to distinguish an object that is nearly the same density as its background is referred to as:

Helical Scanners

Allows continuous rotation of the x-ray tube while the patient table moves inside the gantry and produce different types of images

Drip Infusion

An injection technique where an IV line is initiated and contrast medium is allowed to drip in during a period of several minutes

Cone beam effect

Artifact where lines appear in a windmill formation

EDGE GRADIENT EFFECT

Artifact which results in streak artifact or shading (both light and dark) arising from irregularly shaped objects that have a pronounced difference in density from surrounding structures.

o Physics-based o Patient-based o Equipment-induced

Artifacts can be broadly classified as:

· History of adverse reaction to contrast agents · Use of metformin · Chronic renal diseases · Dehydration · Sickle cell anemia

CONTRAINDICATIONS TO IV CONTRAST

filtration, calibration correction, and beam hardening correction software.

CT systems use 3 features to minimize beam hardening:

monthly

Contrast resolution evaluation is performed _____ in most programs.

Radiation Absorbed Dose (rad)

Describes the amount of energy absorbed per unit mass. SI unit is gray (Gy).

medical physicist

He/she is required to obtain necessary dosimetric data.

shorter scan times

Higher mA settings allow _________

MULTI-DETECTOR ROW COMPUTED TOMOGRAPHY (MDCT)

In these type of scanners, a single rotation can produce multiple slices.

700

In third generation systems approximately _____ detector elements were arranged in an arc.

120

Increases beyond ____ kVp should be avoided, except when imaging obese patients.

Undersampling

Insufficient projection data (for instance, when the helical pitch is greatly extended) is known as:

Aliasing

It causes inaccuracies related to reproducing sharp edges and small objects and results in an artifact known as:

Patient motion

It creates blurring in the image and degrades spatial resolution.

Spiral or helical scanning

It is a continuous acquisition scanning mode developed in the 1990s, a system that eliminated the cables and thereby enabled continuous rotation of the gantry.

Medical History

It is a vital aspect of any CT examination. It will help to ensure a patient's safety, guide the selection of examination protocol, and offer the radiologist valuable diagnostic information.

Contrast Resolution

It is also known as low contrast sensitivity.

Dose Equivalent (rem/Sv)

It is the product of the absorbed dose and a radiation weighting factor (wR).

MODULATION TRANSFER FUNCTION

It is the ratio of the accuracy of the image compared with the actual object scanned. It indicates image fidelity

Image noise

It is the undesirable fluctuation of pixel values in an image of a homogeneous material. It has a "Salt and pepper" look on an underexposed image.

decrease

Keeping all other scan parameters the same, as pixel size decreases, the number of detected x-ray photons per pixel will _______

· Toxicity

Lethality of a compound when injected to the body

overscan and partial scan modes, software correction, and cardiac gating.

Manufacturers have built features into the CT systems to reduce motion artifacts such as:

phantoms

Many aspects of image quality can be evaluated using _______

1. Stat 2. Semi-urgent 3. Executive check-up 4. Regular in-patient 5. Out-patient

PRIORITIES IN SCHEDULING PATIENT IN ORDER are:

Water phantom

Phantom used to measure noise and uniformity

20

RADIATION WEIGHTING FACTOR (wR) of Alpha particles, fission fragments, heavy nuclei

2

RADIATION WEIGHTING FACTOR (wR) of Photons and charged pions

Image Quality

Relates to how well the image represents the object scanned.

· CT technologists and medical physicists.

Responsibility for performing and documenting quality control tests is often shared between:

o Increase mA o Increase kV o Increase slice thickness

Solution to decrease photon starvation are:

axial scanning, conventional scanning, or serial scanning

Step-and-Shoot scanning is commonly referred to as:

F

T or F A long scan time is critical in avoiding image degradation as a result of patient motion.

T

T or F As in any x-ray imaging procedure, larger focal spots cause more geometric unsharpness in the image and reduce spatial resolution.

F

T or F Drip infusion is recommended for scans of the neck, chest, abdomen, or pelvis

T

T or F Filter increases, the lower the px dose

T

T or F If objects are large, not many will fit in a given length and they are said to have low spatial frequency.

T

T or F If the beam rotates 360 degrees or has a rotate-rotate geometry, there is higher radiation exposure

F

T or F If the objects are smaller, many more will fit into the same length. These are said to have low spatial frequency.

T

T or F In CT, image quality is directly related to its usefulness in providing accurate diagnosis.

F

T or F In CT, the appropriate selection of mAs and kVp is not critical to optimize radiation dose to the patient and image quality.

T

T or F Increasing the DFOV increases the size of each pixel in the image.

T

T or F Increasing the x-ray tube potential increases both the radiation dose and penetration of the x-rays through the body.

T

T or F Less-efficient detectors will require a higher radiation exposure to produce an adequate image.

T

T or F Mechanical Injection Systems can be single barrel and double barrel

F

T or F No arrangement by phone will be honored except those done through the following: o Secretary on duty of the CT department. o Floor manager or the technologist on duty. o Nurse

F

T or F Provided that each of the other factors remain constant, the following results in increase image quality: Increase in matrix size Decrease in pixel size Increase in DFOV

T

T or F Redundant or multiphase studies should be performed only when clinically indicated.

F

T or F Scanner calibration and tube warm-up procedures should be done while the room is full of both patients and staff.

F

T or F Small bodies require a greater dose, and large bodies require a lesser dose.

F

T or F Small patients or thin body parts require radiographic techniques that increase the radiation dose to avoid an unacceptable level of image noise.

T

T or F The higher the mAs and kVp settings used to create the image, the higher the dose to the patient.

T

T or F The pitch has a direct influence on patient radiation dose because as pitch increases, the time that any one point in space spends in the x-ray beam is decreased.

T

T or F The relevant risks, benefits, and uncertainties related to each alternative must be included in the discussion of the informed consent

T

T or F The thicker the slice, the more flattening is necessary.

gantry rotation speed the number of detector channels in the system the speed with which the system can record changing signals.

Temporal resolution is controlled by 3 factors:

milliseconds (ms).

Temporal resolution is typically reported in ___________

service engineers.

The analysis of the spread of information within the system using the MTF is done by _________

smaller

The greater the total pixels present in the image, the ________ each individual pixel the better the image quality.

blood urea nitrogen (BUN) and serum creatinine (most common).

The laboratory values most frequently reviewed before routine CT examinations are:

Scout image

The localization scan performed before scanning is referred to as:

quantum noise, or quantum mottle.

The most prevalent cause of noise

Pixel size = DFOV/Matrix size

The relationship between pixel size, matrix size, and DFOV is apparent in the equation:

Pitch

The relationship between slice thickness and table travel per rotation during a helical scan acquisition.

linear

The slice thickness has a _______ effect on the number of x-ray photons available to produce the image- a 5mm slice will have twice the number of photons as a 2.5 mm slice

purpose

The true test of the quality of a specific image is whether it serves the _________ for which it was acquired.

Spatial Resolution Contrast Resolution

The two main features used to measure image quality are:

METALLIC ARTIFACTS

These artifacts can lead to severe streaking artifacts.

Quality Factor

This factor accounts for the different biologic effects produced from different types of ionizing radiation. The quality factor is 1 for the diagnostic x-rays that are used in CT

cupping artifact appearance of dark bands or streaks between dense objects in the image.

Two types of artifact can result from beam hardening:

Mechanical Injection Systems

Type of bolus technique wherein injections are consistent and can be reproduced in subsequent examinations.

Hand Bolus Technique

Type of bolus technique which is subject to many factors, including syringe size, contrast viscosity, IV catheter size, and operator strength.

LOCM

Type of iodine based CM that has lower risk to patient

HOCM

Type of iodine based CM that increase risk to patient

SINGLE DETECTOR-ROW COMPUTED TOMOGRAPHY (SDCT)

Type of scanners used in both third- and-fourth generation systems.

: Iodine based (commonly used in CT) and Barium

Types of contrast media

roentgen (R).

Unit of x-ray exposure in air is the

· Iodine content

Volume distribution of iodine in the CM

xenon gas detectors

What detectors are less efficient at absorbing x-rays?

seventh generation systems

What generation of systems use MDCT?

Radiation Equivalent Man (rem)

When the quality factor has been applied to the radiation absorbed dose the new quantity is called the

increasing

_________ mAs will improve contrast resolution, but at the cost of a higher radiation dose to the patient.

Laser lights

___________ located both inside and outside the gantry are used extensively for patient positioning and alignment.

Volumen

a low HU-value oral contrast

· Miscibility

ability of a medium to mix with other fluids

effective dose, or effective dose equivalent

attempts to account for the effects particular to the patient's tissue that has absorbed the radiation dose.

· Osmolality

concentration of a solution

DFOV (Display Field of View)

determines how much raw data will be used to reconstruct the image.

10mm

highest slice width

1.25mm

lowest slice width

line pairs phantom

measuring the spatial resolution directly, using a _________, is a simpler method and is often performed by a technologist.

Quantum mottle

occurs when there are an insufficient number of photons detected.

· Persistence

refers to the amount of time CM stays in the body.

pixel size

reflects how much patient data is contained within each square

· Viscosity

resistance of fluid to movement

Contrast Resolution

the ability to differentiate between objects with very similar densities as their background. (white, black and grey areas, differences in the shades of grey)

spatial resolution

the ability to resolve (as separate objects) small, high-contrast objects.

Signal-to-noise ratio (SNR).

the number of x-ray photons detected per pixel is also referred to as:

Technologists

typically perform and record routine quality control tests; testing done by physicists is typically annual or semiannual

TUBE ARCING

· A common cause of equipment-induced artifact occurs when there is an undesired urge of electrical current within the x-ray tube.

Iterative reconstruction

· A newer method of image reconstruction known as ________ has been recently introduced for the use in CT image reconstruction and, compared with standard filtered back-projection methods, can reduce the dose by as much as 50%

name

· Always get the _______ of the person you are talking with.

pitch

· Another useful method for reducing radiation dose with helical scanning is to increase the _____ of the examination.

patient motion

· Artifacts from _________ typically appear as shading, ghosting (objects appear to have a shadow), streaking or blurring.

OUT OF FIELD ARTIFACTS

· Caused by anatomy that extends outside of the selected SFOV.

ARTIFACTS

· Defined as anything appearing on the image that is not present in the object scanned.

1mm

· For a slice thickness of 5mm or greater, the slice thickness should not vary more than ________ from the intended slice thickness.

0.5mm

· For a slice thickness of less than 5mm, the slice thickness should not vary more than ______

1

· If the image reproduced the object exactly, the MTF of the system would have a value of ____

0

· If the image were blank and contained no information about the object, the MTF would be ___

Filtration

· Low energy photons only contribute to the px dose and it will be eliminated

SLICE THICKNESS ACCURACY

· Measurements of selected slice thickness are determined using a phantom that includes a ramp, spiral, or step-wedge.

360°

· Most scanners use a _____ tube arc to compensate for radiation beam divergence and patient motion.

PARTIAL VOLUME ARTIFACT

· Occur when dense objects lie to the edge of the SFOV and are only present in some of the views used to create the image.

PHOTON STARVATION

· Potential source of serious streaking artifacts which in highly attenuating areas and insufficient photons reach the detectors.

Automatic Tube Current Modulation

· Some systems have an option that will make changes in tube current (mA) based on the estimated attenuation of the patient at a specific location, which is called:

1. Direct measurement using a phantom. 2. Data analysis is known as the modulation transfer function (MTF).

· Spatial resolution can be measured using two methods:

o LOCM (Low-osmolality contrast media) o HOCM (High-osmolality contrast media)

· TYPE OF IODINE BASED DYE

use thinner slices.

· The best method of reducing partial volume artifacts is to

MODULATION TRANSFER FUNCTION

· The most commonly used method of describing spatial resolution ability, not only in CT, but also in conventional radiography.

RING ARTIFACTS

· They are caused by imperfect detector elements- either faulty or simply out of calibration.

LINEARITY

· This refers to the relationship between the CT numbers and the linear attenuation values of the scanned object at designated kVp value.

CT dose index (CTDI) measurement

· Using the __________, the physicist must calculate several descriptors of dose for an adult head, pediatric abdomen, and adult abdomen examination.

BEAM HARDENING

· X-ray beam passes through an object, lower-energy photons are preferentially absorbed, creating a "harder" beam.

Voluntary motion

· can be reduced or eliminated by adequately preparing the patient for the examination.

Uniformity

· refers to the ability of the scanner to yield the same CT number regardless of the location of an ROI within a homogenous object.

Referral

· the prescription, order or request coming from the physician

4800

·Fourth-generation systems used as many as ____detectors in a single row arranged in a complete ring (detector ring)

F

·T or F For a larger patient, the exit radiation is much more intense as a result of its attenuation through more tissue.

F

·T or F The MTF scale is from 0 to 2

Quality Control program

·These are designed to ensure that the CT system is producing the best possible image quality using the minimal radiation dose to the patient.