Principles of CT

What generation of CT scanners are mainly used currently?

3rd

Which scanner generation demonstrates high temporal resolution, making it suitable for imaging moving structures such as the heart?

5th

Which generation of CT scanner has a dual-energy source?

6th

factors affecting dose in CT include:

Beam energy: KVP Tube current: mA rotation or exposure time: seconds section/slice thickness: beam collimation Object thickness Attenuation: patient size

Oral contrast for CT should be mixed so it is: Select one: a. the same as for general diagnostic examinations b. more concentrated than for general examinations c. less concentrated than for general examinations d. contrast media is not necessary

C

the display monitor

CT image may be displayed on a monitor in a recognizable form, the digital CT data must be converted to a gray scale image the process is achieved by conversion of each digital CT number in the matrix to an analog voltage the brightness values of the grayscale image correspond to the pixels and CT numbers of the digital data present

This represents the relative comparison of x-ray attenuation of a voxel of tissue to an equal volume of water.

CT number

Describe the concept of CT numbers. a. Explain how CT numbers and tissue types are related. How does know CT numbers of specific tissues aid in diagnosis?

CT numbers compare different tissue densities to water, visualized on a gray scale. Ct numbers and tissue types are related in comparison to that of water (0). Anything denser than water is represented with a positive number, and any tissue less dense than water is represented with negative numbers. Positive numbers appear white or bright, while negative tissues appear dark gray or black. Knowing the CT number of a specific tissue aids in diagnosis by allowing the radiologist to differentiate between tissues by comparing it to water. Knowing that anything denser than water will appear white and anything less dense will appear dark, helps the radiologist to rule out any foreign body, like tumors or cysts.

Why is contrast material used for CT imaging?

Contrast material is used for CT imaging to improve images of inside the body and contrast material helps the radiologist to distinguish between normal and abnormal conditions.

After reading the material found on the Radiology Info website, please respond to the following: What are contrast materials and why are they used? What different types of contrast materials are there? What exams are each type used for? Have you ever been given contrast material as part of an imaging exam? What was the experience like? Are there risks involved with receiving contrast material? What are the risks?

Contrast materials are substances that enhance pictures and are used to distinguish internal organs. Different types of contrasts include: Iodine and barium sulfate, used during x-rays and CT. They are available in many different forms as well. Gadolinium is most commonly used during MRI, to alter water molecules. Saline and gas are most commonly used during ultrasound exams, specifically the heart. No, I have not been given contrast material as part of an imaging exam. I don't think I would like the chalky, paste-like texture of it either. Yes, contrast material can cause adverse reactions, ranging from mild-severe. Digestive issues like constipation and diarrhea are considered to be mild, while anaphylactic shock and cardiac arrest are the most severe reactions to contrast material. Some patients may have moderate symptoms like hives and swelling.

contrast resolution: what it means

Contrast resolution is fundamentally how well the difference is rendered between an object and its surroundings or, alternatively, how well an object stands out from the background. Contrast resolution is pivotal to the property of conspicuity in imaging. For example, if an object blends with the background, the object cannot be seen. Hence, low-contrast resolution results in an inability to detect objects in an image. Contrast resolution is a metric to determine how well the imaging system faithfully renders the actual contrast in the structure being imaged. This could be viewed as the signal in the measured signal-to-noise ratio.

Why is contrast resolution important in medical imaging? What factors affect contrast resolution?

Contrast resolution is one of the five important parameters that characterize image quality in medical imaging. The others are spatial resolution, noise, temporal resolution, and, when applicable, radiation dose.

What are two drawbacks to routine radiography?

The best answers: Routine x-ray images have alot of superimposition of structures. Routine x-ray has lower contrast resolution.

What is the goal of quality assurance in CT? Name at least 3 quality assurance tests for evaluating CT performance.

The goal is to produce the best possible images at a low dose 1. Weekly/biweekly maintenance 2. Daily tests on water phantoms by CT Techs to measure the consistency of CT numbers 3. Air calibration

The most common reporting method of dose reporting on present scanners is:

Dose-length product (DLP)

what determines the amount of data displayed on the monitor?

FOV

invention of CT is accredited to:

Godfrey N. Housnfield 1970-1971 prelim work done by oldenforf in 1961

What is a Hounsfield Unit? What is it used to describe?

Hounsfield unit is a series of numbers also known as CT numbers, that provide a range of positive to negative numbers of structures and tissue that is compared to water that is set at 0. + meaning denser than water & - meaning less dense than water. It is used to describe and compare x-ray tissue attenuation to water (0).

Dose reporting methods

MSAD- multiple scan average dose CTDI- computer tomography dose index DLP: dose length product

spiral helical CT

Scans a volume of tissue instead of individual slices Advantages Respiratory motion minimized Detects small lesions Good for uncooperative, combative, trauma, and pediatric patients Decreases amount of contrast material used (increases safety, decreases cost)

The patient dose in CT is determined by the : 1. prepatient collimator 2. postpatient collimator 3. predetector collimator

1

Which of the following CT image quality factors is affected by x-ray beam energy levels? 1. contrast resolution 2. spatial resolution 3. noise

1 and 3

CT image noise is affected by: 1. matrix size 2. slice thickness 3. patient size

1,2 &3

The advantages of multislice spiral or helical CT include: 1. greater anatomic coverage 2. faster exam times 3. improved spatial resolution

1,2 &3

As compared to routine radiography, CT results in: 1. images with no superimposition of structures 2. images with improved contrast resolution 3. reduced scatter radiation

1,2&3

Factors that affect image quality 1. Spatial resolution 2. Noise 3. Contrast resolution

1,2,3

Briefly describe the basic operation of a CT scanner.

1. A patient lies on a bed that slowly moves them under the gantry. 2. The x-ray tube will then rotate around the patient shooting narrow beams of x-rays. 3. Each time the bed moves, the scanner circles again, a computer analyzes the information received by the x-rays and constructs an image of the slice of the body

What are at least two (2) CT image quality factors under the direct control of the technologist?

1. FOV 2. Scan time

What are three ways in which contrast material can be administered to a patient?

1. Iv 2. oral 3. Rectal

Describe the basic operation of a CT scanner. Address the following: a. What advantages does CT imaging have over routine radiography? b. How does patient dose in CT imaging compare with routine radiography patient dose? c. Name/describe at least three (3) things that can be done to keep patient dose to a minimum for CT imaging.

The operation of a CT scanner: Patient lies on a bed that slowly moves them under the gantry.The x-ray tube then rotates around the patient shooting narrow beams of x-rays. Each time the bed moves, the scanner circles again. A computer analyzes the information received by the x-rays and constructs an image of the slice of the body. The advantages of CT imaging over routine radiography are images with higher contrast resolution and structures with less superimposition. Dosage within CT and radiography are both dependent on attenuation/patient size. Distance and technique are factors that affect both CT and radiography. Patient dose in CT compares to routine radiography in unit as well, both are measured in Sieverts. To minimize dose in CT, the technologist can adjust the technique by lowering the mA. The technologist can also alter the slice thickness with a single or multi- detector. Keeping exposure time to a minimum will also reduce patient dose.

Why are these units important for a Radiologist when interpreting a CT scan? How can knowing the HU of structures/tissues aid in diagnosis and treatment?

The use of the HU to measure tissue density has aided radiologists in the interpretation of images and diagnosis of disease.

high resolution CT scans are made using

Thinner sections or slices

Metallic objects such as dental work or jewelry may result in what?

artifacts

_______________ ___________________ is a measure of how different tissues are penetrated by radiation

attenuation coefficient

all of the following affect radiation dose in CT:

beam energy tube current section thickness

computer component of the CT scanner & its 4 basic functions and how each works

computer: has 4 basic functions: control of data acquisition, image reconstruction. storage of image data and image display data acquisition is the method by which the patient is scanned. the tech must select numerous parameters such as scanning in conventional or helical the DAS is involved in sequencing the generation of x-rays, turning detectors on and off and transferring data the reconstruction of the image is accomplished by using an array processor that performs mathematical calculations rapidly the host computer has limited storage, so image data is stored temporarily. to allow for long term storage and retrieval it is transferred to another storage medium, like an optical disk

Window width is a control used to adjust image:

contrast

This is referred to as the ability to differentiate between tissue densities on an image.

contrast material

CT imaging has excellent _____ because of the narrow beam collimation.

contrast resolution

This image quality characteristic is described as the visible difference between two adjacent structures with similar tissue density.

contrast resolution

What is the characteristic of CT imaging called that allows it to differentiate between tissues with similar density?

contrast resolution

the ability to differentiate between small differences in density within the CT image is called:

contrast resolution

which 2 quality factors is affected by the x-ray beam energy

contrast resolution and noise

Indicate which plane each of the following is in

coronal

what accessory device is added to the CT table and why

cradle, for head CT , helps hold the head still helps hold beyond the table, in doing so it minimizes artifacts or attenuation

which of the following contribute to image noise in CT?

detector aperture width matrix size patient size

major components of a CT scanner

gantry, computer and operators console and patient table

FOV

how much tissue was seen in an image This determines the amount of data displayed on the CT monitor.

what is the most common imaging artifact within CT

image noise or quantum mottle and it can be minimized by sequentially increasing the tube current- time product and the tube voltage.

how can knowing the CT numbers help to diagnosis and treat patients?

knowing the ranges allows comparison to water which is set to 0

What was the main disadvantage of 1st generation CT scanners?

long scanning time patients would have to sit still for long periods

An array of numbers arranged in a grid of rows and columns is called a:

matrix

uses brightest pixels from 2D-3D data, commonly used for CT angiography

maximum intensity projection

Which 3D-imaging technique is commonly used for CTA?

maximum intensity projection CTA uses IV

Which 3D-imaging technique is commonly used for CTA?

maximum intensity projection uses an IV procedure

What are the three (3) common techniques used in creating three-dimensional CT images?

maximum intensity projection (MIP) shaded surface display (SSD) Volume rendering (VR)

housnfield unit HU:

measures x-ray attenuation -20 to -150 : adipose tissue 20-50 HU for kidney

Recommended QC tests include:

o Contrast scale and mean CT number of water o High-contrast resolution o Low-contrast resolution o Laser light accuracy o Noise and uniformity o Slice thickness o Patient dose

What type of configuration do 4th generation CT scanners use?

only tube rotates/detectors are stationary

what term is applied to a single square picture element in the image display matrix?

pixel

Which of the following terms refers to a single square in an image display matrix?

pixels

Shaded surface display

pixels having an equal or higher CT number than threshold value selected for 3D image provides depth perception to reconstructed image

Tissues that are denser than water are given what type of CT number?

positive

Why does CT imaging have the capability of differentiating between tissues with similar density better than conventional imaging?

reduced amount of scatter radiation in CT makes it better at differentiating between similar tissue densitys

In 3rd generation CT scanners, the x-ray tube and detectors moved simultaneously (at the same time) around the patient. What kind of motion is this described as?

rotate/rotate

Indicate which plane each of the following is in

sagittal

The principle advantage of 2nd generation CT scanners over the 1st generation was a reduction in what?

scanning time

terms like cut and slice can cause patient distress, using words like

sections is better

Dose in CT is considered to be:

size dependent

3 things selected at the operators console

slice thickness table index imagine technique factors

This CT image quality factor is determined by the degree of blur on an image.

spatial resolution

the amount of blurring in a CT image is termed

spatial resolution

What CT acquisition method involves continuous gantry rotations combined with constant table movement through the gantry aperture (opening)?

spiral or helical CT

This is the ability of a CT system to freeze any motion of a scanned object.

temporal resolution

gantry & table, how they function

the gantry is a circular device that houses the x-ray table, DAS & detector array the components houses in the gantry collect the necessary attenuation measurements to be sent to the computer for image reconstruction. the gantry can be tilted forward or backward up to 30 degrees to compensate for body angulation, the opening is called the aperture (28inches) the table is an automated device linked to the computer and gantry. it is designed to move in increments according to the scan program, the table is programmed to move in and out of the gantry depending on the exam. ( made of low density carbon fiber composite) both of which support the patient without causing image artifacts

each pixel represents a volume of tissue that was imaged, voxel has to do with

the thickness of each individual slice

Scan parameters used to improve CT image appearances include:

thinner slices: shorter scan times scan diameter: FOV

the operators console in the CT components, how does it function

to initiate the process to store or archive the images for future viewing PACS used to store and retrieve soft copy digital images keyboard: for entering patient data graphic monitor: for viewing images allows the operator to control and monitor scan parameters, imaging technique factors and slice thickness a mouse and touch display screen may also be found

First generation CT scanners worked on a process known as what?

translate/rotate

Indicate which plane each of the following is in

transverse axial

A bolus is the amount of radiopaque contrast material injected rapidly per IV administration to visualize high-flow vascular structures and most often injected using a power injector.

true

CT provides more diagnostic information than any other ionizing radiation modality

true

Oral contrast material must be used for CT imaging of the abdomen.

true

Spiral CT, also known as helical CT, is a scanning method that combines continuous gantry motion and table movement to form a spiral scan data path.

true

T/F: Early CT was performed only for brain imaging and was capable of producing only transverse or axial images.

true

IV contrast material is used in CT to demonstrate:

vascular structures

depicts anatomic relationship between vasculature and viscera incorporates entire volume of data into 3D image preserves the dynamic range throughout the image

volume rendering

A volume element is called what?

voxel

the range of CT numbers that are used to map signals into shades of gray is called the:

window width

what important technique allows the technologist to alter the contrast of the displayed image

windowing or gray-level mapping window width- range of CT numbers used to map signals into shades of gray narrow window: few shades of gray= higher contrast, short scale page 216 merills volume 3

positive & negative CT numbers

§ These numbers are manipulated by the computer during image processing and gray scales are assigned. Each tissue is assigned a shade of gray based on its CT number § Positive numbers (dense tissues) will be bright or white § Negative numbers (less dense tissues) will be dark gray or black