real-time imaging display

real-time imaging

each frame is created and displayed very quickly, providing the impression of constant motion

with real-time imaging, the transducer is responsible for:

sending out scan lines across a defined plane

frame

one complete ultrasound image

scan lines

created when one or more pulses of sound return from the tissues containing info related to the depth and amplitude of the reflectors

temporal resolution

pertains to “accuracy in time” and describes the ability to precisely position moving structures from instant to instant

temporal resolution is determined by:

frame rate

how are temporal resolution and frame rate related?

• directly

• high frame rate (high number of images per sec), improves temporal resolution

• low frame rate, reduces temporal resolution

the units for temporal resolution are:

hertz

frame rate (FR)

the number of frames per second

frame rate is determined by:

• sound’s speed in the medium

• the depth of imaging

the frame rate is equal to:

the PRF divided by LPF (lines per frame)

frame time (Tframe)

the time required to make a single image

how are frame rate and frame time related?

inversely

if you increase depth, what happens to the PRF and FR?

PRF and FR decrease

what minimum frame rate needs to be maintained or the image will flicker?

15 Hz

how are imaging depth and frame rate related?

• inversely related

• shallow imaging increasing frame rate and improves temporal resolution

• deeper imaging decreases frame rate and degrades temporal resolution

how are PRF and FR related?

directly

how are pulses per frame and FR related?

• inversely

• high frame rates occur when each image is made with fewer pulses

• lower frame rates occur when each image is made with more pulses

what 3 factors determine the number of pulses needed to create an image?

• number of pulses per scan line (multi-focus vs single focus)

• sector size

• lines per angle of sector (line density)

single focus

• one pulse per scan line

• shorter frame time

• higher frame rate

• better temporal resolution

• poorer lateral resolution

multi-focus

• many pulses per scan line

• longer frame time

• lower frame rate

• diminished temporal resolution

• improved lateral resolution

sector size (field of view)

when this is increased/expanded, more pulses are required to create an image

how are field of view and frame rate related?

inversely

narrow sector

• fewer pulses per frame

• short frame time

• higher frame rate

• superior temporal resolution

wide sector

• more pulses per frame

• longer frame time

• lower frame rate

• inferior temporal resolution

line density

number of scan lines per frame

low line density

• widely spaced lines

• fewer pulses per frame

• shorter frame time

• higher frame rate

• high temporal resolution

• poor spatial resolution

high line density

• tightly packed lines

• more pulses per frame

• longer frame time

• lower frame rate

• low temporal resolution

• excellent spatial resolution

what is the main advantage of high line density?

the gaps between the lines are smaller which improves the accuracy of each individual image

what are ways to improve temporal resolution?

• shallower imaging

• single focus

• narrow sector

• low line density

newer image display technologies provide:

• higher frame rate

• more lines per image (high line density)

M-mode (motion)

• as a sound pulse is emitted by the transducer, the reflections move at a constant speed from right to left across the screen

• various squiggly lines that represent the changing depth of the reflecting surfaces are produced on the screen

• a line that moves up and down on the display indicates that a reflector is moving closer to or farther away from the transducer

• x-axis: time

• y-axis: depth

• used in cardiac

A-mode (amplitude)

• as a sound pulse is emitted by the transducer, a dot moves at a constant speed across the systems display

• when a reflection returns to the transducer, it is processed and the moving dot is deflected upward on the screen

• the height of the upward deflection is proportional to the amplitude of the returning echoes

• x-axis: depth

• y-axis: amplitude

• used in ophthalmic imaging

B-mode (brightness)

• as a sound pulse is emitted by the transducer, an invisible dot moves at a constant speed across the systems display

• when a reflection returns to the transducer, it is processed and the invisible dot is turned on

• the brightness of the dot indicates the strength of the reflection

• x-axis: depth

• z-axis: amplitude

displays used as ultrasound monitors:

• cathode ray tubes (CRT)

• liquid crystal display (LCD)

cathode ray tubes

• funnel shaped, glass vacuum tube

• electrons that contain video info are emitted from an electron gun in the narrow end of the tube

• the electron beam then travels through magnetic fields which focus and sweep the beam across the wide end of the tube

• the interior surface of the screen is coated with phosphors which glow when struck by the electrons

liquid crystal display/flat-panel display

• works with a light source positioned behind 2 polarized filters with liquid crystals sandwiched between them

• images are presented on computer displays because they represent digital info stored in the image (computer) memory

x-ray film

single emulsion x-ray film

thermal processors (thermal printers)

use a paper medium to record the image

laser imaging

• automated film handling and developing

• 15 or more images per sheet of film

• higher resolution

• better grey scale with less distortion

digital recording device

stores images on computer disks or memory and allows viewing on monitors and film transfer

videotape player

used to record motion or real-time imaging such as video home system (VHS) which uses a magnetic tape

magnetic-optical disk

safely stores info on an optical disk and disk can be rewritten and erased

picture archiving and communications system (PACS)

describes the digital ultrasound laboratory in which images and additional medical info are digitized and stored on a large computer network

redundant array of independent disks (RAID)

used by PACS systems to store large quantities of data

digital imaging and computers in medicine (DICOM)

a set a rules or protocols that allows imaging systems to share information on a network

elastography

a sonographic technique used to evaluate the stiffness of a mass or tissue

what is elastography used to asses?

• differentiating malignant and benign neoplasms

• identifying early traumatic changes in muscles and tendons

• aiding in deciding the biopsy site more accurately

• assessing liver fibrosis

strain/static/compression elastography (SE)

• measures tissue strain (change in tissue length/size) due to compression

• operator dependent

• low strain/stiffer tissues are more likely to be malignant

appearance of strain elastography:

• grayscale elastograms show areas of low strain (stiffer tissues) as dark and softer tissues as bright

• color elastograms will have a color assigned to low strain

acoustic radiation force impulse elastography (ARFI)

uses acoustic radiation force to compress the soft tissue

shear wave/transient elastography (SWE)

• compression is produced by the sound wave

• sound causes vibrations which produce shear waves

• used to measure tissue displacement caused by shear waves

what is SWE used to assess?

often used to measure liver stiffness in pts with chronic/diffuse liver disease

intravascular ultrasound (IVUS)

• a medical imaging methodology using a specially designed catheter with a miniaturized ultrasound probe attached to the distal end of the catheter

• it is placed inside vessels to evaluate plaque and other vascular conditions

• mostly done on the arteries of the heart (coronary arteries)