[Ultrasound Physics] IMAGE PROCESSING

[Contents]

1. Preprocessing

2. Postprocessing

3. Persistence (Frame averaging)

4. Edge Enhancement

5. Spatial compounding

6. Panoramic Imaging

1. Preprocessing [★]

 

1) Manipulation of scan data before storage in the memory after scan conversion

- Any function performed by receiver is preprocessing

 

2) Write Magnification (preprocessing)

- Before storage in the scan converter

: Scans anatomy creates image

: Image converted from analog to digital

: Sonographer Identify ROI (region of interest). 

: System discards existing data in scan converter.

: US rescans only the ROI and writes new data into scan converter (All new info acquired)

- # of pixels, scan lines in ROI is greater than ROI of original image

· More pixels = better spatial resolution

· Pixels are the same size as original

 

3) Pixel Interpolation/Fill-in Interpolation (Preprocessing) [★★]

- A way of filling in gaps of data undetected by the observer

- With a sector shape image: scan lines separate more at increasing depths

: Interpolation fills in the data missed due to increased space between scan lines

: without interpolation, image results in a series of scan lines with blank data between the lines

- Uses gray scale of surrounding pixels to predict missing information

: Increases line density

: Improves spatial resolution

2. Postprocessing

1) Data manipulations performed with the receiver

- Manipulates data after it has been stored in scan converter, prior to display

- Processing a frozen image, after memory

- Operator controllable

- Improves contrast resolution

 

2) Read magnification (post processing) [★]

- After image information is stored in the scan converter

               : performance on frozen image

: Scans anatomy and creates image

: Image converted from analog to digital and stored

: Identify region of interest

: Pixels are enlarged, ROI fills screen

               - Resolution loss

- Reads & displays only original data from ROI

- gray-scale map assignment

- # of scan lines & pixels are same as original

· Just magnification causes larger pixels

· Spatial resolution remains the same because same # of pixels

: Undergoes digital to analog conversion for the display

3. Persistence (Frame averaging) [★]

1) Continues to display information from older images

2) Number of previous frames are superimposed on the most current frame

- Positives: Improves image quality during real-time acquisition

: Smoother image w/ reduced noise (Higher signal to noise ratio)

- Limitation

: Reduces frame rate

: Reduces temporal resolution

- Most effective with slow moving structures

4. Edge Enhancement

1) Image processing method that makes image look sharper

2) Increases image contrast in area immediately around sharp edges

- Creates subtle bright and dark highlights on either side of those boundaries

- Makes boundaries more defined

5. Spatial compounding [★★]

1) uses information obtained from several imaging angles to create one image

- Averages frames obtained from different angles due to steering by employing time delays

- Overlap frames to form single real-time image

2) help demonstrate tissue boundaries that are not perpendicular to sound beam

               (improve border definition)

3) More frames = better image quality (improve border definition)

- Reduces speckle, reduce refraction and enhancement

- Minimizes shadowing, artifacts

4) Limitations

- Reduces frame rate (Reducing temporal resolution)

- electrical steering is used → only phased array Transducers

6. Panoramic Imaging

1) Expands images beyond the normal limits of the transducer field of view

- Retains echo information from previous frames and new echoes are added

2) Limitation

- Decreases frame rate (Reduces temporal resolution)

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler