Ultrasound images are
corrupted by speckle; which is a locally correlated
multiplicative noise. Unlike other noises, texture of the
speckle content carriers some form of diagnostic information
in ultrasound images. This primarily leads to poor contrast
resolution in such images, posing in difficulty in detection of
small lesions as well as micro-calcifications. Hence, it is necessary to apply contrast enhancement after despeckling of ultrasound images.
INtravascular ultrasound contrast agents might be used to higlight lesions and vessels and to evaluate perfusion.
Speckle noise is the primary factor affecting the appearance in ultrasound images or videos. This is a multiplicative noise which requires appropriate modelling and should be filtered out. There are a large number of methods proposed in the literature as well as appropriate software for ultrasound image/video despeckling. By preforming despeckling you also increase image contrast. Maybe you may look at our book:
C.P. Loizou, C.S. Pattichis, “Despeckle filtering algorithms and software for ultrasound imaging,” Synthesis Lectures on Algorithms and Software for Engineering, Ed. Morgan & Claypool Publishers, 1537 Fourth Street, Suite 228, San Rafael, CA 94901 USA, June 2008, ISBN-13: 9781598296204.
Alternatively, also have a look at research gate under my page. There are a lot recent publications made from our group accompanied by software (which can be downloaded) for image and video despeckling. See also our website at http://www.medinfo.cs.ucy.ac.cy/
Contrast agents are not required to achieve diagnostic studies. Ultrasound contrast. IE. Intravenous (room air) gas bubbles or encapsulated microbubbles does exist, it is used for perfusion or vascular studies (for example investigation of cardiac shunts). I perform these studies infrequently in the (veterinary) clinical setting and have actually never used the encapsulated bubbles in the clinical or research setting. I’m not sure how frequently they are used in day-day human medicine.
As an imaging modality, inherent ultrasound contrast and spatial resolution are generally poor when compared to a modality such as film mammography. Contrast (in imaging) manifests as grey on the image display. Contrast for a modality, depends on how the modality’s energy interacts with the different tissues of the patient .
To the untrained eye, ultrasound can look pretty homogenously “grey” or like TV screen static. MRI, on the other hand, has high contrast resolution due to sharp delineations between shades of grey. To the trained sonographer, the tissues displayed have different “echogenicities” or brightness and “texture”. Deviation from what is expected in the normal setting is based deviation from normal shape, size, echogenicity, echotexture, margination or “number” - of a given tissue. Then, a list of differentials is then generated by the clinician.
For ultrasound, tissue contrast depends on the acoustic properties of a tissue and how the speed of sound changes as it travels through the different tissue interphases. The difference between the tissues is seen as a contrast difference, especially when the acoustic impedance differences between the tissues are large.
Also, the wavelength of the ultrasound transducer can limit the spatial resolution (or the ability to resolve tiny structures). Although not the only thing affecting resolution, pixel size will also set limits on what can be “seen” on an image unless signal (from the thing/object/tissue) is very high, leading to high contrast within a pixel, such that a grey-scale difference is seen within that pixel.
Bit depth is another limitation. One byte or 8 bits, is the max number of picture element matrix components within a pixel. State of the art transducers have preamplifiers max bit depths of 8-12 bit depths and high sampling rates. Typically, contrast resolution provided by a modality determines bit depth. Ultrasound is a 6, 7 or 8 bit modality and hence, considered a lower contrast modality (when you compare it to CT, typically a 12 bit modality) for example. Too few bits = lower contrast resolution. The sonographer must also understand ultrasound artifacts, which can significantly degrade image quality and negatively hamper image interpretation or cause false lesions.
Ultrasound contrast agents during neurosurgical procedures are still far from being a standard procedure. However its usage has been included in the new updated guidelines of the European Federation of the Societies for Ultrasound in Medicine and Biology (EFSUMB) for non-liver application. In general I would like you to consider this: when using CT or MRI for neurosurgical diagnostic procedures, planning or follow-up, almost invariably contrast agents are used. Why not then for ultrasound?
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