Physics

Please See Core Concepts First!

They are, in fact, Physics.

Resolutions: The Foundation of Physics

Everything in sonography physics comes back to resolutions. Resolutions determines how well we can see. Resolutions are the foundation of image clarity and diagnostic accuracy.

4D

Ultrasound is a unique modality with a special ability: we can image in 4D. What does “4D” mean? That means we scan in four dimensions. We scan 3-dimensional structures. What are the three dimensions?

Transducer → Beam-Shape → Spatial Resolutions

The shape of the transducer creates the shape of the sound beam.

The shape of the sound beam determines the resolution of the images.

The resolution of the images determines the clarity of the picture, the diagnostic accuracy, the removal of various artifacts, and helps distinguish borders.

Resolution is everything.

The smaller the beam is in its three dimensions, the better the sound beam can fit between the structures we are wanting to resolve.

The better the sound beam can fit between the structures we are wanting to resolve, the better we can resolve them and see them as distinct entities.

The height of the transducer (technically the thickness of the crystals in the transducer), in addition to the pulser, creates the spatial pulse length which creates the axial resolution. Remembering that the axial resolution originates from within the probe and projects out from the probe is helpful in your studies.

The length of the transducer face creates the beam width which determines the lateral resolution. I like to think of the lateral resolution (lateral means “side” and another name for this resolution is side-to-side) as the resolution that extends from one’s side to the other like a waist. Lateral resolution is best at the focal zone, which I like to think of as a belt, narrowing the waist.

The width of the transducer face creates the beam thickness which determines the elevational resolution. We want the elevational resolution to be paper-thin. Just like an image is printed on a piece of paper, if the elevational resolution is poor, we can see text on the other side of the page which makes seeing our text or image difficult.

The 5 Resolutions

3 Spatial:

Axial
Lateral
Elevational

Others:

Temporal
Contrast

An ultrasound system must be able to resolve two pixels in order to be able to show them as separate and distinct, or else the two dots will appear as one dot.

Resolution simply means to show as two separate things.

The sound beam, in essence, must be able to fit between the two dots to resolve them.

And then, the machine’s processing speed must be able to keep up in order to resolve the frames as you scan in real time.

Finally, the shades of gray have to be different enough to show variances in the tissues being assessed.

Take a look at the resolutions testing phantoms illustration. Seeing how the resolutions fail helps drive home the concept.

Resolutions-testing phantoms illustration

Contrast Resolution: real life example incoming***

Axial Resolution

Superficial to Deep

Ultrasound that can resolve pixels to form images is pulsed wave (as opposed to continuous wave which only makes a spectral waveform line, almost like an electrocardiogram tracing). The shape of the beam is a single short pulse that travels from the transducer and away. The pulse must fit between two dots to resolve them.

Axial resolution is the parameter that we as sonographers can effect the most.

Axial resolution is the spatial pulse length divided by two. The spatial pulse length is determined by the number of cycles and the wavelength of the sound beam. The wavelength is inversely proportional to the frequency. As nearly everything in ultrasound physics comes back to resolution, the majority comes back to frequency. This is why. We effect frequency effects wavelength effects spatial pulse length effects axial resolution effects image quality. More notes and relationships on axial resolution follow.

Lateral Resolution

Side-to-Side

Lateral resolution corresponds to the waist of the ultrasound beam—the thinner the waist, the better the resolution. We mostly adjust the lateral resolution by adjusting the focal zone location on machines that do not run all foci all the time. More notes about lateral resolution follow.

Elevational Resolution

Toward Me; Away from Me

This is the resolution that is not in the 2-dimensional image. This is the dimension that, if you were to put on 3D glasses, would appear to jump out at you. Elevational resolution corresponds to the slice thickness of the ultrasound beam: think of the thickness of the crust of a slice of pizza. Our image should be one sheet of paper, but sometimes we can see the ink on the next page through our current page (like seeing a drawing that is on a page behind the sheet of paper, only we do not want to see that drawing)! Elevational resolution is especially noticeable when working with fluid-filled areas and seeing tissue—is the tissue within or behind the structure? Additional examples will follow.

Temporal Resolution

Frame Rate

Temporal resolution is not just helpful to see a fetus moving around mom’s womb for a bonding moment. Temporal resolution helps us detect accurate color flow and blood flow velocities to identify pathology. Temporal resolution helps us safely guide interventional procedures in real-time. Temporal resolution helps us accurately measure heartrates. If temporal resolution is degraded enough, even fanning through the area of interest will blur the image on the screen. Recall that the Frame Rate is shown on the screen layout.

Contrast Resolution

Shades of Gray

Contrast resolution helps us see the different shades of gray between normal anatomy and abnormal anatomy. Pathological lesions can be a subtle shade different, and if they are not different enough (or even not at all), we will not be able to see them as separate from the surrounding normal tissue.

Application of the Resolutions to Measurements

The three spatial resolutions directly effect the accuracy of the measurements that we take. Your understanding of how which resolution influences which measurement will empower you to use your physics to optimize your borders to be able to visualize where to measure. Let us take a look.

The ovaries are highlighted in purple:

Orient yourself to these ovary images. Anterior (A) to posterior (P), cephalad (CEPH) to caudal (CAUD), and RT (right) to LT (left) directions are demonstrated.

Note that both images are from A to P, meaning the ovary’s height is the same in both images. The height of the ovary does not change, and we are viewing the ovary from the same angle in both pictures.

Consider these measurements and what you have now learned about axial, lateral, and elevational resolutions. Which resolutions effect length, width, and height?

Axial effects height; lateral effects length and width! Where is elevational?

When optimizing borders for measurements, consider which resolution is affected. As you study your physics, pay attention to how you could make your axial borders pop, how you could make your lateral borders pop, and how you could make both pop! So where is elevational resolution? Think fluid. If there were a cyst in this ovary, and we needed to determine if the cyst is a simple cyst (benign, no follow-up required), has blood products (endometrioma, benign but problematic), or solid (worrisome, needs a workup), we would depend on our elevational resolution to determine if we saw any echoes within that space or not.

In the left image this breast cyst of mine looks to have solid areas (suspicious for malignancy); however, as seen on the right, once angled into the elevational plane, one can see those solid areas that appeared to be within the cyst are gone. One could fan through and match those solid areas to the tissue on the outside of the cyst beyond the scan plane. Other tricks include speckle reduction, frame average, edge enhancement, and compound imaging. Be wary of rejection which eliminates real solid areas.

Always scan through an area with color on slowly, in two planes, to see if the vessel is in the area of interest (AOI) or *beside* the area instead!

Physics of Resolutions

Sonography physics and instrumentation (SPI) almost always comes back to resolution and how each setting effects one of the resolutions. We optimize by improving resolution!

The following tables are summaries of notes from Miele (2013), Edelman (2012), and Davies (2015). Refer to these while you study physics. Pay attention to the “if, then” relationships. For example, if we increase frequency, then we reduce the wavelength. Keep going. If we reduce the wavelength, then we reduce the spatial pulse length (SPL). Almost there… If we reduce the SPL, then we increase the axial resolution! And there is the gold! These tables help illustrate these relationships for you and also highlight key concepts important for the SPI exam.

Physics Practice Problems (coming soon)

From simple wave equations to complex Doppler calculations, these posts present worked examples and exam-style challenges to build fluency and confidence.

A meticulously arranged study desk dedicated to ultrasound physics, featuring an open textbook filled with Doppler waveforms and annotated diagrams, a spiral notebook with neatly written formulas, and a digital tablet showing a color flow image. A small model of a sound wave diagram etched into a transparent acrylic block sits to one side. The desk surface is a light wood grain, with minimal, tidy accessories. Soft daylight from a nearby window washes across the scene, casting gentle shadows and subtle highlights on the pages. Photographic realism, shot from a slightly elevated angle with moderate depth of field, keeping all educational materials sharp. The overall atmosphere is focused, organized, and academically driven, ideal for serious ultrasound students.

Reference Formula Hub (coming soon)

This area compiles core formulas, clear definitions, and labeled diagrams so students can quickly locate and review essential ultrasound concepts.

View formulas