Medical Imaging 101 pt 5: US

Here’s part 5 in my medical imaging series. I was going to follow up the PET post with SPECT but ultrasound was closer to home this week. You’ve probably heard of sonar to detect objects underwater, e.g., submarines, mines, torpedoes, etc. You probably know that sonar uses sound waves and that some marine mammals also use sonar or echolocation. For medical imaging, why do we need ultrasound (US), i.e., high frequency? The wavelength (lambda) is related to the frequency (f) and velocity (u) as follows: lambda = u/f. In order to measure objects in the millimeter scale, we need a frequency in the MHz range, i.e., ultrasound. The frequency is inversely proportional to the wavelength.  For medical imaging, 1 to 15 MHz are used. I don’t use US so I’m not as familiar with it. Also this post is for Peo, who got sick and missed trick-or-treating on Halloween.

Typical velocities [m/s] and impedances [g/cm^2-s]:

Water 1496 and 1.49 x 10^5

Fat 1476 and 1.37 x 10^5

Muscle 1568 and 1.66 x 10^5

Bone 3360 and 6.20 x 10^5

Air 331 and 4.13

Physics for raw data acquisition

Most US systems use a transducer to transmit and receive. A piezoelectric device is used in the transducer. A material that has piezoelectric  properties, deforms when a voltage is applied and can change the voltage of a circuit if it is deformed. So you can send a pulse of voltage changes, in the MHz range, and the transducer will pulse like a small speaker. The sound waves will propagate and reflect differently depend on what tissue is encountered. The transducer then will pulse in response to the reflected waves. Think of it as a speaker during transmission and as a microphone during reception. Gel is often used to “couple” the transducer to the imaging surface because sound travels faster through the gel rather than air. Also, the angle of the transducer plays a role and some skill is required to obtain good images. Higher frequencies are used for higher spatial resolution (recall the relationship between wavelength and frequency). However, this is at the expense of depth of penetration.

Contrast

Recall with CT, we get contrast due to the differences in densities, e.g. muscle vs. bone. With MRI, we get contrast due to differences in relaxivity and water content of tissues. For US we get contrast due to differences in impedance to the sound waves. For injectable contrast agents, microbubbles are used.  In the two microbubble images below, the microbubbles are used for both contrast and therapy. In the cartoon, you can see how the US is used to not only break the microbubbles, releasing the drug, but also to cause microporation of the vessels so that the drug can distribute locally, e.g. where the operator targets. In the other image, you can see the drug release after a pulse from the US system.

How is the 3D image made?

The majority of US images are 2D. 3D images can be obtained by taking multiple US images at different angles of the transducer position. The images can be processed similar to CT.

Strengths

Ultrasound’s strengths are its portability, lack of radiation, and relatively low expense.

Weakness

Ultrasound’s weaknesses are its relatively low spatial resolution (compared to MRI and CT) and relatively lower contrast compared to the other modalities discussed so far in this series.

Something unique: FUS, Duplex, and Photoacoustic

Focused ultrasound (FUS) can be used for therapy and/or imaging. Higher energy US can be used with a focused beam, rather than the typical “fanned” out beam. This is typically done with concave transducer, rather than a flat transducer. The beams can be focused on a tumor, for example, to heat up the tumor. That’s called thermal ablation. It can be combined with MRI for guidance. Philips makes a HiFU/MRI system for treating uterine fibroid tissue.

Duplex is when US is combined with Doppler technology. You can Google the Doppler effect if you aren’t familiar with it but I’ll give you a hint to refresh your memory: train whistle. In a duplex system, the sound waves can calculate blood flow, for example, in addition to imaging the heart or blood vessels.

Photoacoustic imaging uses a laser to heat the tissue to be imaged. Some of the energy will be converted to an ultrasonic emission due to the expansion of the tissue. The US part receives those waves and an US image can be obtained. The amplitude of the waves can be correlated to physiological properties such as oxygenated vs. deoxygenated blood.

Example

Last month we found out that my dog has a heart murmur. We just recently took her to have an echocardiogram, aka, ultrasound of her heart. In this YouTube video, you can see her heart along with blood flow via the Doppler (duplex) mode. She has 1% backflow in her mitral valve which is nothing to worry about for now.

Ana echocardiogram 2013

The movie formatting wasn’t that good so I quickly collated them. Sorry the quality and editing isn’t so good.

References:

Medical Instrumentation: Application and Design 3rd Ed.

John G. Webster, Editor

Ultrasound microbubble technology targets localized drug delivery

http://goo.gl/raIobb

Philips and GlyGenix Therapeutics team up to research ultrasound-mediated gene therapy

http://goo.gl/LV7eg9

#CHMedicalImagingSeries #ScienceSunday