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Posts Tagged ‘medical ultrasonography’


Cardiovascular Imaging

Author: Justin D. Pearlman, MD, PhD, FACC

 

WORK IN PROGRESS

Multiple physical means are used for cardiovascular imaging: ultrasound, electromagnetics, radioactive agents, xrays and optical.

Ultrasound vibrations pass waves through tissue by exciting an array of piezo-electric crystals (e.g., lead zirconate) with alternating current. The speed of sound wave propagation through tissue is ~1540 meters/second, and the frequency of vibration used for imaging ranges 1-20 Megahertz (million cycles per second). Higher frequencies can report finer details but they attenuate faster (signal drop out with depth).  Sound waves are reflected back by tissue interfaces that represent a change in sound transmission impedance.

In its simplest mode, called A-MODE, the amplitude of return signal is displayed as a function of time. The A-MODE signal for a beam of sound passing through the center of an orange immersed in water will return an A-MODE signal peak for the time of the round trip to the first contact with the orange, another for transition from orange peal to the inner pulp, and another pair for the pulp to peel and peel to exodus at the far side of the orange. Immersion in water solves the problem of 99% attenuation (stoppage) of sound propagation at an air-fluid interface (very high impedance). Thus the distance between the first two peaks reports peel thickness, and the inner distance between the paired peaks from peel report the inner diameter of the orange.

The next more advanced mode of ultrasound imaging, called B-MODE, converts the amplitude of returned sonic energy to a bright dot, so that the display of data from a linear probing of tissue by sound wave reflections is reported on a screen as a line with bright spots for the crossing of each tissue interface. Multiple B-MODE displays can be combined at different positions corresponding to the angle and position of the sonic probe to construct composite mode of display, called C-MODE. Aimed at a pregnant abdomen, the result shows contours of fetal head and limbs, which produced the first dynamic fetal two-dimensional images. The excitement generated a mechanical mimic of a sweeping series of angulations, called SECTOR SCANNING. In sector scanning, the sonic probe pathway sweeps back and forth like a windshield wiper, to produce a triangular 2D image called a sector.

With the invention of phased arrays of sonic crystals, a combination of small sources staggered in time produces a composite sonic beam that is electronically steered by adjusting the timing of activation, so that sector scanning can be accomplished electronically without mechanical moving parts. A linear array is now the preferred means to perform sector scanning. Furthermore, with a 2D phased array, the sonic beam can be swept both vertically and horizontally, to build 3D images (4D if you include the time dimension). Current clinical ultrasound uses phased arrays, sector scanning, and optional 3D views that are acquired within one heartbeat (one cardiac cycle).

Johannes Doppler described a shift of phase due to motion of a wave sensor: if the detector is moving towards a source of sound, it will pick up more peaks per second the faster it approaches. The difference between the number of peaks per second (frequency) with the sensor not moving, and the higher frequency observed with the sensor approaching the source at a velocity V,  is called frequency shift (Doppler shift). The frequency shift is proportionally to the rate of approach (component of velocity towards the source of sound waves).  With red blood cells moving through heart valves. Doppler shift reports their velocity towards the sound wave sensor (the ultrasound transducer). The kinetic energy is easily derived (KE=½ m v2), and the pressure drops across a valve or a vessel narrowing (stenosis), which is substantially from the change in kinetic energy, is thus estimated by 1/2 m V2 = 4 V2 for blood cells reporting mmHg pressure change.

Color Doppler display of ultrasound modifies gray images by applying a red-blue scale to image data according to the frequency shift, with red indicating velocity towards the transducer, and blue away. Thus blood movement can be visualized within the heart, as color contained in the gray walls.

A simple graphic plot of computed velocity versus time for a linear beam of ultrasound is called “continuous wave”. The maximum height on such a plot reports the maximum pressure gradient observed along the beam. Thus continuous wave Doppler can find the peak gradient across a narrowed (stenotic) aortic valve to help determine the severity of narrowness. Seriously small aortic valve area with an otherwise normal heart, meriting valve replacement surgery even at age 90, generally produce a peak gradient well above 50 mmHg.

It is tempting to correct the amplitude according to the cosine of the angle of the beam to the direction of blood flow, and many textbooks and ultrasound machine manufacturers manuals have incorrectly recommended that. The error is that the red blood cells reporting velocities are individual, so the sonic image is a bit like an image of a swarm of bees. The individual elements have different directions of motion, so changing angles to see less of some sees somewhat more of others, making the fall-off with angulation less than the cosine law predicts (the cosine rule overcompensates).

The Doppler interrogation may be “pulsed” (briefly “on” then “off” in a repeated cycle separated by a time gap). Then the return signal shift will represent the velocity corresponding to the timed round-trip transit, or a multiple thereof. Thus, the velocity sampling can be limited to a specified sampling location depth (and multiples thereof) rather than summarize the peak of all velocities encountered along the entire beam transit. By setting the depth to the visualized depth of the aortic valve, velocity can be sampled there, then the depth can be reduced to sample the velocities in the outflow track just before the aortic valve, for comparison. The change in estimated kinetic energy estimates the pressure drop due to the valve stenosis (narrowness).

Estimation of the peak velocity of blood leakage through an austensibly closed tricuspid valve reports the pressure drop across the tricuspid valve, i.e., pressure drop from the right ventricle (RV) to the right atrium (RA) during peak right ventricular contraction is estimated from the peak velocity of tricuspid valve regurgitation. If the RA pressure is assumed to be 10 mmHg and the 4 V2 reports a pressure stop of 50 mmHg, then the systolic pressure in the RV is estimated to be 50 mmHg. In systole (contraction time), the pulmonary valve (PV) is open, so if that is a normal passage, then there will be no significant step across the PV and the pulmonary artery peak systolic pressure will be well represented by the RV peak systolic pressure. High pulmonary pressure values impair blood delivery from right to left, and if not treated, can lead to death within 2 years. A silent killer more easily missed than arterial pressure, because the pulmonary pressure is not accessible by a blood pressure cuff.

 

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Author and Curator: Dror Nir, PhD

 

That is the question…

We are all used to clichés such as “seeing is believing”, “seeing is knowing”, “don’t be blind” and so on. Out of our seven (natural and supernatural) senses we tend to use and trust our eyes the most. Especially, when it comes to learning, accumulation of experience and acceptance of information as correct. On the other hand, we are taught from childhood to be aware of illusions and not to judge according to looks but rather according to matter. The problem is, does one recognise the substance inside an image? To answer this, a wide-ranging discipline of image interpretation was developed alongside with imaging technology. In order not to fatigue the innocent reader, I’ll review the state of the art of imaging in medicine in subsequent posts, each dedicated to a specific modality. This post is dedicated to…

Current main trends in ultrasound imaging in cancer patients’ management;

The most used imaging modality in medicine is ultrasound. This is due to the fact that it is noninvasive, practically harmless, relatively inexpensive and fairly accessible; i.e. everyone can operate it, even a layman! No formal training or certification is required!

Interesting enough, ultrasound is labeled by the regulatory agencies, FDA and CE, as a diagnostic medical device! This is real demonstration of the aforementioned tendency to believe our eyes, even if these eyes do not see well or the brain behind them is lacking the experience required for ultrasound image interpretation.

Since “ultrasound imaging in medicine” is the subject of many text books and articles I found it  appropriate, for the sake of this post, simply  to refer the reader to Wikipedia’s page (http://en.wikipedia.org/wiki/Medical_ultrasonography) on ultrasound in medicine: “Diagnostic Sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendonsmuscles, joints, vessels and internal organs for possible pathology or lesionsObstetric sonography is commonly used during pregnancy and is widely recognized by the public. In physics, the term “ultrasound” applies to all sound waves with a frequency above the audible range of normal human hearing, about 20 kHz. The frequencies used in diagnostic ultrasound are typically between 2 and 18 MHz.”

When it comes to cancer patients’ management, ultrasound provides real-time imaging of body organs at a relatively cost effective workflow. However, it suffers from lack of sensitivity and specificity, especially if the investigator is still fairly inex­perienced. Therefore, no diagnosis is confirmed without biopsy of the suspected lesion discovered during the ultrasound scan. As mentioned in my previous post; identification of suspicious lesions in the prostate during TRUS is so inconclusive that in order to reach diagnosis biopsies are taken randomly.

Did we hit the target?

To improve prostate cancer detection, various biopsy strategies to increase the diagnostic yield of prostate biopsy have been devised: sampling of visually abnormal areas; more lateral placement of biopsies; anterior biop­sies; and obtaining an increased number of cores, with up to 45 biopsy cores [1-5].

In recent years, new features such as 3D and contrast-enhanced sonography, elastography and HistoScanning were added to the basic video image in order to improve the quality of ultrasound based investiga­tion of cancer patients.

3-D Sonography.

3-D ultrasound allows si­multaneous biplanar imaging of the organ with com­puter reconstructions providing a coronal plane as well as a rendered 3-D image. This promises to improve the detection and pre-clinical grading of cancer lesions. Still, the interpretation is very much “image quality” and “user experience” dependent.

3D imaging of breast using ABUS by Siemens; using the coronal view to better investigate a lesion.

  

 

3D imaging of breast using Voluson 730 by GE; three planes are presented for review by the radiologist.

 

 

 Contrast-Enhanced Sonography.

Using intravenous micro-bubble agents in combination with color and pow­er Doppler imaging contributes to increase in the signal obtained in areas of increased vascularity. The underlying assumption is that vascularization in the tumor’s area will be more pronounced than in normal tissue. Hot off the press: The UK National Institute for Health and Clinical Excellence (NICE) has published guidance that supports the use of contrast-enhanced ultrasound with Bracco’s SonoVue ultrasound contrast agent for the diagnosis of liver cancer [6].  The main use of contrast-enhanced ultrasound is directing biopsies to the “most suspicious” areas; i.e. those who presents higher vascularity. Never­theless, in reported clinical studies [7] targeted biopsies’ sensitivity on contrast-enhanced ultrasound was only 68%.

 

Elastography.

Elastography is an imaging technique that evaluates the elasticity of the tissue. The underlying assumption is that tumors present greater stiffness than normal tissue and therefore will be characterized by limited compressibility. The first person to introduce this concept was  Professor Jonathan Ophir, University of Huston, Texas [http://www.uth.tmc.edu/schools/med/rad/elasto/]:
Estimation of differences in lesions’ stiffness relies  on computing the level of correlation between consecutive imaging frames while the tissue that is being imaged is subjected to changing compression, usually applied by the sonographer who manipulates the ultrasound probe. Since malignant and benign lesions exhibit similar elasticity, elastography is not suitable for lesion characterisation. Therefore, as in the previous example, elastography’s main use is identifying suspicious areas in which to take biopsies [8, 9].  Furthermore, users’ experiences related to elastography reveal a lot of controversy.  For example, according to Prof. Bruno Fornage of MD. Anderson [http://www.auntminnie.com/index.aspx?sec=sup&sub=wom&pag=dis&ItemID=99028]; “current commercially available scanners are confounded by a lack of intraobserver reliability, so that it’s not unusual to produce an opposite result on repeat testing a few seconds later”. “There are very few evidence-based non-industry sponsored studies reporting substantial superiority [of elastography] over standard grayscale ultrasound,” he said. “In fact, a sensitivity of 82% in the diagnosis of breast cancer has been reported for elastography, versus 94% for conventional grayscale ultrasound. More disturbing is that even if the technology of elastography worked flawlessly, the huge overlap in breast pathology between very firm solid benign lesions and less firm malignancies gives this technology no practical place in the differential diagnosis of solid breast masses.”

 

HistoScanning.

HistoScanning™ is a novel ultrasound-based software technology that utilizes advanced tissue characterization algorithms to address the clinical requirements for tissue characterization. It visualizes the position and extent of tissue suspected of being malignant in the target organ. In this respect its design is unique and superior to other ultrasound based-technologies [10, 11]. HistoScanning’s first clinically available application (since 2009) is in the management of prostate cancer patients.

 

 

HistoScanning indicating suspicious lesions superimposed on 3-D ultrasound of the prostate. The three imaging plans and 3D reconstruction of the segmented prostate are presented.

 

 

 To conclude; if we are looking to improve the current state of the art in ultrasound-based cancer patients’ management we should strive to introduce systems which will enable the medical practitioners to rule in or rule out suspicious lesions at imaging before they biopsy them. Using ultrasound just as a tool for directing biopsies as done today is not enough. Indeed, this requires capability of ultrasound-based tissue characterisation in addition to detection of ultrasound-based abnormality (i.e. circumstantial evidence for cancer). To-date, the only available system that bears the promise to provide such improvement is HistoScanning. Obviously, the level of confidence in the Negative Predictive Value of HistoScanning and future systems alike must be built to become high enough to provide the medical practitioner the reassurance and comfort that he is not missing any significant cancer by not taking a biopsy. Such confidence can only be built by subjecting these systems (i.e. HistoScanning and alike) to properly designed clinical studies and, not less important, by reporting the experience of early adopters who will test them in a controlled routine use.

 

References

  1. Flanigan RC, Catalona WJ, Richie JP, Ah-mann FR, Hudson MA, Scardino PT, de-Kernion JB, Ratliff TL, Kavoussi LR, Dalkin BL: Accuracy of digital rectal examination and transrectal ultrasonography in localiz­ing prostate cancer: results of a multicenter clinical trial of 6,630 men. J Urol 1994; 152: 1506–1509.
  2. Eichler K, Hempel S, Wilby J, Myers L, Bach­mann LM, Kleijnen J: Diagnostic value of systematic biopsy methods in the investiga­tion of prostate cancer: a systematic review. J Urol 2006; 175: 1605–1612.
  3. Delongchamps NB, de la Roza G, Jones R, Jumbelic M, Haas GP: Saturation biopsies on autopsied prostates for detecting and charac­terizing prostate cancer. BJU Int 2009; 10: 49–54.
  4. Rifkin MD, Dähnert W, Kurtz AB: State of the art: endorectal sonography of prostate gland. AJR Am J Roentgenol 1990; 154: 691– 700.
  5. Chrouser KL, Lieber MM: Extended and sat­uration needle biopsy. Curr Urol Rep 2004; 5: 226–230.
  6. http://www.auntminnieeurope.com/index.aspx?sec=nws&sub=rad&pag=dis&ItemID=607068&wf=284
  7. Yi A, Kim JK, Park SH, Kim KW, Kim HS, Kim JH, Eun HW, Cho KS: Contrast-en­hanced sonography for prostate cancer de­tection in patients with indeterminate clini­cal findings. Am J Roentgenol 2006; 186: 1431–1435.
  8. König K, Scheipers U, Pesavento A, Lorenz A, Ermert H, Senge T: Initial experiences with real-time elastography guided biopsies of the prostate. J Urol 2005; 174: 115–117.
  9. 32 Pallwein L, Mitterberger M, Struve P, Hor-ninger W, Aigner F, Bartsch G, Gradl J, Schurich M, Pedross F, Frauscher F: Com­parison of sonoelastography guided biopsy with systematic biopsy: impact on prostate cancer detection. Eur Radiol 2007; 17: 2278– 2285.
  10. SALOMON (G.), SPETHMANN (J.), BECKMANN (A.), AUTIER (P.), MOORE (C.), DURNER (L.), SANDMANN (M.), HASE (A.), SCHLOMM (T.), MICHL (U.), HEINZER (H.), GRAFEN (M.), STEUBER (T.).Accuracy of HistoScanning for the prediction of a negative surgical margin in patients undergoing radical prostatectomy. Published online in British Journal of Urology International (BJUI). 09/08/2012.
  11. SIMMONS (L.A.M.), AUTIER (P.), ZATURA (F.), BRAECKMAN (J.G.), PELTIER (A.), ROMICS (I.), STENZL (A.), TREURNICHT (K.), WALKER (T.), NM (D.), MOORE (C.M.), EMBERTON (M.).  Detection, localisation and characterisation of prostate cancer by Prostate Hist°Scanning; Published in British Journal of Urology International (BJUI). Issue 1 (July). Vol 110, P 28-35.

 

 Written by Dror Nir

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