Friday, March 29, 2013

Yolc in Ultrasound

3D ultrasounds imaging my give additional data to functional Doppler studies of research in developmental anatomy and embryology. This method allows a detailed morphologic and volumetric analysis of extraembryonal static structure. Conventional methods for measuring volume of fluid-filled spaces include modeling of shapes (e.g. using and ellipsoidal approximation).
Using 3D planar mode, the position of the yolk sac wall is accurately spatially assessed. Measurement of the volume, rather than estimation from the simple geometric model, increases the accuracy of the measurement. Growth and appearance of the yolk nsac have been correlated with the outcome of the pregnancy. “Kupersik and Kurjak” measured gestational sac volume and yolk sac volume and vascularity in pregnancy between 5 and 12 weeks of gestation. Regression analysis revealed exponential growth of the gestation volume throughout the first trimester of pregnancy.
With the formation of the extraembryonic celomic cavity at the end of the 4th weeks, the primary yolk sac is replaced with newly formed secondary yolk sac. During organogenesis and before the placental circulation is established, the yolk sac is the primary sources of exchange between the mother and the embryo. It has nutritive, metabolic, endocrine, immunological, excretory and hematopoietic functions. At the beginning of the 5th weeks it become is visible as the first structure inside the chorionic cavity. At this time, a circular, well-defined, eco free area measures 3 to 4 mm in diameter, while the gestational sac measure about 8 to 10 mm. the yolk sac grows sliwly until it reaches a maximum diameter of approximately 5 to 6 mm at 10 weeks. Its stalk can be followed from its origin all the way to the embryonic abdomen. As the gestational sac grows and the amniotic cavity expands, the yolk sac as an extraembryocic structure is gradually separated from the embryo. Different theories exist about the destiny of the yolk sac. Until recently, it was assumed the it gets caught and compressed between amnion and chorion and than ultimately disappears by the end of the 11th weeks.
Recent studies emphasized that instead of getting compressed; the yolk sac degenerates first and than consequently disappears. The ultrasound appearance of the yolk sac has already been proposed as a prognostic parameter for the outcome of pregnancy. Kurjak and coworkers established some criteria for distinguishing between normal and abnormal yolk sac appearance. In their experience, yolk sac should always be visible before viable embryo, it measures 4.0 to 5.0 mm in diameter until 7 to 8 weeks of gestation and reaches 6.0 to 6.5 mm by the end of the 9th week. It is evident that sonographic detection of abnormal yolk sac morphology may predict abnormal fetal outcome. Absence of yolk sac, its abnormal size, echogenicity, shap and number are predictive indicators of early pregnancy failure. All these parameter should be defined and assessed prior to 10 gestational weeks. Abnormal yolk sac size may be the first sonographic indicator associated failure. Primarily, the presence of an embryo without the visible yolk sac before 10th gestational week is mostly an abnormal finding.
According to some authors, the inner diameter of the yolk sac is always less than 5.6 mm in a normal pregnancy before the 10th week of the gestational age. Lyons established that for a mean gestational sac diameter of less than 10 mm, the yolk sac diameter should be less than 4 mm. in 15 patient who had abnormally large sacs, six had no embryo, five aborted spontaneously and only one conceptus survived. Out of nine other with embryo and large yolk sac, eight patients aborted and in one patient trisomy 21 was detected at the 24th gestational week. The yolk sac can be too small, and this is accepted as a marker of poor pregnancy outcome. Green and Hobbins reported a group of patient distributed between 8 and 12 weeks gestational age, with a yolk sac diameter less than 2 mm associated with an adverse outcome.
It is unknown whether abnormalities of the yolk sac are related primarily to the yolk sac or secondary to the embryonic maldevelopment. According to the present data, it seems that the yolk sac plays an important role in materno fetal transportation in early pregnancy. Change in size and shape could indicate or reflect the significant dysfunction of this system, and therefore, could influence early embryonic development.
Currently, major benefits of the sonographic evaluation of the yolk sac are:
1. Differentiation of potentially viable and nonviable gestation
2. Confirmation of the presence of an intrauterine pregnancy against decidual cast, and
3. Indication of a possible fetal abnormally

Tuesday, March 26, 2013

Transducer Basics

The one indispensible part of a diagnostic imaging system is the transducer. transducers come in many shapes, frequency and size.  In this post I will present a basic intuitive model of a piezoelectric transducer to describe its essential acoustic and electrical characteristic. The simples transducer is apiece of piezoelectric material with electrodes on the top and bottom. Unlike the drawing at the top of this figure, the top has across-sectional area (A) and sides that are much longer (>10 X ) than the thickness ( d ) . Piezoelectric material is dielectric; therefore ., it has a clamped capacitance.

Because of forces generated by the transduce, the electrical impedance looking through the voltage terminals is affected. A radiation impedance, ( Z A ) is added to the capacitive reactance so that an equivalent circuit for the overall electrical impedance is

Z T =Z A- i(1/ωC0 ) = R A (f) + I [X A (f) – 1/ωC0

Here Z A is radiation impedance, of which R A and  X A its real and imaginary parts. What  is R A ? to first order, it can be found from the total real electical power flowing into the transducer for an applied voltage (V) and current (I)

W E = II* R A / 2 = │I 2│ R A /2

where current is I = iωQ = iωC 0 V. The total power radiated from both sides of the transducer into surrounding medium of spoesific acoustic impedance , Z C = ρ c A (equal to that of the crystal ) is

W A = ATT’ / (2Z C / A ) = A 2 │F (f)/ A│2 / 2Z C = │hC O V sin ( πf/ f0│2/ 2Z C

Setting the Eqs and equal we can solve R A,

R A (f) = R AC sinc 2 ( f / 2 fo )

Where sin C( x ) = sin ( πx) / (πx) and

RAC = kT   / 4f 0 C 0  = d 2 k 2 T / 2A Ƹ s

The electroacoustic coupling constant is kr , and kr = h / √ C D / Ƹ S , Interesting properties of RAC include an inverse proportionality to capacitance and area of the transducer and a direct dependence on the square of the thickness ( d ). Note that resonance,

RA ( f0) = k 2 / π 2 f o C o

Networks theory requires that imaginary part of an impedance be related to the real part through a Hilbert transform ( Nalamwar and Epstein, 1972)

Saturday, March 23, 2013

Color Doppler Flow and Displacement Imaging


Ultrasonic B-mode real-time imaging can be combined with Doppler in a scanner so that the scanner is capable of providing not only anatomical information but also blood flow data. Both sets of information are displayed simultaneously. A cursor line is typically superimposed on the B-mode image to indicate the direction of the Doppler beam. An FFT algorithm is used to compute the Doppler spectrum that is displayed in real time. This type of scanner is called a duplex scanner; a duplex image is shown in Figure below. Alternatively, blood flow data can be displayed in real time and superimposed with the B-mode image if the data acquisition rate and image processing algorithms are sufficiently fast.

ultrasoud
where the magnitude and phase of H(τ) are an even function and an odd function, respectively. The symbol A is used to represent the magnitude of H(τ) here. where the dot operation represents the first derivative=∂H(τ)/∂τ. Let< ω > denote the mean of ω and, from the definition of mean angular frequency and Equation However, limited by the frame rate and field of view, the pulse repetition frequency in most color Doppler systems is between 8 and 16 kHz. As a result, aliasing frequently occurs with color Doppler in cardiac imaging. To overcome these
problems, one can reduce the image size or use M-mode color Doppler, in which the beam is fixed in one direction. In the heart, the myocardium is in motion during a cardiac cycle. Tissue color
Doppler images of this motion can be acquired with color Doppler methods previously described as well. The difference lies in that myocardial motion is slower than blood flow and myocardial echoes are stronger than blood. The spurious Doppler signals from blood in this case can be eliminated by thresh holding the echoes. A tissue Doppler image of the heart in which the color indicates the velocity of myocardial motion is shown in Figure 6.5. Many clinical applications have been found for color Doppler flow imaging, including diagnosing tiny shunts in the heart wall and valvular regurgitation and stenosis. It considerably reduces the examination time in many diseases associated with flow disturbance. Problematic regions can be quickly identified first from the flow mapping. More quantitative conventional Doppler measurements are then made on these areas. Color Doppler has been now widely used in a variety of medical disciplines; however, it has several shortcomings:

  • Flow perpendicular to the beam cannot be reliably detected.
  • Higher blood flow velocity results in aliasing.
  • Its spatial resolution is poorer than B-mode gray-scale imaging.
  • The mean velocity estimated is the average velocity within a pixel or voxel.
  • Because the color Doppler image is overlaid over the gray-scale B-mode, the overlay process is determined arbitrarily by thresh holding, which may result in vessel wall overwrite obscuring the slow blood flow signal near the wall.
  • Large echoes due to slow moving tissues can cause the “color flash artifact” because they overlap echoes from flowing blood.

Friday, March 22, 2013

Sonographic Diagnostic : Cystic Masses & Physiologic Ovarian Cysts

This discussion of the transvaginal sonographic differential diagnoses of pelvic masses is organized according of the most frequently seen sonographic appearance of particular types of a pelvic mass  . If a particular pelvic mass has a spectrum of sonographic appearances, it is mentioned in more than one category.

This differentiating scheme should be used only as a general approach to the sonographic characterization of a pelvic mass. Sonographic findings must be correlated with the clinical ones. The sonographic depiction of morphology is helpful in determining the change that a mass is malignant. The presences of wall or septal irregularity, or papillary excrescences, correlate with the changes of malignancy.

Cystic Masses

Pelvic masses appearing as cystic adnexal masses on transvaginal sonography most often include physiologic (follicular or luteal) ovarian cysts, hydrosalpinges, endometriomas, and paraovarian cysts. Even with the similar sonography appearance of several types of cystic adnexal masses, the diagnostic possibilities can usually be narrowed to one or two entities based on clinical presentation and evaluation. In general, most cystic masses that arise within the pelvis are of ovarian origin. Depending on the referral population, physiologic ovarian cysts or hydrosalpinges will be the most common cystic pelvic masses encountered by the sinologist.

Physiologic Ovarian Cysts

Since functional cysts are usually asymptomatic, their precise incidence is unknown. They are most common during the reproductive years, but may occur at any age. Several types of cystic masses can result from abnormalities that occur at different stages of folliculogenesis. In general, follicular cysts occur either due to failure of a mature follicle to rupture at any time of ovulation, or following the collection of blood within the follicle after ovulation occurs (corpus luteum cysts). In most women, a mature follicle average size ranges from 15 – 20 mm.7,15 Follicular cysts of the ovary are usually larger than a mature follicle, ranging from 3 to 8 cm in size. Luteal cysts, compared to follicular cysts, usually have a thicker wall and tend to contain hemorrhagic areas. Patient with hemorrhagic cysts may experience the abrupt onset of lower abdominal or pelvic pain Because this story can also pertain o cases of ruptured ectopic pregnancy, it is important to obtain an accurate pregnancy test in phase patient.

Tuesday, March 19, 2013

Color Duplex Imaging Limitation

As a result of the vast amount of information to be processed, color duplex scanning has a much poorer spatial and temporal resolution than pure B-mode imaging. Axial resolution is proportional to the wavelength in B-mode imaging while it is dependent on the number of sample volumes placed along the color Doppler scan line in the color duplex mode. The use of smaller sample volumes improves axial resolution but at the expense of sensitivity and accuracy in Doppler shift evaluation as the signal-to-noise ratio deteriorates. Lateral resolution in color-coded duplex scanning is determined by the number of color Doppler lines processed per centimeter. The frame rate decreases as the number of Doppler lines increases, resulting in a lower temporal resolution, in particular at greater scan depths. As a result of these limitations, the axial resolution of color duplex ultrasound on the order of 0.4–1.0 mm with a lateral resolution of only 1.0–2.0 mm, which is 4–10 times lower than B-scan resolution (Widder 1995). The frame rate in the color duplex mode ranges from 50–200 ms (corresponding to an image repetition rate of 5 Hz), depending on the scanning depth and size of the color box. When a low pulse repetition frequency is selected, the speed at which the color Doppler scan lines sweep the sector is similar to or slightly below the mean flow velocity in arteries. Therefore, a single ultrasound scan may simultaneously depict systolic flow (e.g. displayed in red) and early diastolic flow(e.g. displayed in blue) . Due to the low temporal resolution, however, the color coding does not fully reflect the pulsatile character of flow. Slow flow produces smaller Doppler frequency shifts, which have to be extracted from short echo pulse packets for each scan line consisting of a number of individual pulses. The scan lines must be processed successively. Though the insonation angle should ideally be as small as possible for optimal Doppler scanning, this is not always practical because there will be a longer delay when the color box is tilted (beam steering). This is why one must find a compromise, in particular when examining deeper vessels. Tilting the color box 20° and 30° prolongs the echo arrival time by 13% and 31%, respectively.

Color duplex imaging, like all diagnostic ultrasound techniques, is impaired by scattering and acoustic shadowing caused by bowel gas or calcified structures (bone or calcified plaques on vessel walls). The examiner can circumvent such interfering structures by moving the transducer, but this is frequently achieved only at the cost of a longer echo arrival time due to a greater distance from the structure of interest. Strong reflectors that are oblique to the beam axis act like mirrors and generate phantom images in another area of the scan. Such mirror images can be identified by angling the transducer, which will make the mirror artifacts disappear or appear in a different location. When the ultrasound beam strikes interfaces of high acoustic impedance at a right angle, reverberations (repeat echoes) may occur with the ultrasound pulses being reflected to and fro, resulting in a kind of ping-pong effect. Slight angulation of the transducer prevents reverberations but will also reduce reflection from the interface and thus degrade image quality.

Color Doppler, Tissue Doppler & Multigate Doppler

Tissue Doppler & Multigate Doppler

In the Doppler signal processing chain  , if only large amplitude echoes of lower Doppler frequencies from tissues such as myocardium or heart muscle are retained and low amplitude echoes of higher Doppler frequencies are suppressed, the motion of the tissues can be monitored. This will be touched upon again in the next chapter. An amplitude threshold can be set to allow only the larger echoes to pass through. Tissue Doppler has been proven a clinically useful tool for assessing the state of myocardium. In conventional pulsed Doppler, only one gate is used to measure blood flow within the sampled window or sampling volume confined by the beam width and the gate duration. If blood flow velocities at multiple points along the ultrasound beam need

Doppler Flow Measurements to be measured, pulsed Doppler flow meters with multiple gates (e.g., 8 or 16 gates) have been developed. These devices allow the measurement of velocities in real time across the lumen and thus have been used frequently to determine the blood flow velocity profile in arteries.

Color Doppler

A new way of displaying color Doppler information, i.e., “power mode” or “energy mode” imaging, has been introduced to minimize some of the color Doppler problems (Rubin et al., 1994; Zagzebski, 1996) and has been well accepted by the clinical community. All top-of-the-line scanners now have this option. Instead of the mean Doppler shift, the power contained in the Doppler signal is displayed in this approach. There are several advantages to doing so:

• A threshold can be set to minimize the effect of noise.

• The data can be averaged to achieve a better signal-to-noise ratio.

• The images are less dependent upon the Doppler angle.

• Aliasing is no longer a problem because only the power is detected. As a result, signals from blood flowing in much smaller vessels can be detected. The images so produced have an appearance similar to that of x-ray angiography.

Monday, March 18, 2013

Application of 3D and 4D ultrasound for Evaluation Abnormal Pregnancy

The first trimester is characterized by many important landmarks with regard to ultimate outcome of pregnancy and is mostly defined by the first 100 days pregnancy. Women becomes aware of her pregnancy after missing her last period and in that time she is already at least weeks pregnant. A positive pregnancy test opens Pandoras box offering more questions than answers. Ultrasound evaluation of an early pregnancy includes detection of the pregnancy location (extra uterine or intrauterine, the type of pregnancy, one fetus or multiple pregnancy, molar pregnancy), the viability of the pregnancy and establishment of gestational age. Evaluating pregnancy, the ultrasonographer also recognizes the complications that may occur in first trimester. Ultrasound examination has become the golden standard in follow up the development and complications of early pregnancy. With introduction of transvaginal sonography (TVS), a possibility for early morphological and biometrical ultrasound examinations has been significantly improved. Application of color Doppler ultrasound has enabled functional hemodynamic presentation and evaluation soon after implantation .basic ultrasound markers for normal pregnancy are intrauterine gestational sac, morphologically, normal embryo and its heart action. Normal embryonic echo in 90% of the cases suggest normal pregnancy. Application of 3D and 4D ultrasound seems to be advantageous in determining the point for differentiation of the embryo and the fetus.

Early Pregnancy Failure
Early Pregnancy Failure is defined as a pregnancy that ends spontaneously before the embryo is detected by ultrasound at the gestational age in which visualization of viable embryo should occur . the most common pathogical symptom of the early pregnancy failure is the vaginal bleeding.

One of main problems in diagnosis of early pregnancy failure is why vaginal bleeding occurs. When it happened, all clinicians should answer several questions that can radically alter the management :

  • Is the patient pregnant?
  • Is the embryo viable or not?
  • What is gestational age?
  • Is there any evidence to suggest that the pregnancy is ectopic?
  • If an abortion occurs, is it complete or incomplete?

Is there any associated pelvic mass?Only differentiation and accurate estimation of the pregnancy status and embryo/fetus status make it possible to obtain appropriate therapeutic measures to cases where a normal outcome of the pregnancy can be expected. At this moment, ultrasonography is considered to be the best diagnostic method for detection of early pregnancy complications. For these patients the skill of the ultrasonographer is very important, since the accurate diagnosis of pregnancy failure will often result in surgical intervention. clinical presentation of the symptom such as vaginal bleeding and abdominal paint, with or without the expulsion of the product of conception is suspected of a spontaneous abortion. For ultrasound evaluation, it is important to distinguish threatened, complete and incomplete abortion

Safety of Routine Ultrasound

 

Diagnostic ultrasound of the developing fetus has largely been considered safe without apparent deleterious effect. The potential teratogenecity of sound energy conversion to thermal energy and mechanical bio effect of cavitation's have not been proven or ascribed to diagnostic ultrasound.The American Institute of Ultrasounds(AIUM) 1998 conference on mechanical; bioeffect encouraged continued research regarding ultrasound safety, especially in tissues with known gas . i.e. lung and intestine. The conference did conclude there is no known risk of lung or intestinal hemorrhage in the fluid filled human fetal lung or intestine that exposed to diagnostic ultrasound during routine obstetrical examination. In, the AIUM stated although there are no confirmed biological; effect from ultrasound at the present time, the possibility exist that such as biological effects may be identified in the future. Other researcher about it: the American College of Obstetrics and Gynecology (ACOG): does not support. The US Preventive Services Task Force, Randomized Controlled Trials of Routine Ultrasound An early prospective randomized trial of routine obstetric ultrasound was performed by Bennet et al in London, in study, 1062 women underwent routine ultrasound at16 week menstrual age. The fetal biparietal diameter BPD was measured and dates corrected in 25% of the patients, mostly due to an overestimation from the last menstrual period. In response to the 1984 NIH consensus Conference, the routine Antenatal Diagnostic Imaging with Ultrasound (RADIUS) trial was created as the largest prospective randomized controlled trial routine versus selective antenatal ultrasound. The conclusion that routine mid trimester ultrasound is an effective diagnostic tool. Establishment of accurate gestational age, detection of fetal anomalies and multiple gestations and reduction are proven benefits of routine ultrasound. The benefit burden calculus of routine antenatal ultrasonography support its use and fulfillment of ethical principles of beneficence and respect for patients autonomy (Geeta Sharma, Stepen T Chasen , Frank A Chevernak)

Wednesday, March 6, 2013

Fetal Posture Presentation by Ultrasound

fetal structureThroughout normal pregnancy the fetus swims freely in amniotic fluid, being able to assume any position, presentation, or posture necessary to adapt to intrauterine environment. Fetal activity, an event of little consequence in the absence of placenta prevail, uterine anomalies or fetal malformations, can be easily observed with 2D, 3D and especially by using 4D. The latter allows the visualization of opening the mouth and other subtle movement of special interest in cases of some malformations like arthrogryposis multiple congenital.

Between weeks 28 and 32 pregnancy the fetus adopts a longitudinal position due to the change in uterine shape from spherical to avoid (Reynolds transformation phenomenon). Most fetuses will acquire a chepalic presentation, which is likely to remain up to the time of delivery. A few fetuses will be in breech presentation and this may even during labor.

Most fetuses re in attitude of flexion so that the chin appears to rest in the upper chest, the arms are folded, and the legs somewhat bent at the knees and crossed at the ankle level. Because of this position , it is difficult to observe the face prior to 12 or 14 weeks of gestation. Beyond this gestational age it becomes easier to observe the fetal attitude and features with 2D and even more so with 3D. Beyond the third trimester the fetus has features and attitudes that are identical to those of a child. some fetuses seem to < refuse to be photographed>(figure) allowing to observe their posture details , fingers, hands and articulation. These details are important for the evaluation of malformations that involve the extremities, a subject to which we have dedicated full post.

In our experience, 3D –4D imaged if the fetus are much realistic and of better quality than 2D images and allow clearer visualization of the details that are necessary to arrive at a more accurate prenatal diagnosis of fetal malformations