Sistem Parametrelerinin Ultrasonik Medikal Görüntüler Üzerindeki Etkilerinin İncelenmesi

Abstract

1877 yılında Fransız fizikçi Pierre Curie’nın piezoelektrik kristalleri keşfi ile başlamış olan Ultrason un gelişimi; 1930 larda tıp alanında yaygın bir şekilde kullanılmaya başlanması ile ilerlemiş ve ultrason günümüz modern görüntüleme teknikleri arasında yerini almıştır. Ultrasonografik görüntülemenin; non-invasiv dediğimiz; girişim içermeyen bir görüntüleme tekniği olması; radyasyon içermemesi; hava ihtiva etmeyen (mide; akciğer hariç) tüm organların görüntülenmesinde kullanılabilmesi gibi avantajları görüntülemenin sıklıkla ve hastaya ekstra riskler içermeden yapılabilmesi imkanını sağlamaktadır. Günümüzde ses dalgaları; piezoelektrik kristallerin elektriksel olarak uyarılması suretiyle belirli bir frekans bandı aralığında üretilmektedir. Ses dalgalarının frekansı; görüntülenmek istenen organların akustik empedansları ve bu parametrelerin birbirleriyle olan ilişkileri; ultrasonografik görüntülemedeki karşılaşılan sınırlamalar olarak karşımıza çıkmaktadır. Optimum görüntüler elde edebilmek için bu ilişkileri dolayısı ile ultrason ve doppler fiziğini bilmek gerekmektedir. Benzer bir şekilde görüntüleme için kullanılmakta olan dönüştürücülerin yapıları ve şekilleri de elde edilen görüntülerin kalitesi ile yakından bağlantılıdır. Burada en önemli parametre kullanılmakta olan piezoelektrik kristallerin dizilimleri ve şekilleri olarak karşımıza çıkmaktadır. Bu bağlamda; ultrasonografik görüntülemede limit olarak görünen dönüştürücüden gelen bilgi miktarını arttırmak ana araştırma konusu olarak karşımıza çıkmaktadır. Son yıllarda özellikle silikon bazlı komponentlerle piezoelektrik kristallerin yer değiştirmesi üzerine çalışılmaktadır. Diğer üzerinde çalışılan konular ise; dinamik bir inceleme olan ultrasonda olabilecek kullanıcı hatalarını minimuma indirmek ve ultrasonografinin tanısal özelliğini arttırmanın yanında, tedavi amaçlı kullanılabileceği uygulamalarında geliştirilmesi şeklinde özetlenebilir. Hazırlanmış olan Yüksek Lisan Tezi nin son kısmında ise; CIRS marka bir görüntüleme fantomu kullanılmak suretiyle; dönüştürücü şekillerinin; sistemlerde kullanılan kazanç ve zaman kazanç dengeleme ayarlarının; kullanılan frekansın; odak nokta ve sayısının ve dokuların akustik empedansının görüntüye etkileri incelenmiştir. Doppler uygulamaları ile ilgili olarak da; doppler açısının; darbe yenileme frekansının etkileri pratik uygulamalar ile gösterilmeye çalışılmıştır. Sonuç olarak; ultrasonografik görüntülemenin teorik bilgileriyle; pratikteki uygulamaları birleştirmek ve gerçek hayattaki kullanımda; örneğin renkli doppler görüntüleme de açının doğru olarak ayarlanması vb. gibi; kullanıcılar tarafından nedeni ve etkileri bilinerek yapılması gereken ayarlarının elde edilen görüntülerin kalitesinde; dolaylı olarak da tanısal hassasiyette ne derece önemli olabileceği vurgulanmaya çalışılmıştır.

In 1877, French physicist Pierre Curie discovered piezoelectric crystals; that is also acknowledged as the beginning of the story of the diagnostic ultrasound. In the 1930 s ultrasound became a widely used tool in the medical field. On the other hand; the first physician that has used ultrasound as a diagnostic device was Karl Dussik at 1940. Dussik has successfully located the brain tumors and cerebral ventricles by using ultrasound waves. In 1948; a technological milestone; B-mode imaging has been invented by Douglas Howry. Starting with the invention of CW Doppler by Robert Rushmer, Dean Franklin and Don Baker at 1956; ultrasound has been rapidly developed. Late 1960 s and the beginning of 1970 s was the years that echocardiography, PW Doppler was invented and ultrasound become one the indispensable imaging modalities of today s diagnostic medicine. Today ultrasound is used for a wide variety of examinations of different organ of the human body. The only limitation is the lack of imaging of the organs which has air inside; such as stomach and lungs. Except this limitation ultrasound is an imaging modality which is non-invasive; radiation free and easy to perform in every circumstances. One other advantage of the ultrasound is the cost of the ownership compared with other imaging modalities. An ultrasound systems contains four modules. These are the transducer; central processing unit; monitor and optional print out units. The most important unit among of these four is the transducer that generates the ultrasound waves via piezoelectric crystals. Piezoelectric crystals start to resonance whenever there is a voltage applied to them. They also have the ability to convert any pressure applied to them to electric signals. Basically an ultrasound wave at special frequency is generated by applying voltage to the piezoelectric crystals that is located in the transducer. These waves propagate through human body by passing different tissues. During this propagation some of these waves are scattered from different tissues and these scattered and reflected waves return back to transducer and apply pressure to same piezoelectric crystals that is used to generate the ultrasound signal at the first place. Each tissue has a different acoustic impedance value that is directly related with the speed of the sound wave and the density of the tissue. The frequency of the ultrasound waves is an important parameter that effects of the resolution and penetration capabilities. Higher frequency corresponds with lower penetration but higher resolution. Lower frequency corresponds higher penetration but lower resolution. Axial and lateral resolution is directly related with the frequency. There are five different imaging modes. These are A (amplitude)-mode; B (brightness)-mode; M (movement) - mode; Doppler mode and 3D/4D mode which improved dramatically in the near past. A-mode is based on imaging the amplitude of the reflected wave on time domain. This mode is one of the first modes that has been used for imaging; but as of today there is no practical advantage to any other modes. So it is no more used. B-mode image is obtained by giving different gray levels to different intensities of reflected waves and combining of all informations from different piezoelectric crystals to get a 2D image. This mode is the gold standard on ultrasound imaging. But on the other hand the movement of the organs like hearth is also very important to diagnose certain illnesses that points out the M-mode. M-mode is used for to observe the differences of B-mode images in time domain. Doppler mode is mainly to examine the hemodynamic in human body. The speed of the blood flow in hearth or vessels is examined via Doppler mode which also contains three sub groups as CW (continuous wave); PW (pulsed wave) and color Doppler imaging. All of these hemodynamic measurements are based on using Doppler effect to determine the speed and direction of the flow in the specific organ. The last imaging mode is 3D/4D which is basically constructing volume image by using a set of B-mode images. Today s technology enables to obtain these volume images close to real time; so new terminology of 4D imaging has been started to be used especially on obstetric imaging. As it is stated before the most important component of an ultrasound system is transducer. We can easily group the transducers according to the crystal arrays or the physical shapes of the transducer. Considering the crystal arrays there are three sub groups which are linear; annual and phased arrays. Linear arrays; has crystals that has isolated from each other on a linearly lined transducer aperture. These crystals are triggered as sub groups sequentially with certain time delays in order to obtain focusing. Annual arrays have circular shaped crystals. Compared with linear and phased arrays the number of the crystals is very limited. But because of the shape of the piezoelectric crystals; these arrays has the opportunity to do two dimensional focusing as an advantage. Phased array transducer has a smaller aperture compared with the linear array transducers. Opposite to the linear arrays; all crystals are triggered at the same time with certain time delays in order to obtain focusing. The major advantage of phased arrays is the capability to steer the sound waves in order to image wider area on the far field. Therefore these arrays are used mainly at sector transducers. One of the major issues on both linear and phased array transducers is the maintain focusing on elevation plane. There are two ways; one of them is to use special concave piezoelectric crystal shape, the other one is using more them 1 crystal array parallel to each other. Regarding the shapes of the transducers there are three groups which are linear, convex and sector. Each of them has different advantages. For instance linear transducer is ideal for to image wider area at the near field. Convex is mainly used for abdominal and obstetric because of wider image view in the far field. Finally sector is used for imaging from a narrow acoustic window and to have an access to a very wide imaging area on the far field. That is exactly clinician needs to examine hearth from a narrow intercostal acoustic window. There are still rooms to improve on today s ultrasound technology. Transducer technology is the main area that the major research and developments is done is the. As ultrasound resolution is limited with the number of piezoelectric crystals on a limited aperture; all researchers are actively trying to replace these crystals with alternative sound generator. Therefore the silicone transducers are invented and still work in progress. One other area is the capability of using the ultrasound device as an therapeutics device besides diagnostic area. The goal is to do organ specific medicine delivery by using contrast agents. Also having additional information about the tissue morphology and characterization is one of the areas to improve. The last subject is the maintaining objectivity. As we indicated before ultrasound is a dynamic examination. Usually the diagnose has to be done during the scanning. Therefore the operator dependency is very high. Any malpractice can create serious problems at the end. Therefore researchers are trying to add more intelligence to the systems in order to detect the human errors. The last area that is recently improved is fusion imaging. Fusion imaging is enabling to do cross modality imaging synchronized electronically with each other. As a resume; ultrasound is one of the leading imaging modalities due to ease of use; accessibility; and the wide variety of usage in human body. On the other hand; a successful ultrasound exam is directly related to the operator. The most important issue for the operator is to know the limitations of the ultrasound and to have the fundamental knowledge on optimising the image during the scanning.