Sesötesi uygulamalar için yapımı kolay ve düşük maliyetli fantom tasarımı
Ultrasound applications for easy and low cost phantom design
Yazar: AYDIN UĞUR KORU
Danışman: PROF. DR. TAYFUN AKGÜL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı / Biyomedikal Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Biyomühendislik = Bioengineering ; Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering
Dizin: Fantomlar = Phantoms ; Ultrasonik dönüştürücü = Ultrasonic transducers ; Ultrasonik hız = Ultrasonic velocity ; Yumuşak doku = Soft tissue

Özet:
Sesötesi cihazlar ülkemizde teşhis amaçlı olarak yaygın bir şekilde kullanılmaktadır. Sesötesi teşhis cihazlarının pozisyon doğrulama (kalibrasyon), mevcut sistemlerin sinyal ve gürültü oranlarının iyileştirilmesinde, kullanıcıların cihaz kullanımları öncesi pratik çalışmalarında ve uygulama esnasında alınan görüntülerin bilinirliği ve yorumlama kolaylığı sağlanması amacıyla fantomlara ihtiyaç duyulmaktadır. Sesötesi uygulamalarında insan dokularının özel karakteristiğini tasvir edebilecek test materyallerine fantom veya doku simülatörü denir.

Literatürde çeşitli dokuların tasvir edilmesi amacıyla değişik materyaller kullanılarak fantomlar oluşturulmuştur. Özellikle yumuşak dokuların tasvir edilmesi amacıyla agar, jelatin, magnezyumsilikat, yağ jel, poliüretan, epoksi reçine, polivinil alkol (PVA), polisakkarit jelleri gibi materyaller ile oda sıcaklığında sertleşebilen silikonlar (RTV) kullanılanlardandır. Su temelli materyaller özellikle agar ve jelatin gibi hidrojel olarak adlandırılabilecek malzemeler iyi performans, herhangi bir zararlı kimyasal içermemeleri, kolay kullanım, uygulamada sağladıkları esneklik ve akustik özellikleriyle öne çıkabilmektedirler.

Fantomların üretilmesinde en önemli kriterler simule edilmek istenilen dokuların akustik özelliklerine sahip olunmasının sağlanabilmesidir. Bunların başında dokudaki ses hızı, akustik empedansı, zayıflatma ve gerisaçılım katsayıları ile dokunun basınç altındaki değişime tepkisidir. 

Ticari olarak üretilen fantomlar birçok pazar ve belli başlı uygulamalar için üretilmiş olup kişiselleştirilememektedir. Birçok dokuyu ve organı temsili olarak oluşturan bu fantomların fiyatları yüksek değerlere ulaşabilmektedir. Bu sebeple uygulama yapılacak dokulara en yakın özellikte, kullanımı kolay ve maliyeti düşük fantomların üretilmesi önem taşımaktadır. Jelatin gibi materyaller ile oluşturulmuş fantom veya  doku modellerinde yaşanan en temel sorunlar; mikrobiyolojik unsurların (bakteri ve mantarların) zaman içinde gelişmesi ve hassas yapıları nedeniyle fantomların fiziksel ve akustik özelliklerini kaybedebilmeleridir.

Çalışmada yapımı kolay, herhangi bir zararlı kimyasal içermeyen hayvansal bir ürün olan jelatin kullanarak, maliyeti düşük sesötesi fantomlar oluşturarak ölçümler alındı. Ölçümlerin alınmasında darbe-yankı metodu ve A tipi tarama kullanılarak, fantomların içerisine yerleştirdiğimiz kitle ve kist tasvirlerinin konum, boyut gibi özelliklerinin tespit edilmesine çalışıldı. Fantomdaki ses hızının ölçümü ve bu hızın insan yumuşak dokularındaki hız değerine (≈1540 m/s) ulaştırılması için çalışma yapıldı. Ayrıca; doku içerisinde bulunması muhtemel saçıcı ve dağıtıcı tasvirlerini de katmak suretiyle ölçümlerimizin gerçekliği arttırılmaya çalışıldı.

Abstract:
Medical imaging is one of the fastest developing areas in medicine. One of the major factors of this development is examining the tissues without physical intervention. The most frequently used technique in medical imaging is the use of ultrasonic systems due to its damage-free nature to tissues, easy to use, getting real time images, and non-radiation technique, which is relatively low cost comparing with other imaging methods.

Ultrasound systems use sound waves to examine tissues. When sound waves meet different impedance inside the human body, they are scattered or backscattered where the same transducer collects the backscattered waves. When one transducer makes this process, is called A type scan. In practice instead of a single transducer an array of transducers are used whose outputs can be combined to produce two-dimensional images, which are called B scan. The B scan works as A type scan mode but plots the strength of the scattered or backscattered signals as changes in brightness. Strong reflections are brighter than the weaker reflections. While transducer is moving on human body, a two-dimensional image is built by these reflections. Today, mostly ultrasonic system imaging uses B type scan.

Ultrasound phantom or tissue simulators are testing objects that illustrate or simulate the special characteristics of human tissues in ultrasonic applications. Hospitals, clinics and research centers use phantoms to calibrate and initial testing of their systems, additionally training of ultrasound technicians prior to the practical work. Optimizing signal to noise ratios of the existing systems and getting familiar before real applications are also necessary.

In literature, various materials and many techniques have been proposed to produce phantoms. Specifically, for mimicking soft tissues agars, gelatin, magnesium silicate, oil gel, polyvinyl alcohol (PVA), polyester and epoxy resins, and room-temperature vulcanizing (RTV) silicone are frequently used. Water based substitutes, agars and gelatin based soft tissue substitutes (also called hydrogels) are the most widely used materials that have advantages of being easy to prepare, and easy to use. Moreover, well performance, containing no harmful chemicals, flexibility and acoustic properties come into prominence at usage. 

The ideal ultrasound phantom should have same acoustical properties as those simulated tissues. The most important characteristics are sound speed in tissue, acoustical impedance, attenuation, backscatter coefficient and nonlinearity parameter. The speed of sound in tissue typically determined by the time of flight measurements through a material of given thickness. Acoustic impedance can be calculated by speed of sound in tissue and measured density of material. The attenuation coefficient can be measured using through-transmission method, especially for low attenuation materials.

Acoustic properties of soft tissue phantoms are highly dependent on preparation technique and handling. Mostly measurements of acoustic properties of tissues are taken in room temperature, however these will be highly dependent on tissue substitutes and temperature varies. The most important problem encountered in the use of such materials in phantoms is microbiological factors (i.e., bacteria and fungus) which may evolve over time due to the sensitivity nature of losing physical specifications and acoustical characteristics that change with time.

Many commercially produced phantoms, which are used by a wide range of markets, are not customized but manufactured for particular applications. Customize designed phantoms are either expensive or cannot simply developed by the user. To overcome the above-mentioned disadvantages and to have the closest features to the tissues one may need to produce his/her own easily produced and low cost phantom.

In this thesis; gelatin, which is an animal product that contains no harmful chemicals, has been used as low-cost, homemade ultrasonic phantoms. The pulse-echo measurements of A type scanning is preferred. Note that the pulse-echo method is the most frequently used and simply method. This method relies on sending sound waves from a transducer and getting echoes from the same one.

Among various available 1 MHz surface type transducers in our laboratory, a transducer with the best frequency response has been chosen for experiments whose center frequency was at 870 kHz with a bandwidth of 485 kHz. Vegetable oil and vaseline are used as couplant. As these couplant materials cause defect during the tests, simply water is used also. For those, the selection criteria should be the same as the impedance value with applied tissues additional non-reacting chemical selection. In this study, the cartridge of silicon has been considered as the tissue mimicking phantom material, which can be easily found in the market.

200 bloom of beef gelatin is used in this study, which is low cost, contains no harmful chemical and can easily be found in vendors. To simulate a mass and a cyst in a tissue, an olive and a grape have been chosen respectively. Both of are considered in literature also. Moreover, water filled balloons may be used for depiction of cyst and fruit or vegetable pieces may be used for depiction of soft tissue mass also. The probable scatters within tissue are simulated (to strengthen the reliability of the measurements) by local dehumidifier silicagels.

Existing laboratory tools such as an ultrasound pulse sender/receiver, a surface type transducer, an oscilloscope and by labview program controlled computer have been used as the experimental setup. Ultrasonic velocity measurements and position determination of foreign objects (such as money) are studied for our homemade produced phantom. Then, in another homemade produced phantom cyst depiction is formed and its location is determined. Similarly, mass simulated measurements have been performed.
 
In order to achieve the same range of ultrasonic velocity (approx. 1540 m/s) in human soft tissues ethanol (70%) is added to the phantom. Ethanol addition to measurements shows an effect of rate increase. Another ethanol added gelatin mixture has been prepared for re-measurements to simulate scatters in the tissues by using silicagels, which are distributed as possible as homogenously whose radial dimensions are 2-4 mm. In fact, it is rather difficult to create a desired distribution of silicagels in the gelatin and ethanol liquid mixture homogenously. The silicagels that are added to create the noise in the phantom but did not. Silicagel's radial dimensions are much bigger than sound wavelength, since backscattered waves amplitude are decreased; instead of silicagels, more highly reflective materials should be used is concluded.
In this thesis, measurement of depth has achieved correctly for depicted materials in soft tissue phantoms, which are made by gelatin. Additionally, adding ethanol to gelatin mixture has increased the sound speed in the phantom to as approximate as real soft tissue values. Further study may be necessary to investigate the effect of increasing the concentration of ethanol mixture to phantoms to see the effect of sound speed.