Because of its smaller convex contact surface, the curvilinear or convex sector-array fits much better on the abdomen and allows for a wider field of view than does the linear-array configuration. Work on the fabrication of an electronic convex array had started in the late 1970s in the larger Japanese companies such as Hitachi® (publishing their convex attachment to their EUB-10A scanner in 1978); Aloka® (filing their patent on the convex scanner in 1980), as well as in U. S. companies notably the North American Philips® and the Picker Corporation®, who had filed their patents for convex arrays and processors in 1979 and 1980 respectively. The first commercially available convex array transducer apparently only debuted in 1983 in a scanner from Kontron Instruments® in Europe, the Sigma 20, which was designed especially for use in Obstetrics and Gynecology. Hitachi® in Japan marketed their model EUB-40 with their new convex array later on in the same year.

Toshiba® introduced a similar array in 1985, in their new scanner model SAL-77A. Interestingly, the design actually replaced an earlier model (by only about 9 months) the SAL-90A which boasted a new "trapezoid" linear array in which the face of the transducer was flat but a trapezoid-shaped image was produced from the 128 transducer elements using phased electronics. American machines were apparently still using linear arrays by 1985, although very shortly they were quickly replaced by the new convex configuration. By about 1987, convex arrays are standard on every new scanner, whether or not it is configured for use in Obstetrics and Gynecology.

Advertisement of Hitachi's first convex-array scanner the EUB-40 in early 1984.

Explaining the advantage of the convex-array probe in the same advertisement. .

Toshiba's advertisement of the SAL-90A in early 1985, which used
a linear array probe producing a "Trapezoid" image.

Toshiba's advertisement of the SAL-77A in mid 1985, which used for the
first time a convex-array probe.

The second page of the same advertisement in 1985.

Because of its convex shape, the curvilinear or convex sector array fits better on the abdomen and scans a wider field of view than does a linear-array configuration. The curvilinear array operates in the same manner as the linear array in that the scan lines are directed perpendicular to the transducer face. As many as 512 elements constitute a convex sector array in currently available scanners. An aperture containing as many as 128 elements is selected to function at a given time. Similar to the linear arrays the acoustic beams are focused but not steered.

The curvilinear or convex sector array became popular and completely replaced the linear configuration in the late 1980s.

convex sector systems are capable of lateral resolution on the order of less than 1 mm. Axial resolution of 1 mm is always possible depending on the frequency of the system. A "wide aperture" array design means that pulses from a large number (say 128) or all the elements are used to form each scan line. At each line, a different delayed pulse sequencing of the whole array of elements is required to form the unique interference pattern, resulting in a highly focused ultrasound beam perpendicular to the transducer face. Since a unique delay pattern for all the elements is required to produce each scan line, highly sophisticated computer-controlled electronics are required. Lateral resolution of less than 0.5 mm can be achieved.

Back to History of Ultrasound in Obstetrics and Gynecology.