应当就是该装置的主电脑，控制计算机

在计算机术语里，主机一般都写为host（反之亦然）。简单地说，主机就是电脑的机箱那一部分，在这个基本意义上，主机是相对于电脑的外设而言的。这也是主机区别于计算机的一个主要特征。另外，由于主机的目的是执行计算、管理、数据存储或下载等操作，它甚至可以不带常见外设（如显示器、鼠标、键盘等），用户交互即可以在本机外设中进行，也往往可以在其它电脑中执行。不过很多情况下我们并不需要关注这种区别，可以简化理解，认为主机（host）就是计算机（computer）。主机不需要写为host computer。

host在商务和日常生活中常见的意思是是主人、主办方、宿主的意思。之所以叫“主机”，可能是因为它是电脑多个设备中最主要的一个，也可能是因为它是主持操作、业务（比如主持、执行大型运算的大型主机），主承数据（比如做为下载服务器用的独立主机或虚拟主机）的机器的意思。主机并不需要指多个计算机中为主的一台，其实和其它计算机基本可以认为没有关系，或没有主从关系。所以主机的主是指主持，而非一组计算机中主从关系中的主。同样也就容易理解为什么主机就是host了。

当然，有时确实有主机、从机的关系，但在你的例子中，超声波诊断仪是这台主机的外设之一（可以认为是与主机联接的打印机、鼠标、键盘、显示器等外设对等关系的外设之一），就是我上面说的这种意思。如果你在相关描述中找不到从机或类似的词汇，一般就可以确认。

参考：
http://www.google.com/search?um=1&complete=1&hl=zh-CN&lr=&ne...
http://www.google.cn/search?um=1&complete=1&hl=zh-CN&lr=&new...
http://baike.baidu.com/view/23880.html?wtp=tt

Main frame 这个可以是大部分仪器的主机。

http://www.virtualsciencefair.org/2003/guilh3m/public_html/c...
Central Processing Unit (CPU)
The CPU is the hub of an ultrasound machine. The CPU is like a computer that contains the microprocessor, memory, amplifiers and power supplies for the microprocessor and transducer probe. The transducer receives electrical currents from the CPU and sends electrical pulses that are created by returning echoes. The CPU does all of the calculations to produce an image on the monitor, and also stores the processed information on a disk.


http://www.dimensionsofdentalhygiene.com/ddhright.asp?id=797
The main components of ultrasound equipment are the display/output device; the transducer, which is responsible for emitting and receiving sound waves; and the central processing unit.

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Note added at 15 hrs (2008-06-21 16:42:30 GMT)
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P.S. main frame isnot applicablehere,because ultrasounds are all stand alone machines and are not interconnected (LAN/internet/etc.).

Summarily from what other answerers have conceived, I find the most appropriate target term to be the simplest: main computer. Real-time imaging/processing of ultrasonography in general clinical applications, is operated using a main computer [1] with a frame-grabber or an imaging software. The transmission or analysis of a large quanitity of diagnostic data in larger clinical settings, where other instruments/operators are involved, requires a "mainframe computer" or a "multiprocessor workstation" [2]. The latter two architectures integrate other devices interconnected by direct communication lines [3].

Thus in this case, the term "Main computer" appears to be the most suitable choice.

1. http://www.jultrasoundmed.org/cgi/content/abstract/19/1/15
2. http://www.freepatentsonline.com/6423009.html
3. http://en.wikipedia.org/wiki/Mainframe_computer