English to Russian Medical Medical: Instruments

Руководство по медицинскому оборудованию MiraQ производства компании Medistim. Это оборудование используется при операциях на сердце и сосудах (например, при аортокоронарном шунтировании)

Контекст:

Transit time flow measurement (TTFM)

The TTFM principle

The TTFM principle is based on measuring the difference between upstream and downstream transit time of a wide ultrasound beam.
The transit time difference is directly proportional to the blood volume flow.
This measurement principle gives an accurate quantification of the real time volume flow that compliments the ultrasound imaging.
Tasks during procedure by surgeon
The surgeon will judge the diameter of the vessels to be measured and inform the operator which probe(s) to connect.
The operator needs to be informed of which type of conduit, connection, and target vessel that is being measured.
Arterial conduits such as the internal mammary arteries have to be skeletonized for a length corresponding to the width of the probe, approximately 1cm.
When the TTFM probe is placed onto a conduit, the system will generate a simulated Doppler sound, indicating that flow has been detected.
The volume of this sound is adjustable, using the on-screen buttons.
To ensure accurate flow measurement, it is important that the probe is placed perpendicular to a straight segment of the measured vessel.
All parts of the vessel should be inside the probe as shown in .

Пожалуйста, посмотрите картинку, где показано, что такое "upstream and downstream transit time of a wide ultrasound beam" - вот ссылка на нее:

https://www.dropbox.com/s/p7nxv3d57chwquj/BR00IN005-v2_MiraQ...

На 2-ой странице этой брошюры см. картинку слева.

Пожалуйста, помогите перевести всё это предложение:

The TTFM principle is based on measuring the difference between upstream and downstream transit time of a wide ultrasound beam.

Мой вариант перевода:

Принцип TTFM основан на измерении разницы транзитного времени между потоком идущими по течению и потоком идущим против течения по ширине ультразвукового луча.

Но он мне не очень нравятся.
Пожалуйста, предложите свои варианты.

Интересует, как правильно и точно перевести "difference between upstream and downstream transit time of a wide ultrasound beam"

Как перевести "upstream and downstream transit time"?