[Intl_DxMedPhys] MRI Question about declining Tx

[Zhang Zhongwei]

Hi Peter,

It appears that there may be an issue developing in the 1st MRI scanner RF
transmit chain. If the Transmitter Reference Voltage is gradually
decreasing, then the corresponding reference power is also decreasing,
based on P=V^2/R with a 50 Ω reference plane. Under your 1st MRI scanner
conditions, the expected transmitter power should be on the order of
~1600–2200 W. Both the reference voltage and power can be verified in the
scanner log files and, in some cases, in the DICOM headers.

Assuming identical testing conditions across all three scanners, a gradual
reduction in reference voltage most commonly points to increased coil
loading or degraded impedance matching. In this scenario, the coil absorbs
more RF energy and/or mismatch increases reflected power, resulting in
reduced forward voltage at the reference plane. Other potential
contributors include RF power amplifier (RFPA) drift or thermal effects,
where amplifier gain decreases with temperature or long-term aging, leading
to lower output voltage. Cable or connector losses within the transmit
chain, which can introduce additional attenuation and should be inspected
if feasible.

One additional diagnostic approach is to intentionally position the
transmit coil with the load offset (say ~5-10 cm, but not too much) from
isocenter and record the resulting transmitter reference voltage. A
significant change under off-isocenter loading can help distinguish
coil-loading effects from upstream RF chain issues.

I agree that Transmitter Reference Voltage or Transmitter Reference
Amplitude should be used here. Referring to this parameter as “gain” may be
misleading, as gain is conventionally expressed in dB, while this quantity
represents an absolute voltage (or linear amplitude) referenced to a 50 Ω
system.

Hope this helps.


Best regards,
Zhongwei


--------------------------------------------

Zhongwei Zhang, MD, PhD, DABR, DABMP
Associate Professor of Radiology
Mallinckrodt Institute of Radiology,
Washington University School of Medicine

510 South Kingshighway Blvd., MSC 8131-43-1220A,
St. Louis, MO 63110

Office: 314-273-8305
Email: *zhongweiz at wustl.edu <zhongweiz at wustl.edu>*




On Thu, Jan 29, 2026 at 11:30 AM Hardy, Peter A. via
Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu> wrote:

> Hi I have a site where the transmit voltage on the head coil has been in
> steady decline over past few months. The site is one of three mobile units
> we have. Charts of Tx voltage vs date for the unit in question and two
> other mobile units are
>
> Hi
>
>
>
> I have a site where the transmit voltage on the head coil has been in
> steady decline over past few months. The site is one of three mobile units
> we have. Charts of Tx voltage vs date for the unit in question and two
> other mobile units are shown below. The 2nd and 3rd units do not show
> this behavior. I can understand an increase in Tx over time as a
> consequence of a gradual failure of a component in the head coil requiring
> increased Tx gain to overcome the situation. A decrease in Tx seems odd.
>
> Any suggestions of what could be going on here?
>
>
>
>
>
> *Peter Hardy, PhD, DABMP*
>
> Associate Professor of Radiology, Neuroscience and Biomedical Engineering
>
> Faculty, Division of Diagnostic & Nuclear Medical Physics
>
> Department of Radiology
>
> University of Kentucky College of Medicine
>
> UK HealthCare
>
> 800 Rose Street, Room HX-307
>
> Lexington, KY 40536-0293
>
> Phone: (859) 323-2954
>
> Email: peter.hardy at uky.edu
>
>
>
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