Ultrasound Output Display Standard Copyx

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Ultrasound	
  Output	
  Display	
  –	
  What	
  do	
  the	
  numbers	
  mean?	
  
G.	
  Wayne	
  Moore,	
  B.Sc.,	
  MA,	
  FASE	
  
July	
  13,	
  2015	
  
	
  
The	
  ultrasound	
  Output	
  Display	
  Standard*	
  (ODS)	
  has	
  a	
  specific	
  technical	
  definition	
  
but	
  that	
  definition	
  is	
  generally	
  found	
  standing	
  in	
  contextual	
  isolation.	
  Specifically,	
  
how	
  do	
  these	
  dimensionless	
  MI	
  and	
  TI	
  numbers,	
  displayed	
  on	
  the	
  ultrasound	
  system	
  
monitor	
  (see	
  photo	
  below),	
  relate	
  to:	
  (1)	
  the	
  laws	
  of	
  ultrasound	
  physics,	
  (2)	
  FDA	
  
prescribed	
  regulatory	
  constraints	
  placed	
  on	
  the	
  limits	
  of	
  the	
  transmitted	
  acoustic	
  
energy,	
  (3)	
  the	
  other	
  user	
  controls	
  on	
  the	
  ultrasound	
  system,	
  and	
  (4)	
  the	
  ALARA	
  
principle.	
  Related	
  to	
  the	
  potential	
  for	
  bio-­‐effects	
  occurring	
  at	
  certain	
  levels	
  of	
  
ultrasound	
  acoustic	
  intensity,	
  the	
  MI	
  and	
  TI	
  indices	
  displayed	
  serve	
  as	
  a	
  guide,	
  and	
  a	
  
reminder	
  to	
  the	
  clinical	
  end-­‐user	
  that	
  although	
  ultrasound	
  is	
  generally	
  regarded	
  as	
  a	
  
safe	
  imaging	
  modality,	
  one	
  must	
  always	
  use	
  the	
  minimal	
  amount	
  of	
  energy	
  
necessary	
  to	
  obtain	
  a	
  diagnostic	
  quality	
  image	
  (including	
  Doppler).	
  MI,	
  or	
  
mechanical	
  index,	
  relates	
  to	
  the	
  potential	
  of	
  non-­‐thermal	
  effects	
  from	
  ultrasound	
  
occurring	
  within	
  tissue	
  (e.g.,	
  cavitation).	
  TI,	
  or	
  thermal	
  index,	
  relates	
  to	
  the	
  potential	
  
for	
  tissue	
  temperature	
  increases	
  within	
  the	
  acoustic	
  beam.	
  There	
  are	
  three	
  types	
  of	
  
TI	
  that	
  may	
  be	
  displayed	
  on	
  the	
  monitor,	
  depending	
  on	
  the	
  clinical	
  application	
  
selected:	
  (1)	
  TIS,	
  for	
  soft	
  tissues,	
  (2)	
  TIB,	
  for	
  bone	
  tissue	
  (shown	
  in	
  the	
  photo	
  below),	
  
and	
  (3)	
  TIC,	
  for	
  transcranial.	
  	
  	
  	
  	
  	
  	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  
	
  

	
  

	
  

When	
  designing	
  an	
  ultrasound	
  system	
  there	
  is	
  a	
  classical	
  macro-­‐level	
  engineering	
  
trade-­‐off	
  that	
  must	
  be	
  dealt	
  with	
  and	
  resolved;	
  balancing	
  image	
  resolution	
  and	
  
depth	
  of	
  penetration.	
  Both	
  of	
  these	
  parameters	
  are	
  related	
  to	
  the	
  ultrasonic	
  
frequency	
  being	
  used	
  and	
  are	
  in	
  opposition.	
  Specifically,	
  the	
  higher	
  the	
  frequency	
  
being	
  used	
  the	
  better	
  the	
  resolution,	
  but	
  at	
  the	
  expense	
  of	
  the	
  depth	
  of	
  penetration.	
  
That’s	
  the	
  physics	
  of	
  it.	
  Although	
  a	
  tad	
  more	
  complicated	
  an	
  easy	
  math	
  example	
  will	
  
explain;	
  ultrasound	
  energy	
  is	
  lost	
  in	
  soft	
  tissue	
  at	
  the	
  rate	
  of	
  approximately	
  
0.7dB/cm/MHz,	
  where	
  MHz	
  is	
  the	
  ultrasonic	
  frequency.	
  So	
  at	
  3MHz	
  the	
  attenuation	
  
would	
  be	
  2.1dB/cm,	
  while	
  at	
  8MHz	
  the	
  attenuation	
  would	
  be	
  at	
  5.6dB/cm.	
  This	
  
relationship	
  means	
  that	
  	
  
	
  
*	
  The	
  official	
  title	
  of	
  the	
  ODS	
  is:	
  “Standard	
  for	
  Real-­‐Time	
  Display	
  of	
  Thermal	
  and	
  
Mechanical	
  Output	
  Indices	
  on	
  Diagnostic	
  Ultrasound	
  Equipment”	
  
as	
  the	
  frequency	
  (let’s	
  relate	
  this	
  to	
  the	
  probe	
  frequency)	
  is	
  increased	
  and	
  image	
  
resolution	
  improved,	
  the	
  depth	
  of	
  penetration	
  will	
  decrease,	
  unless	
  we	
  increase	
  the	
  
ultrasound	
  system’s	
  output	
  power,	
  which	
  the	
  FDA	
  says	
  is	
  a	
  no-­‐no	
  beyond	
  an	
  
estimated	
  in-­‐situ	
  intensity	
  of	
  720mW/cm2.	
  So	
  ultrasound	
  systems	
  and	
  their	
  
attendant	
  probes	
  are	
  designed	
  with	
  the	
  intended	
  clinical	
  application	
  clearly	
  in	
  mind;	
  
striking	
  the	
  optimal	
  balance	
  between	
  resolution	
  and	
  required	
  depth	
  of	
  penetration	
  
within	
  the	
  FDA	
  acoustic	
  output	
  power	
  constraints.	
  So	
  within	
  each	
  clinical	
  application	
  
there	
  is	
  a	
  range	
  of	
  MI	
  and	
  TI	
  numbers.	
  The	
  MI	
  and	
  TI	
  “values”	
  change	
  as	
  the	
  clinical	
  
user	
  changes	
  various	
  parameters	
  of	
  the	
  image	
  or	
  modes	
  of	
  operation	
  (e.g.,	
  B-­‐mode,	
  
M-­‐mode,	
  spectral	
  Doppler,	
  or	
  color	
  flow	
  Doppler),	
  but	
  especially	
  the	
  “power”	
  knob.	
  
Enter	
  the	
  ALARA	
  principle;	
  As	
  Low	
  as	
  Reasonably	
  Achievable.	
  Even	
  though	
  through	
  
regulation	
  there	
  is	
  a	
  maximum	
  upper	
  limit	
  on	
  acoustic	
  output,	
  user	
  groups	
  have	
  
developed	
  the	
  ALARA	
  principle	
  with	
  the	
  guiding	
  premise	
  that	
  the	
  clinical	
  user	
  should	
  
only	
  use	
  (that	
  is,	
  only	
  expose	
  the	
  patient	
  to)	
  as	
  much	
  acoustic	
  power	
  required	
  to	
  
obtain	
  a	
  diagnostically	
  acceptable	
  image;	
  hence	
  the	
  range	
  of	
  MI	
  and	
  TI	
  values	
  
adjustable	
  by	
  the	
  acoustic	
  output	
  knob.	
  Original	
  Equipment	
  Manufacturers	
  (OEMs)	
  
are	
  required	
  by	
  the	
  FDA	
  to	
  submit	
  testing	
  results	
  in	
  the	
  form	
  of	
  acoustic	
  output	
  
power	
  tables	
  for	
  each	
  probe	
  and	
  mode	
  of	
  operation	
  for	
  which	
  they	
  seek	
  510(k)	
  
market	
  clearance.	
  This	
  testing	
  is	
  an	
  on-­‐going	
  requirement	
  throughout	
  the	
  product	
  
life-­‐cycle	
  of	
  any	
  given	
  probe	
  on	
  the	
  cleared	
  system	
  to	
  ensure	
  that	
  no	
  significant	
  
changes	
  have	
  occurred	
  in	
  the	
  manufacturing	
  process,	
  or	
  if	
  new	
  acoustic	
  materials	
  

	
  

	
  

are	
  being	
  substituted**.	
  This	
  ensures	
  that	
  the	
  output	
  power	
  of	
  any	
  given	
  probe	
  is	
  still	
  
within	
  the	
  statistical	
  range	
  of	
  the	
  reported	
  MI	
  and	
  TI	
  values	
  displayed	
  on	
  the	
  
monitor,	
  and,	
  therefore,	
  still	
  a	
  valid	
  indicator	
  to	
  the	
  clinical	
  user	
  relative	
  to	
  how	
  
much	
  power	
  they	
  are	
  inputting	
  into	
  the	
  patient.	
  	
  
	
  
Safe	
  and	
  efficacious	
  are	
  the	
  by-­‐words	
  for	
  an	
  FDA	
  cleared	
  ultrasound	
  system	
  and	
  
probe	
  sold	
  in	
  the	
  United	
  States.	
  Unfortunately	
  there	
  are	
  some	
  probe	
  “repair”	
  
companies	
  in	
  the	
  United	
  States	
  that	
  replace	
  the	
  acoustic	
  stack	
  of	
  a	
  probe	
  with	
  an	
  
array	
  manufactured	
  by	
  someone	
  other	
  than	
  the	
  OEM	
  without	
  performing	
  acoustic	
  
power	
  testing.	
  This	
  practice	
  has	
  the	
  potential	
  for	
  rendering	
  the	
  MI	
  and	
  TI	
  values	
  
displayed	
  as	
  unreliable	
  indicators	
  for	
  the	
  clinical	
  user,	
  and	
  may	
  lead	
  to	
  higher	
  than	
  
necessary	
  acoustic	
  exposure	
  to	
  the	
  patient,	
  and/or	
  producing	
  sub-­‐diagnostic	
  images.	
  
Replacing	
  OEM	
  arrays	
  with	
  un-­‐tested	
  and	
  un-­‐validated	
  arrays	
  manufactured	
  by	
  non-­‐
OEM	
  companies	
  also	
  renders	
  the	
  probe	
  as	
  an	
  “adulterated”	
  product	
  as	
  defined	
  by	
  
the	
  FDA**.	
  Before	
  giving	
  permission	
  to	
  any	
  probe	
  repair	
  company	
  to	
  replace	
  an	
  
acoustic	
  array,	
  the	
  user	
  should	
  first	
  demand	
  to	
  see	
  the	
  acoustic	
  power	
  testing	
  
results	
  for	
  that	
  array,	
  the	
  name	
  of	
  the	
  manufacturer	
  of	
  that	
  array,	
  and	
  the	
  
comparison	
  chart	
  of	
  the	
  OEM	
  array	
  test	
  results	
  with	
  those	
  from	
  the	
  non-­‐OEM	
  array.	
  
To	
  do	
  less	
  would	
  be	
  a	
  disservice	
  to	
  the	
  patient.	
  
	
  
	
  
**

Guidance	
  for	
  Industry	
  and	
  FDA	
  Staff	
  “Information	
  for	
  Manufacturers	
  Seeking	
  
Marketing	
  Clearance	
  of	
  Diagnostic	
  Ultrasound	
  Systems	
  and	
  Transducers”	
  Document	
  
Issued	
  on	
  -­‐	
  09/09/2008	
  	
  	
  

	
  
	
  



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File Type Extension             : pdf
MIME Type                       : application/pdf
PDF Version                     : 1.3
Linearized                      : No
Page Count                      : 3
Title                           : Microsoft Word - Ultrasound Output Display Standard copy.docx
Producer                        : Mac OS X 10.10.4 Quartz PDFContext
Creator                         : Word
Create Date                     : 2015:07:21 22:34:41Z
Modify Date                     : 2015:07:21 22:34:41Z

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