BIA and advanced imaging modalities Flashcards
impedance is composed of these 2 frequency dependent parameters:
resistance (R) and reactance (Xc)
opposition to the flow of current when passing thru body
resistance
delay in conduction caused by cell membranes, tissue interfaces, and non-ionic substances
reactance
muscle and body water act as ____ while fat act as ____
conductor; insulator
resistance is ___ to length of conductor, ____ to Cross sectional area
proportional; inversely proportional
what is ohm’s law?
resistance = volume/current
volume of body =
length (height) x area
1 cylinder model assumes:
assumes body constitutes single cylinder having uniform conductivity and distr. of FFM and FM for any cross-sectional area (not true)
segmental bia regards body as ____ model
5 cylinder
when use segmental BIA?
in disease states with edema/ascites
these advanced imaging techniques for Body comp assessment are highly _____
precise
DXA derives body comp via 3-comp model:
bone mineral tissue, fat tissue, lean tissue
how does DXA work?
low dose x ray beams passed thru tissues of body
why are CT images cross-sectional?
allows for precise quantification of tissues and organs
cross sectional images in CT are composed of ___ which has a ___ unit
pixels; houndsfield
a houndsfield unit is a measure of _____
radiodensity
HU of ___ for air (black) and HU of ___ for water (white)
-1000; 0
in CT, the yellow is ___, green is _____, blue is ____
visceral adipose tissue; intramuscular adipose tissue; subcutaneous adipose tissue
red in CT is:
muscle
MRI uses ___
magnetic fields
analytical technique that complements MRI
MRS
MRI vs MRS?
uses signals from H+ protons to create 2D images vs. uses 1H signals to determine relative concentrations of target metabolites
MRS used for?
neurological (originally predominant), skel muscle, liver
definition of biological impedance:
ability of bio tissue to impede electric current
what is active response?
bioelectricity; occurs when bio tissue provokes electricity from ionic activities inside cells (ie. signals in heart, from brain)
what is passive response?
occurs when bio tissues are simulated thru an external electrical current source
3 types of bioimpedance devices:
single frequency, multiple frequency, bioimpedance spectroscopy
similarities between diff BIA?
safe, noninvasive, inexpensive, precise
diffs between diff BIA?
underlying theory, potential applications, validity in various clinical populations
what is impedance?
(Z); the obstruction to the flow of an alternating current, composed of R (resistance) and Xc (reactance)
what is resistance?
pure opposition of conductor to flow of current; determined by shape (length and SA) and material type
what is reactance?
opposition to current flow caused by capacitance produced by cell membranes
what is capacitance:
ability of non conducting object (ie. cell membranes) to save electric charges
in general, ____ is ability of current to flow through fluid and tissues, and ____ reflects interaction of current with membrane interfaces like cell men
resistance; reactance
when current applied at low frequency, what happens?
current does not penetrate cell membrane, which acts as an insulator ; therefore current passes thru the ECF which is responsible for measured reactance of body–>impedance is purely resistive and is function of ECW
when current applied at high frequency, what happens?
with alternating current, cell membrane charges and discharges at rate of frequency; as frequency ^ the amount of ICF measured increases and reactance (Xc) increases
at an infinitely high frequency (>10000kHz) what happens?
charge and discharge of current thru the cells becomes so fast that effects of cell membrane capacitance becomes insignificant and impedance becomes purely resistive; (both ECW and ICW fully measured = TBW)
fundamental assumption of TBW?
73% of FFM is ICF+ECF
___ considered non-conductor of electrical charge whereas ____ is the conducting volume that helps the passing of electric current due to conductivity of electrolytes dissolved in body water
fat mass; fat free mass
what is single frequency BIA?
just apply one frequency of 50kHz, current passed between electrodes placed on hand and foot in standard tetrapolar position; measures weighted sum of ECW and ICW (not measure TBW)
SF-BIA used to assess ____ in normal hydrated subjects, but cannot differentiate between ______
TBW and FFM; ECW and ICW
analysis of bioimpedance obtained at multi frequencies, based on premise that ECW and TBW can be assess by exposing it to low and high frequency electric currents (low to predict ECW and high to predict TBW)
MF-BIA
body characteristics and their influence on assessment and recommendations for the execution and interpretation of the test
ethnicity, age, gender, hydration, obesity, severe malnutrition, neuro disorders
takes measurements at 250 diff frequencies from 5-1-1000 kHz
BIS
instead of linear equations, BIS uses ___ to calculate resistance at 0 and infinite frequences; these are applied to _____ to generate estimates of _____
complex modelling ; complex algorithms; ICW and ECW (subsequently FFM and FM)
optimal testing conditions for BIA
no caffeine /alcohol/exercise 24 hours before testing, fasting 8 hours before testing, remove metal, void bladder, normal fluid/electrolyte status, arms separated from trunk by 30 degrees and legs by 45 degrees, taken at 5 minutes after assuming supine position
limitations of BIA equations
integrates various body segments with variable physical effects of hydration, fat fraction, geometrical boundary conditions etc. on tissue conductivity ; ECW:ICW ratio is factor known to limit applicability of predictive equations generated by BIA to populations with varying hydration; difficulties of validating BIA in diff age and ethnic groups and clinical conditions with abnormal hydration states
prediction equation for BIA should have prediction error (SEE) of ____ in men and ____ in women
2-2.5kg; 1.5-1.8kg
actual error of ___kg considered ideal
0-1.8
BIA equations should not be used without prior verification against reference methods in subject population studied, such as:
DXA and doubly labelled water
why BIA one of the most common and widely used methods to assess body comp?
low p.t./client burden, expedient, more precise than skin folds, portable, relatively inexpensive, training is minimal, precise technique, non invasive,
same instrument for BIA should be used with consistent replication of measurement conditions to control for ____
inter and intra subject variation
3 aspects for body comp analysis implied by whole body BIA approach
1) changes of impedance are closely related to changes of FFM of limbs 2) changes of FFM of trunk not adequately described by whole body impedance measurements 3) large changes in fluid volume within ab cavity have only minor influence on measurement of FFM or BCM
advantages of DXA?
short scan time, allows for regional analysis and whole body, can put animals in without anesthesia
disadvantages to DXA?
exposure to radiation, unable distinguish specific tissue depots, changes in BW reflected in DXA as changes in lean tissue
advantages of CT?
short scan time, able diff. between tissues, good imaging resolution, less risk motion artifact due to scan speed, great for clinical populations
disadvantage CT?
radiation, analysis laborious and requires training, CT viewing cutoff with morbid obesity
advantage MRI?
provides better soft tissue detail vs CT, no radiaiton
disadvantage MRI?
longer scan time (30min), more sensitive to human mvmt, can’t be used with pacemaker /metal implants, claustrophobia
advantage of MRS:
can detect functional and metabolic heterogeneity within single tissue, non invasive but assess in vivo metabolic changes , higher sensitivity to detect changes
disadvantage of MRS:
only really for research, many MRI systems not equipped with spectroscopy systems, currently no methodology/technique to measure O2 metabolism in vivo
