pourquoi T2* plus sensible pour hémosidérose
T2*-weighted and other gradient-echo sequences are the most sensitive to magnetic susceptibility effects of ferritin and hemosiderin because of their lack of a 180-degree refocusing pulse.
explique pourquoi dans mx polykystique rein tu peux voir kyste hyperechogene au lieu anechogene
Because of their small size, some of the cysts are hyperechoic (instead of anechoic) with parallel echogenic lines trailing from the far walls. This is due to reverberation artifact.
Reverberation artifact is caused by repeated reflections of intracystic sound energy between the near and far walls of the cyst, creating aberrant “copies” of the cyst wall in the tissue beyond.
–> dans le fond, transducer pense que chaque echo recoit une seule réflection –> donc les echos qui ont plus que une reflection cree une copie plus distale
This is due to a faulty assumption by the transducer that each echo it receives only encountered a single reflection. Because the distance of the reflector from the transducer is mapped based on a calculation contingent on the fixed speed of sound, echoes that are reflected more than once are artificially mapped deeper in tissue than they actually are. Multiple reflections create multiple lines deep to the cyst depending on the number of reflections each echo encounters (more reflections for a given echo equate to a greater mapped distance from the transducer).
zone IRM
ZONE I : sans restriction (outside world)
Zone II refers to the interface between unrestricted zone I (i.e., the outside world) and restricted zone III (i.e., the control room immediately outside the scan room). Zone II is used for intake of patients scheduled to undergo MR imaging. It is a place where patient histories can be obtained and MR screening can be performed.
zone 3 : controle room, immédiatement dehors du scan
Zone IV is the maximum zone level and refers to the room that contains the magnet.
Locks or other barriers are required between zone II and zone III.
Unscreened individuals are not allowed to enter zone III or zone IV.
If a medical emergency occurs in zone IV, the patient should be relocated to a lower zone to permit safe resuscitative measures outside the magnetic environment.
quelle parametre impact atténuation en CT
kVP
low kVp imaging is closer to the k-edge of iodine, resulting in increased attenuation at lower kVp within iodine-containing (i.e., enhanced) structure
Lower kVp imaging is associated with greater image noise, less radiation dose, and greater measured iodine attenuation than higher kVp imaging.
paramètre de temps dans dual echo, quelle est plus long et pourquoi
When selecting echo times for dual-echo gradient-recalled echo imaging,
it is important that the in-phase echo time be longer than the opposed-phase echo
time.
This criterion permits microscopic fat to be differentiated from susceptibility
artifact.
If the in-phase echo time is shorter than the opposed-phase echo time,
microscopic fat and susceptibility artifact will both cause signal loss on the opposed phase images (i.e., microscopic fat because fat and water protons are in opposite alignment, and susceptibility artifact because of the opposed-phase echo time is
longer than the in-phase echo time [not desirable]).
(a 3T, 1 de plus; a 2T 2sec de plus)
Along what vector do the protons align in
conventional inversion recovery imaging immediately following the preparation
radiofrequency pulse (applied before the excitation pulse)?
With conventional spin echo–based inversion recovery imaging, a 180-degree preparation radiofrequency pulse is applied before a 90-degree excitation radiofrequency pulse
stepladder artefact ?
Reverberation artifact occurs when echoes are repeatedly reflected between two highly reflective interfaces. This results in multiple equidistantly spaced linear reflections which have a stepladder appearance.
predicteur brulure peau en fluroscopie
Cumulative air kerma is an expression of radiation at a point, so it would best predict skin injury. Kerma area product is an expression of total energy deposition across the entire exposed skin so it would best predict risk of radiation induced cancer.
sepration frequent gras et eau pour calculer chemical shift
fat/water resonant frequency
separation (3.5 ppm)
Shift = (Fat/water resonant frequency separation [in ppm])×(field strength [T] ×42MHz/T
What organ is best used as an internal reference standard to determine the degree of signal loss within an adrenal nodule
RATE
The spleen has been shown to be the best internal control on chemical shift imaging when attempting to determine the degree of signal loss within the adrenal gland. This is because the spleen does not have internal fat content that might confound interpretation (such as is often present in the liver).
artefact qui cause pseudorehaussement dans petite lesion renale
Erroneous beam hardening correction
double energy ct basé sur quel principe
DIFFÉRENCE K-EDGE ÉLÉMENT UNIQUE IMAGÉ À kVP DIFFÉRENT
Dual-energy CT is based on the principle of distinguishing elements that
have sufficiently unique K-edges by imaging them with two different kVp settings.
V ou F, aucune image en IRM réflete purement T1 et T2 ?
V
Remember that no image reflects purely T1 information or purely T2 information. MR images are a combination of both, with “weighting” toward one or the other based on the desired image contrast.
effet concentrated gadolinium-based
contrast material (GBCM)
indirectly shorten the T1 time of adjacent protons, but it also shortens T2 times.
This effect is not noticeable on most T1 weighted images because the T1 shortening effects dominate
gado = Gadolinium is a paramagnetic agent
However, when gadolinium becomes highly concentrated (e.g., within a dilated renal pelvis, or in the dependent urinary bladder), the T2-shortening effects take over and produce a signal void.
risque cancer induit avec dose effective 10 msv chez adulte et ped
The risk of cancer induction in an adult patient who receives an effective dose of 10 mSv is estimated to be roughly 0.01% (1 in 1,000).
The risk likely is higher for young adults, children, infants, and fetuses. The risk to a newborn infant is estimated to be roughly 0.4% (1 in 250) per 10 mSv dose.
c’est quoi le principe de doppler shift
The reflected ultrasound frequency always increases when the ultrasound
beam interacts with a structure moving toward it.
Likewise, the reflected ultrasound
frequency always decreases when the ultrasound beam interacts with a structure
moving away from it.
The ultrasound probe emits sound at a known frequency (f₀).
Moving structures in the body (e.g., red blood cells) reflect the waves back.
If the blood cells are moving toward the probe: The distance between the wavefronts is shortened.
Reflected frequency (fᵣ) is higher than the transmitted frequency.
If the blood cells are moving away from the probe: Wavefronts are stretched.
Reflected frequency is lower than f₀.
relation frequence et wavelenght
As the frequency increases, the
wavelength decreases (i.e., becomes more compressed),
as the frequency
decreases, the wavelength increases (i.e., becomes more elongated).
quelle parametre statistique varie avec prévalence maladie
VPP
VPN
the positive and negative predictive values depend on the prevalence of disease in the population; therefore, the PPV and NPV will be different if the test is applied to a population with different disease prevalence.
quelle effet prédomine dans paramètre suivant
LONG TR, LONG TE
COURT TR, COURT TE
heavily T2 weighted (static fluid) = long TR, long TE, fast spin echobased sequence)
-> Fluid has a very long T2; therefore, on a heavily T2-weighted sequence, the signal from all other structures is suppressed while the signal from fluid is preserved.
T1 weighted (short TR, short TE,
gradient recalled echo-based sequence).
What is the primary function of statistical iterative reconstruction methods
to reduce image noise.
This is accomplished by matching a simulated CT projection to the acquired CT projection in an iterative (repeated) fashion to better model the image noise.
Reduced image noise can translate into radiation dose savings, but the effect is indirect
one can obtain noisier images
with less radiation exposure, and then “clean up” the noisy images with statistical
iterative reconstruction after they are acquired.
frequence nyquist
represent The analog frequency that is equal to half the sampling frequency (fs)
fN = 0.5 × fS
Before analog data can be displayed in a digital format, it must first be converted
to digital data using an “analog-to-digital” converter (ADC). The sampling
frequency (fs) of the ADC must be at least twice that of the analog frequency to
preserve the integrity of the data.
pour maximiser T1
tu veux augmenter ou diminuer TE ?
puis tu veux grand ou petit flip angle ?
PETIT T1, LARGE FLIP ANGLE
Short echo times (TE) minimize T2 (T2*) contrast because when the TE is short, there is insufficient time for spin–spin relaxation to occur, and so therefore all substances, regardless of their T2 time, will appear similar
Large flip angles promote T1 weighting by converting a large fraction of longitudinal
magnetization into transverse magnetization. This allows differences in T1 times to
become evident during spin–lattice relaxation.
catégorie de Susceptibility
Susceptibility reflects the tendency of an object to become magnetized when placed into an external magnetic field.
There are four major categories.
1. diamagnetique: paroi vessie
2 paramagnétique: gadolinium
3. superparamagnétique: particule fer oxide
4. ferromagnetique : arthroplastie metalique
comment diminuer flow voids
DIMINUER TEMPS TE
The 180° refocusing pulse occurs at TE/2.
A longer TE → longer time between 90° and 180° pulses.
More time = higher chance moving protons leave the slice (or new ones enter too late).
Therefore: longer TE increases flow voids.
