CMR, CT, Echo Flashcards
ultrasound physics
- All forms of ultrasonic imaging are based on generation of high-frequency (>1MHz) acoustic pressure waves from a transducer consisting of one or more piezoelectric crystals.
- As current is passed across the crystals they deform and generate the ultrasound wave.
- The piezoelectric element also serves as a receiver. Waves returning from objects (e.g. walls, valves) deform the crystals which, in turn, generate a current that can be recorded.
- Because the velocity of sound is constant, object location (spatial resolution) can be determined based on the time it takes for a wave to return.
- The amplitude of the returning signal depends on the angle of incidence (surfaces perpendicular to the ultrasound beam are stronger reflectors) and the interface of acoustic impedances (greater differences such as occurs in the left ventricle at the tissue–blood interface lead to greater reflectivity).
doppler physics
A powerful aspect of echocardiography is the ability to assess the speed of movement of objects with the same equipment that generates 2D and 3D structural images. Quantification of object motion is possible with Doppler-based technologies. The principles of Doppler are as follows.
• Frequencies of returning ultrasound are shifted upwards or downwards by cells depending on whether the cells are travelling towards or away from the transducer, respectively. The amount that the frequency shifts is proportional to the velocity of the object.
• The signal intensity depends on the number of cells moving at a particular velocity.
• Velocity information is depicted as a spectral pattern over time similar to the M mode (continuous and pulsed wave Doppler) or mapped to pixels as colour overlying the 2D or 3D image (colour flow imaging) (Fig. 1.10).
• The ultrasound beam must be as parallel as possible to the target for accurate measures. Off-axis angulation by >30° leads to significant underestimation of velocities.
MRI physics
Magnetic resonance depends on the interaction between atomic nuclei and radio waves in the presence of a magnetic field. The major nucleus of interest is the hydrogen atom, which is present throughout the body in water and fat. When the body is placed in a strong magnetic field the hydrogen atoms align themselves in the direction of the field. A radio-frequency wave is then applied to ‘knock’ some of the hydrogen atoms out of alignment. The hydrogen atoms absorb this radiofrequency energy and then release it as they return to alignment within the field. This -energy release is picked up as a signal. The strength and nature of the signal provides information on the hydrogen atoms within different tissues. By altering the number, timing, and features of the radiofrequency pulse an image can be generated.
gadolinium
Gadolinium contrast has opened the possibility for a range of new sequences with CMR imaging. Gadolinium is an injectable agent which, once fixed to an appropriate carrier, alters the T1 signal characteristic of any fluid or tissue within which it is present. Therefore it can be used to highlight specific areas. Early imaging allows imaging of movement of the agent through the circulation, and because it can pass out into tissue can also be imaged in areas of scar. It is excreted by the kidneys, and there have been recent concerns about increased incidence of systemic fibrosis following gadolinium injection. This is a fatal irreversible condition and appears to occur in those with severe renal disease in whom, presumably, the gadolinium is not cleared effectively. Gadolinium contrast is contraindicated in severe renal dysfunction.
CTA should be considered in those patients:
CTA should be considered in those patients:
• • with low to intermediate pre-test probability of significant coronary artery disease;
• • who do not wish to have an invasive coronary angiogram;
• • in whom an invasive coronary angiogram is likely to be problematic (i.e. severe peripheral vascular disease);
• • in whom there is a high likelihood of aberrant coronary vessels (i.e. in adult congenital heart disease);
• • in those where the risk of invasive coronary angiogrpahy is particularly high (i.e. severe aortic stenosis);
• • in whom an alternative to myocardial perfusion scintigraphy is sought.
MDCT should not be carried out on patients with a high risk of coronary artery disease since invasive angiography can and should lead seamlessly into immediate intervention in this group.
when is MPI indicated?
1) In patients with intermediate pre-test Risk
2) in those with equivocal Exercise ECG
3) in those with abnormal baseline ECG
4) in those with poor exercise capacity
how to manage a negative ETT result?
means the risk is low, manage risk factors + risk factor modifying medications
how to manage unequivocally poositive ETT?
if suitable, patients may benefit from invasive angiography
patients with equivocal ETT results and intermediate post-test risk>
noninvasive MPI with SPECT or PET.
CMR is an alternative in experienced centres.
Summary for IHD prognosis
Ischaemic burden is the most powerful predictor of coronary events
But is incremental to clinical features, exercise time, LV function and coronary anatomy
No ischaemia = <1% risk of coronary events
Predictive power wains with time at a rate depending upon conventional risk factors
Coronary events caused by unstable atheroma
Instability related to degree of stenosis, but non-stenotic atheroma more widespread and can be unstable
BUT remember that prognosis relates to populations rather than individuals
summary for IHD diagnosis
Pre-test likelihood from clinical features (Pryor’s algorithm)
NICE guidance, now also ESC guidance
Perfusion imaging more sensitive than wall motion imaging
Stress functional imaging to investigate symptoms
Anatomical imaging (CTCA) to exclude disease in low likelihood
Functional imaging is cost-effective for diagnosis
in direct comparison of dobutamine CMR and SPECT, in terms of detecting viable myocardium that recovered after revascularisation
In a direct comparison between dobutamine
MRI and thallium and tetrofosmin SPECT in patients with
ischaemic left ventricular dysfunction undergoing revascularisation, MRI had a low sensitivity (50%) but high specificity (81%), whereas the nuclear techniques were more sensitive, but less specific for predicting recovery of regional function.
the frequency of segmental recovery after revascularisation:
Recovery is seen, on average, in
55–60% of dysfunctional segments, even in patients with
baseline ejection fraction below 40%.44
The true prevalence of recoverable dysfunction is
probably underestimated because the completeness of
revascularisation is seldom assessed,45 and because
segments with advanced morphological changes46 may
take up to one year to recover function.
accuracy of echo in predicting recovery of segmental function after revascularisation
The reported accuracy of stress
echocardiography for predicting recovery of segmental
function after revascularisation varies, with sensitivities
of 70% to 85% and specificities of 80% to 90%. This variation
may, in part, reflect the operator-dependence of
the technique
NPV 70%
PPV 85 %
positive and negative predictive accuracies for
predicting recovery of segmental function after revascularisation in patients
with ischaemic left ventricular dysfunction. FOR NUCLEAR
PPV 75%
NPV 80%
