Power Doppler, SWI, Spectroscopy The document discusses various ultrasound and MRI techniques including Doppler shift, aliasing, power Doppler, color Doppler, MRI spectroscopy, susceptibility weighted imaging (SWI), and their clinical applications. It provides detailed explanations of key concepts such as Doppler shift, aliasing, metabolites measured by MRS, magnetic susceptibility effects, and specific SWI signs seen in conditions like Parkinson's disease, multiple sclerosis, PML, brain tumors, and pyogenic abscesses.

1. Power Doppler, SWI,
Spectroscopy
Dr Himanshu

2. Doppler Shift

3. Doppler Shift
• Doppler shift or Doppler effect is defined as the change in frequency of sound wave
due to a reflector moving towards or away from an object, which in the case of
ultrasound is the transducer.
• When sound of a given frequency is discharged and subsequently reflected from a
source that is not in motion, the frequency of the returning sound waves will equal
the frequency at which they were emitted.
• However, if the reflecting source is in motion either toward or away from the
emitting source (e.g. an ultrasound transducer) the frequency of the sound waves
received will be higher (positive Doppler shift ) or lower (negative Doppler shift) than
the frequency at which they were emitted, respectively.

4. A
B
C

5. Aliasing
• Aliasing is a phenomenon inherent to Doppler
modalities which utilise intermittent sampling in
which an insufficient sampling rate results in an
inability to record direction and velocity accurately.
• Unlike continuous wave Doppler, pulsed wave and
color flow Doppler modalities alternate between
rapid emission of ultrasound waves (at a rate
termed the pulse repetition frequency/PRF) and
reception of incident ultrasound waves.
• The ultrasound machine will only record returning
echoes during a certain interval. If Doppler shifts
occur at a frequency exceeding the maximum
pulse interval (1/pulse repetition frequency)
detected phase shifts will be calculated based on
incorrect assumptions.

6. Power Doppler
• Is independent of velocity and direction of flow, so there is no possibility of signal aliasing.
• Is independent of angle of insonation.
• It has higher sensitivity than color Doppler.
• In contrast to color Doppler, where noise may appear in the image as any color, power
Doppler permits noise to be assigned to a homogeneous background color that does not
greatly interfere with the image, permitting higher effective gain settings and increased
sensitivity for low detection
A: Color Doppler B : Power Doppler

7. MRI Spectroscopy
❖ It is the use of magnetic resonance in quantification of metabolites
and the study of their distribution in different tissues.
❖ Rather than displaying MRI proton signals on a gray scale as an image,
depending on its relative signal strength, MRS displays the quantities
as a spectrum.
❖ The resonance frequency of each metabolites is represented on a graph
and expressed as parts per million (ppm).
❖ There are numerous metabolites found in brain. Fortunately, only several
of them are useful in spectroscopic studies. MRS has been a powerful
research tool and provide additional clinical information for several
diseases such as brain tumors, metabolic disorders and systemic
disease.

8. ight to left along the x-axis and the y-axis (height) is the degree of chemical shift as expressed in

9. Major Metabolites in Brain
N- acetylaspartate
• NAA is the marker of neuronal density and viability.
• It is present in both gray and white matter.
• It has the largest signal/tallest peak in normal adult brain spectrum in MRS.
• Its concentration appears to decrease with any brain insults such as infection, ischemic injury,
neoplasm and demyelination process.
• NAA is not found in tumors outside the CNS such as in meningioma.
• It is markedly elevated in Canavan Disease.

10. Choline
• It is the precursor of acetylcholine and phosphatidylcholine.
• Acetylcholine is an important neurotransmitter and the latter is an integral part of cell membrane synthesis.
• Disease processes affecting the cell membrane and myelin can lead to the release of phosphatidylcholine.
• Thus elevation of choline can be seen during ischemic injury, neoplasm or acute demyelination diseases.
• It is low or absent in toxoplasmosis whereas it is elevated in lymphoma, helping to distinguish between the
two.
Creatine
• Acts as a reservoir for the generation of ATP.
• Reduced Creatine levels may be seen in pathological processes such as neoplasm, ischemic injury,
infection or some systemic diseases.
• Most metastatic tumors to the brain do not produce creatine since they do not possess creatine kinase.

11. Lactate
• Lactate levels in the brain normally are very low or absent. When oxygen supply is
depleted, the brain switches to anaerobic respiration producing lactate. Therefore,
elevated lactate peak sign is a sign of hypoxic injury.
• Low oxygen supply can result from decreased oxygen supply or increased oxygen
requirement.
• The former may be seen in vascular insults, or hypoventilation and the latter may be
seen in neoplastic tissue.
Myo-inositol
• It is glucose like metabolite and it is involved primarily in hormone-sensitive
neuroreception. It is found mainly in astrocytes and helps to regulate cell volume.
• Elevated level would be seen where there is glial cell proliferation as in gliosis.
• It is markedly reduced in Hepatic encephalopathy (which also shows reduce choline
but increased glutamine)

12. Lipids
❖ Lipids are incorporated into cell membranes and myelin.
❖ Lipid peak should not be seen unless there is destructive process of brain including necrosis,
inflammation or infection.
❖ Tubercular abscess show increased lipid peak.

13. Magnetic susceptibility
❖ Signal/Image usually in MRI comes from H+ in H2O.
❖ We make an assumption that in MRI, the external
magnetic that we apply to the patient is homogenous.
❖ But in reality the compounds that have paramagnetic,
diamagnetic, and ferromagnetic properties all interact
with the local magnetic field distorting it and thus
altering the phase of local tissue which, in turn, results
in a change of signal.

14. ❖ Paramagnetic substances : They
include oxygen and ions of
various metals like iron,
magnesium and gadolinium.
These ions have unpaired
electrons, resulting in a positive
magnetic susceptibility
❖ The effect on MRI is an increase
in the T1 and T2 relaxation rates
(decrease in the T1 and T2
times).

15. ❖ Diamagnetism is the property of
materials that have no intrinsic atomic
magnetic moment, but when placed in a
magnetic field weakly repel the field,
resulting in a small negative magnetic
susceptibility.
❖ Materials like water, copper, nitrogen,
barium sulfate, and most tissues are
diamagnetic.
❖ Only cause subtle distortion in magnetic
field compared to significant distortion
caused by paramagnetic substances.

16. ❖ It should be noted that calcium atoms, in isolation, as
they have paired outer-shell electrons, are
paramagnetic. However, when calcium is mixed with
many other atoms as is the case in physiological
calcifications, the result is a diamagnetic substance,
which is useful in allowing calcifications to be
distinguished from blood products that are
paramagnetic on susceptibility weighted imaging

17. ❖ Ferromagnetic materials generally contain iron, nickel, or cobalt. These
materials include magnets, and various objects that might be found in
a patient, such as aneurysm clips, parts of pacemakers, shrapnel, etc.
❖ These materials have a large positive magnetic susceptibility and
remain magnetised when an external magnetic field is removed.
❖ Superparamagnetic materials consist of individual domains of
elements that have ferromagnetic properties in bulk. Their magnetic
susceptibility is between that of ferromagnetic and paramagnetic
materials.
❖ Examples of superparamagnetic materials include iron-containing
contrast agents for bowel, liver, and lymph node imaging.

18. Susceptibility Weighted Imaging
(SWI)
❖ SWI is a 3D high-spatial-resolution fully velocity corrected gradient-
echo MRI sequence.
❖ Conventional T2-weighted imaging typically uses spin-echo
sequences that minimize susceptibility artifacts (even with fast
implementations) because of repetitive refocusing of 180° pulses.
❖ However, T2*-weighted, SWI, or SWI-like sequences purposely
enhance the effect of local field variations caused by tissue content
such as blood products (ie, hemosiderin in CMB), iron content (often
in the form of ferritin), calcium content, and deoxyhemoglobin in
venous blood. These processes cause local variations in the
magnetic field that lead to signal loss in the form of T2*.

19. ❖ Following the acquisition, post-processing takes place which includes a
high-pass filter, to remove background inhomogeneity of the magnetic field,
and the application of a phase map to accentuate the directly observed
signal loss.
❖ The most common use of SWI is for the identification of small amounts of
haemorrhage/blood products and calcium, both of which may be inapparent
on other MRI sequences.
❖ Distinguishing between calcification and blood products is not possible on
the post-processed SWI images as both demonstrate signal drop out and
blooming.
❖ The filtered phase images are, however, able to distinguish between the
two as diamagnetic and paramagnetic compounds will affect phase
differently (i.e. veins/haemorrhage and calcification will appear of opposite
signal intensity)

20. ❖ This is, however, not without its own complications as
whether a lesion appears black or white on phase
imaging depends on Handedness of the system
How to determine handedness ?
In right-handed system, veins looks dark on phase images because it is paramagnetic
relative to surrounding tissues. Meanwhile, calcium looks bright on phase images
because it is diamagnetic relative to surrounding tissues ( vice versa in left handed
system)
Generally, the internal cerebral veins are readily identified. If the patient has pineal or
choroid calcification this can also be helpful.

21. ❖ Now look at lesion ?
❖ If it is the same as veins it is paramagnetic and
therefore contains blood products. If it is the opposite,
then it will be diamagnetic and therefore most likely
dystrophic calcification.

23. ❖ Initially, SWI and related sequences were mostly used
to improve the depiction of findings already known from
standard two-dimensional T2*-weighted neuroimaging:
more microbleeds in patients who are aging or with
dementia or mild brain trauma; increased conspicuity of
superficial siderosis in Alzheimer disease and amyloid
angiopathy; and iron deposition in neurodegenerative
diseases or abnormal vascular structures, such as
capillary telangiectasia.
❖ But SWI also helps to identify findings not visible on
standard T2*-weighted images

24. 1. Nigrosome 1 Sign in Parkinson Disease, Atypical
Parkinsonian Syndromes, and Dementia with Lewy
bodies
The N1 territory is located at the posterior part of the
substantia nigra and characterised by a high signal intensity
on high-spatial-resolution SWI-like sequences, flanked by
two hypointense linear regions, which resembles a swallow
tail.
Because of the neurodegeneration, the hyperintense high-
contrast spot of the N1 disappears, and only a single black
region remains.

26. 2. The central vessel sign and peripheral rim sign in multiple sclerosis
Imaging criteria in MS rely on the location of T2-weighted hyperintense
lesions and contrast-enhanced lesions, which have important roles in
diagnostic criteria.
In some cases of patients suspected of having multiple sclerosis, notably in
somewhat older patients with cardiovascular risk factors, it may be difficult
to discriminate vascular-ischemic from demyelinating lesions. The central
vessel sign is characteristically found in multiple sclerosis lesions by
findings that show venous structures as a linear hypointensity on SWI scans
in the centre of the lesion.
A cut-off greater than 45% in brain lesions with central vessel sign was
suggested to discriminate multiple sclerosis from vascular (or other) lesions.

29. 3. Hypointense gyriform rim in PML
On axial FLAIR images (A–C),
multifocal PML lesions are
observed in both frontal lobes,
the capsula interna and externa,
and the right parietal lobe. A
linear, relatively thin
hypointense rim is observed on
the cortical side of the lesions
on axial SWI (D–F) in a long-
term survivor.
One possible explanation for the
specific location of SWI findings
is the greater iron content in
subcortical fibers and high
density of iron-rich
oligodendrocytes

30. 4. Intratumoral susceptibility signals in neoplasms
In brain tumors, SWI findings can reveal features that remain
undepicted at conventional MRI, referred to as intratumoral
susceptibility signals. These are linear or dot-like intratumoral
areas of low signal on susceptibility images, most likely
related to intratumoral microhemorrhage, calcification, and
neovascularization
Intratumoral susceptibility signals occur in a high percentage
of high-grade gliomas but are mostly absent in lymphoma

32. 5. Dual rim sign in pyogenic abcess
6. SWI also has many other applications like detecting microbleeds in
mild traumatic brain injury, detecting vascular malformations, venous
sinus thrombosis, neonatal haemorrhages, metabolic disorders.