Question

describe six different perfusion imaging techniques. State what they are, what they do, what they can be used for, the advantages over regular MRI, and the disadvantages of the technique.

Answer 1

Answer :

Perfusion MRI is a promising tool in assessing stroke, brain tumors and patients with neurodegenerative diseases. MRI has been used in the assessment of a variety of CNS abnormalities, including tumors, metastases, infections, vascular and degenarative diseases. Multiple studies have shown that the optimized use of high-quality contrast media in perfusion MRI can substantially ipmrove detection, charecterization and monitor of CNS diseases.

Perfusion is one of the most imporrant physiologic and pathophysiologic parametersand can be assessed non-invasively with MRI. Today we have several techniques to derive perfusion-related parameters using endogenous contrast methods.

Six types of perfusion imaging techniques are :

1. Positron emission tomography.
2. Single photon emission computed tomography.
3. Xenon-enhanced computed tomography.
4. Dynamic perfusion computed tomography.
5. MRI dynamic susceptibility contrast.
6. Arterial spin labelling.

1. Positron emission tomography ( PET) :

It is a scan is an imaging test that check for disease in our body. The sacn uses a special dye containing radio active tracers, The PET scan can meassured blood flow, oxygen use , how your body uses sugar much more.

What PET do ?

PET works by using a scanning device ( a machine with a large hole at its center) to detect photons, ( sub atomic particles) emitted by a radio nuclide in the organ or tissue by examined. The radionuclieds used in PET scans are made by attaching a radio active atom to chemical substancesthat are used natuarally by the particular organ or tissue during its metabolic process . For example in PET scans of the brain, a radio active atom is applied to glucose to create a radio nuclide called flurodeoxyglucose, because the brain uses glucose for its metabolism. Other substances may be used for PET scanning depending on the purpose of the scan

The radionuclide is administered into a vein through an intravenous line. Next the PET scanner slowly moves over the parts of the body being examined.positrons are emitted by the breakdown of the radionuclide.Gamma rays are created during the emission of positrons , and the scanner then detects the gamma rays. A computer analysis the gamma rays and uses theinformation to create an imagr map of the organ or tissue being studied,The amount of the radionuclide collected in the tissue affects how brightly the tissue appears on the image and indicates the level of organ or tissue function.

They can be used for :

• To diagnose dementias such as alzheimer's disease as well as other neurological conditions
• To locate the specific surgical site prior to sugrical proceduresof the brain.
• To evaluate the brain after trauma to detect blood clot, bleedind and / or perfusionof the brain tissue.
• To detect the spread of cancer to other part of the body fro the original cancer site.
• To evaluate the effectiveness of cancer treatment.
• To evaluate the perfusion to the myocardium as an aid in determine the usefulness of a therapeutic procedures to improve blood flow to the myocardium.
• To further identify lung leision or masses detected on chest X-ray and / or chest CT. etc...

Advantages of PET over MRI Scan

      Unlike CT or MRI scans, PET scans can meassure cellular-level metabolic changes occuring in an organ or tissue.This gives PET an advantage over other technologies in determining whether leisions in the body are benign or malignant, eliminating the need for surgical biopsies if PET scans are negative.

• Dis advantage : a PET scan is less accurate in certain situations : Slow growing, less active tumors may not absorb amuch tracer.
• Small tomores may not be detectable.
• High levels of blood sugarcan cause the cells to absorb this normal sugar rather than the radioactive injected kind.   

​​​​​​​2. Single photon emission commited tomography.( SPECT)

It is a nuclear medicine tomographic imaging technique using gamma race.

It provide 3D information.

What they can do ?

A radio labelled tracer like iodin 123, xenon- 133, flourine 18 etc..It emits gamma rays thet can be detected using CT scanner.

THey are used for :

• Used to view how blood flows through the artery and veins in the brain.
• It can detect reduce blood flow to the injury site.
• Use to dignose seizures, stroke, stress, fracture , infections, tumor in the spine.

​​​​​​​Advantages over MRI.

​​​​​​​MRI shows anatomy of structure of brain. Spect shows how the brain works, shows the area of brain that healthy, over active , under active.

Disadvantages :

Spacial resolution is limitted.

Difficulty in interpritting result in patient with 3 vessel disease and balanced myocardial ischemia.

3. Xenon enhanced Tomography

It is method used for neuro imaging. Subjects inhales xenon gas while CT images are made. The diffusion of gas into tissues show how much blood flow each areas getting.This method used to assess changes in cerebral blood flow in the period shortly after a traumatic brtain injury.

Used to detect or indicate the locatio of stroke.

Advantages over MRI :

• It is safe , non-invasive imaging technique.
• High anatomical spacial resolution,
• readily available instruments.

​​​​​​​Disadvantage :

Anaesthetic effects of xenon gas on patients

great expense for single examination

significant dose of radiation deliverd to the patients.

4. Dynamic perfusion computed tomography.

It is an imagic technique for assessing vascular supply and hemodynamics of brain tumors by messuaring blood flow, blood volume nd permiability.

Uses :

1.Grading brain tumors

2.distinguishing high grade gliomas from other brain leisions

3.differentiatiing true progression from post treatment effects.

4.predictng prognosis after treatments.

How they work ?

dyanamic acquisition of sequential CT sections in cine mode during the iv administration of iodinaised contrast method.

Advantages.

Allow the assessment of cerebral vascular auto regulation and ischemic penumbra.

Multi section dyanamic CT perfusion can used for accute cerebral ischemia over MR perfusion imaging.

Disadvantages :

Only a single or a few sections of perfusion can be acquired.

Application of radiation.

iodine contrast can leads to acute kidney injury.

5. MRI dyanamic susceptibility contrast.

This technique uses MRI contrast ( gadolinium) to induce reduction of T2 intensity while diffusing inside the tissues.

Uses:

• used in cerebral infraction the penumbra has decreasing perfusion.

amount of brain tissue that can be salvagable by thrombolysis or thromectomy can be identified.

used to get cerebral blood volume, blood flow, etc...

Used to find out glyoma.

Dis advantages :

• If there signal loss due to presesnce of calcification or blood products or due to artifacts from dense bone or aerated sinuses, the obtain value will not be reliable.

6. Arterial spin labelling

It is MR imaging technique that enable the meassurement of brain perfusion non-invasively at the tissue level.

Based on the principle of magnitically labeling in flowing arterial blood protons prior to the entry into the tissue of interest.

Uses :

to asses perfusion alteration both in acute and chronic phase.

used in arterio venous malformation and dural arterio venous fistulas.

very sensitive to detect even small degree of shunting.

used to detect epileptic focus

Disadvantages:

contra indicated in agadolinium allergy, renal impairements.

neurologic and psychiatric diseases display alternation on ASL images.