Aim of this study is to validate the use of the pcASL sequence with 1.5 T scanner in the neuroradiological routine in order to obtain perfusion studies in patients who have contraindications to the administration of Gd contrast medium and therefore unable to be studied with the DSCE technique.
pCASL sequence (Pseudo Continuous Arterial Spin Labeling) is the evolution of the less recent PASL (pulsed asl) which was characterized by a pulsed labeling RF with consequent decay of the marked spin signal by virtue of T1 relaxation and consequent loss of SNR .
In pCASL,
labeling is obtained with a train of pulsed RF pulses of a total duration of about 1-2 sec and of the single duration of the msec order.
This results in an immediate double advantage: low SAR and higher signal to noise ratio.
Tip and triks
pCASL is usually performed at the end of the base sequences and before the administration of contrast medium.
It is important to focus on wich brain area should be to studied so that the acquisition package can be exactly positioned.
Crucial moment for a correct technical execution is the correct positioning of the spin labeling layer,
which must be perpendicular to the vessels belonging to the district studied in order to obtain the maximum signal.
To facilitate the correct positioning,
it is advisable to acquire a Phase Contrast survey sequence of the neck vessels in order to visualize exactly the course of the carotid and vertebral artery.
PLD (post label delay) is the time by wich labeled spins reach district to be studied and it's directly related to the presumed rate of blood flow in the carotid-vertebral district.
This speed is influenced by factors such as age and health or pathological conditions.
Currently,
the literature suggests setting the following values: 2000 msec in infants,
1500 msec in children,
1800 msec in healthy adults <70 years,
2000 msec in healthy adults> 70 years.
The most frequent risk is that the PLD is longer than the transit of the marked spins in the district under examination resulting in poor effectiveness of the investigation.
Label Distance: it is the parameter that regulates the distance of the spin labeling layer from the acquisition package,
the increase and reduction of this parameter depend on the anatomical region involved and on the anatomical characteristics of the patient dependent on intrinsic factors such as habitus constitutional. In our experience (adult patients) it is 90 mm.
Labeling duration. It depends on the intensity of the B0 field,
the type of ASL sequence and the expected flow rates.
Labeling total duration for 1.5T brain imaging,
is generally 1500 ms for pCASL (mutiple RF pulses of ms duration).
However,
this is a parameter intrinsic to the sequence and not modifiable.
Acquisition sequences.
In our experience we have used 2D echoplanars,
the latest generation are 3D GRASE or RARE that allow the acquisition of the package marked in a single "shot" with elimination of the artifacts from loss of signal in the cranial slices,
improvement of the SNR and reduction of the total duration of the sequence.
Artifacts in the pCASL
Areas of physiological hyper-perfusion. They can be located at the occipital level due to the activation of the visual cortex.
In the young and middle-age subjects,
a physiological hyper-perfusion pattern was also seen in the frontal position.
Areas of physiological hypoperfusion. They can be located at the peripheral areas of arterial vasculature.
Decay of the signal of the labeled spins. Signal decay in the labeled spins may result in reduced signal in the cranial slices of the acquisition packet.
Intravascular spin label. It is due to hyperintensity of intravascular spins and can give high-signal areas in ASL maps in Silvian fissure,
in the cisterns of the base or at the level of the dural sinus.
Magnetic susceptibility artifacts. Due to metallic devices,
calcifications,
hemoglobin degradation products,
surgical outcomes.
They are responsible for signal drop in the perfusion maps.
Effects dependent on gadolinium. Gadolinium,
resulting in a reduction of T1 in the enhancement areas,
minimizes the signal intensity differences between labeled spin and spin of control images,
generating erroneous areas of hypoperfusion.
This is the reason why pCASL must be acquired before any administration of contrast medium.
Motion artifacts. It is the most frequent type of artifacts,
especially in hospitalized patients.
Baseline magnetization. They occur in case of insufficient suppression of the fund.
Parameters influencing the sequence and technical optimization.
External magnetic field. High B0 magnetic field allows increase of the SNR,
this not means that 1.5 T scanners can not be used to perform this sequence.
Repetition time (TR). It must be long enough to allow a substantial relaxation of the previously marked spin among the various acquisitions: in our experience with 1.5 T scanner it is 4800 ms.
Eco Time (TE). TE must be used as low as possible to preserve the signal from the T2 / T2 * decay,
in our experience with a 1.5 T scanner it is 25 ms.
Spatial resolution.
We use matrices from 64x64 to 128x128 pixels,
with layer thicknesses ranging from 3 to 6 mm,
in our experience we use a 64x64 matrix with a thickness of 6 mm and a 3 mm reconstruction.
Repeated acquisitions. In 2D techniques,
between 30 and 50 signal acquisitions are required,
2-4 signal acquisitions are required in 3D techniques.
Crusher gradients. They are used exclusively in 3D pCASL.
Suppression of the fund.
The suppression of the background slightly reduces the ASL signal (~ 5%).
Duration of the sequence. In our experience is 6 m 33 sec.
Standardization. It is a process that is implemented on post processing to improve the quality of the images.
It may or may not be activated.
If activated,
a reference image is acquired (without the labeling of the spins) that will serve to normalize the post-marking sequence.
Use of multichannel phased array coils and parallel imaging.
Allow a further increase in SNR.
Post-processing.
The images generated by the sequence are in grayscale,
then it is possible to reconstruct chromatic maps quantitatively representing the CBF in mL/100 g.