| MRI of the Pituitary |
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4.2
Clinical indications for MRI of the pituitary The
indications for a MRI of the pituitary include:
(Westbrook,
1999) 4.3
Image interpretation of pituitary fossa MRI As
for brain imaging, the principles mentioned in section 3.3 can be applied
to pituitary MRI. It is also
important to look closely to post-contrast images for areas of enhancement
or non-enhancement in the pituitary gland.
As stated in section 3.4.7 the pituitary gland normally enhances
after Gd-DTPA administration, however, microadenomas are usually
hypointense to the normally enhancing pituitary gland (Chong and Newton,
1993). 4.4
Image optimisation of the pituitary fossa 4.4.1
Equipment Same
equipment as stated in section 3.4.1 (MRI of the Brain) 4.4.2
Image quality The
ideal MRI parameters include a balance between spatial resolution,
contrast resolution and scan time. Images
should be obtained in the appropriate plane with the highest possible SNR,
contrast spatial resolution and shortest time possible. The
pituitary fossa is a relatively small structure, which makes the
identification of microadenomas even more difficult.
Therefore, the use of thin slices (2-3mm) is essential since
spatial resolution is very important in pituitary fossa MRI. As spatial resolution is improved the SNR is decreased.
This effect is minimised by using interleaved thin slices and the
smallest FOV possible. Moreover,
a fine matrix used in conjunction with a high NEX value is also important
in keeping a high SNR value (Chong and Newton, 1993; Westbrook, 1999). 4.4.3
Routine pituitary MRI The
routine pituitary examination at St. Luke’s Hospital includes the
following sequences. At
present, no protocol has been established. ·
3-planar T2* FGRE ·
Axial PD &T2 FSE (whole brain) ·
Coronal T2 FSE (fine cuts – 3mm) ·
Coronal T1 SE (fine cuts – 3mm) ·
Sagittal T1 SE (fine cuts – 3mm) ·
Coronal T1 SE + Gd-DTPA (fine cuts – 3mm) ·
Sagittal T1 SE + Gd-DTPA (fine cuts – 3mm) The
pituitary examination starts with the localiser and followed by a dual
echo FSE axial sequence (more information in section 3.4.3). This is usually performed as general brain assessment, which
is performed at short scan times. In
follow-up investigations this sequence is usually omitted.
All T2-weighted images are done using a FSE sequence due to the
long TR needed. All the
remaining series are performed using SE.
In FSE the scan time is less since the number of phase encodings is
less, however, the spatial resolution is also less when compared to SE.
This effect is undesired in the pituitary where detail is essential
(Westbrook and Kaut, 1993). The
coronal plane is optimal for imaging the sellar and parasellar structures
as it avoids the partial voluming effects from the carotid arteries,
sphenoid sinus and the suprasellar cistern.
The sagittal images are used to assess the midline structures.
High spatial detail resolution requires the use of thin slices
(2-3-mm), a fine matrix (256 x 256) and a small FOV (16 to 20 cm).
Good SNR is used by using 2 to 4 NEX.
However, the likelihood of patient motion is increased with
increased NEX (Chong and Newton, 1993). Westbrook,
1999 recommends the use of volume acquisitions as they allow for thinner
slices with no gap. Also, as
anatomical detail and contrast enhancement are important, an incoherent
(spoiled) GRE sequence is required. However,
3D imaging is prone to motion and truncation artefacts in the two
dimensions that are phase encoded (Chong and Newton, 1993). 4.4.3.1
Fast imaging techniques FSE
reduce the overall scan time by a factor of 5 without changing the TR,
matrix or the NEX. However,
these time saving benefits do have costs evident in the contrast and
spatial resolution of the resulting images.
Since one is now using multiple echo times to generate a single
image, the contrast results can be mixed (Woodward, 2001).
4.4.4
Contrast usage in pituitary fossa MRI Paramagnet
contrast agents as Gd-DTPA enhance areas in which the blood-brain barrier
is absent, not well developed or where it has been rendered incompetent by
tumour or inflammatory process. The
pituitary gland, infundibulum, median eminence, tuber cinereum, cavernous
sinus and the nasopharyngeal mucosa normally enhance (Chong and Newton,
1993). Westbrook (1999)
states that contrast is not routinely required except for diabetis
insipidus and hypothalamic disorders as opposed to the sequence used in
our imaging center (St. Luke’s Hospital) were contrast is used for all
routine pituitary examinations. Westbrook
(1999) also states that studies have shown that a half-dose Gd-DTPA is
optimal for imaging the pituitary gland.
It is common to see a high signal in the posterior lobe of the
pituitary on unenhanced images, especially in patients with diabetis, the
cause of which is as yet undiscovered.
Westbrook (1999) states contrast is sometimes necessary for
Cushing’s disease because microadenomas are often very small, and not
seen on unenhanced scans. However,
it should be noted that eventually all pituitary gland enhances including
the micoadenoma itself. Therefore,
careful timing of post-contrast scan is essential. For this purpose Miki
(1997) the use of dynamic MRI is recommended. 4.4.4.1
Dynamic MRI In
dynamic MRI, successive images with short acquisition time are performed
before and after the rapid injection of contrast.
A SE with short TR or FSE is the recommended sequences for this
imaging technique as GRE is prone to magnetic susceptibility artefacts.
On dynamic MRI, adenomas reach peak enhancement later than the
normal anterior gland due to their sluggish circulation although many
enhance before the anterior due to direct arterial circulation.
By this difference in enhancement patterns between adenomas and the
anterior pituitary, were best contrast can be achieved 1-2 minutes after
injection, the overall sensitivity to micoadenoma detection is of 80-90% (Elster,
1994). Dynamic
MRI is also useful in the assessment of the normal pituitary gland
displaced by a macroadenoma. This
may help surgeons in avoiding post-operative hypopituitarism.
Multiple slices covering the entire tumour should be achieved
because the location of the normal gland may be unpredictable on the
precontrast images. (Miki et al, 1990). A
half-dose Gd-DTPA is also recommended in dynamic MRI of the pituitary.
A better contrast between the pituitary and the cavernous sinuses
may be achieved. Moreover, a
half-dose shortens the time for injection, which is also very practical in
dynamic studies (Miki and Asato, 1991). 4.4.5
Artefacts in pituitary MRI Since
the pituitary fossa is located just anterior and inferior to the circle of
Willis, and therefore flow motion artefacts may be generated. Moreover, since a smaller FOV is selected in pituitary MRI,
the likelihood of aliasing artefacts is increased.
Therefore, oversampling is necessary if anatomy is present outside
the FOV, in the phase direction. Spatial
presaturation bands are placed superior, inferior, left and right of the
FOV to reduce aliasing and flow-artefacts.
Unfortunately, this increases the specific absorption rate (SAR)
and the slice number per TR decreases (Westbrook, 1999). Further still, Miki (1997) states that the application of
these flow compensation techniques increases the TE, which may result in
increase in magnetic susceptibility artefact. Pituitary
imaging is also vulnerable to magnetic susceptibility artefacts, because
bone and air surround it anteriorly, posteriorly and inferiorly.
Therefore, a SE sequence is preferred to GE sequence although the
artefacts cannot be completely eliminated (Elster, 1993). The chemical shift artefact along the readout gradient on MRIs displaces fat. The default setting of the readout gradient in most MR systems is in the superior-inferior direction, where the fat is displaced superiorly. Therefore, the fatty marrow in the dorsum sellae may overlap and obscure the high signal posterior pituitary on sagittal images. To avoid this overlapping, the readout gradient can be set so that the fat is moved posterior on the sagittal MR images. This technique is particularly important when germinoma is clinically suspected, as it may involve the neurohypophysis and because early diagnosis can improve patient life expectancy. On the coronal images, fatty marrow of the sphenoid bone can overlap the inferior aspect of the pituitary by an upward displacement due to chemical shift, which might conceal a tiny adenoma in the anterior and inferior portion of the pituitary gland. The readout gradient on the coronal imaging can be set so that fat is moved inferiorly. This technique, which constantly, displays the inferior aspect of the anterior pituitary, is also important for precise measurement of the height of the pituitary (Sato, Ishikaza, Matsumoto, Matsubara, Tsushima and Tomioka, 1991).
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