With the increased use of minimally invasive parathyroidectomy (MIP), radiologists must provide optimal preoperative localization. Scintigraphic protocols should be tailored to the type of parathyroidectomy performed. This article reviews the anatomy of parathyroid adenomas, analyzes imaging strategies for unilateral neck exploration and MIP, and presents illustrative cases.
is an Assistant Professor, Department of Radiology/Nuclear
Medicine, Wake Forest University School of Medicine,
The utility of parathyroid scintigraphy has increased
dramatically in the past 25 years. Parathyroid scintigraphy was not
used for preoperative localization when bilateral neck exploration
for primary hyperparathyroidism was considered the standard of
care. The 95% cure rate associated with bilateral neck exploration
supported this standard.
At that time, many experts believed that finding an experienced
parathyroid surgeon was the only localization study needed prior to
However, considering that a solitary parathyroid adenoma is the
culprit in 85% of patients with primary hyperparathyroidism, a
bilateral neck exploration to identify all parathyroid glands is
often unnecessary. With the advent of the unilateral neck
exploration-first described in the early 1980s and commonly
performed beginning in the late 1990s-preoperative localization
with scintigraphy or ultrasound became commonplace. Localization of
an adenoma posterior to the right or left thyroid lobe, or in an
ectopic location, allowed for this directed surgery, reducing
potential morbidity and operative time.
Sensitivity of parathyroid scintigraphy for adenoma localization
has been reported at >90%.
More recently, endocrine surgeons are being trained to perform
an even less invasive surgery that is generally called minimally
invasive parathyroidectomy (MIP). Although techniques may differ
among surgeons, a recent survey showed that 59% of endocrine
surgeons are performing a minimally invasive procedure. This often
means an incision as small as 2 cm, placed for optimal
intraoperative localization with minimal dissection and palpation
required. Most of these procedures are performed under attended
local anesthesia and can be performed on an outpatient basis.
Figure 1 illustrates incision sites for both of these surgical
Accordingly, radiologists must provide optimal preoperative
localization for these less invasive surgeries. Scintigraphic
protocols should be tailored to the type of parathyroidectomy
performed. For example, many imaging strategies employed for
general lateralization prior to unilateral neck exploration are
insufficient for MIP. This article reviews the anatomy of
parathyroid adenomas, analyzes imaging strategies for unilateral
neck exploration and MIP, and presents illustrative cases.
Primary hyperparathyroidism and parathyroid
The most common cause of primary hyperparathyroidism in
ambulatory patients is a solitary parathyroid adenoma. Biochemical
evidence of this disease includes elevated serum calcium in the
face of normal or elevated parathyroid hormone levels. Most
commonly, parathyroid glands are found posterior to the thyroid
lobes, 2 in a superior location and 2 in an inferior location.
Most are located outside the thyroid capsule, or extracapsular.
However, parathyroid glands can be located beneath the thyroid
capsule in a subcapsular position. Additionally, they can be
entirely encased in thyroid parenchyma, or intrathyroidal. When
ectopic, parathyroid glands can be located from the carotid sheath
to the mediastinum to the inferior aspect of the heart.
Superior parathyroid glands tend to be found dorsal to the
mid-to-superior thyroid pole and in the retropharyngeal space
(Figure 2). When enlarged and heavy, superior parathyroid adenomas
can also descend inferiorly to the level of the inferior thyroid
pole and posteriorly into the tracheoesophageal groove. Inferior
parathyroid glands tend to be located dorsal to or caudal to the
inferior thyroid pole.
The distinction between the superior and inferior origin of a
parathyroid adenoma can be important, as some surgeons place their
incisions based on the origin of the vascular pedicle of an
adenoma, and not simply on the location of the adenoma.
Technetium-99m (Tc-99m) sestamibi and Tc-99m tetrofosmin are
commonly utilized tracers for dual-phase parathyroid scintigraphy.
These tracers localize in the thyroid gland as well as in
parathyroid adenomas. This makes correlation of the adenoma in
relation to the thyroid gland possible on planar as well as early
single-photon-emission computed to-mography (SPECT) imaging.
Differential washout of the radiopharmaceutical from the thyroid
allows for delayed imaging that often delineates a parathyroid
adenoma. Thyroid-specific imaging with Tc-99m pertechnetate and
I-123 can also be employed using a subtraction technique to
differentiate parathyroid from thyroid activity.
Imaging protocols should be adapted to the type of surgery
performed by endocrine surgeons at your center. Several imaging
goals that must be considered are discussed below (Table 1).
Screen for ectopic adenomas
All imaging protocols should contain anterior planar images from
the ears through the inferior border of the heart to screen for
ectopic adenomas, which can be located anywhere from the carotid
sheath to the inferior cardiac border.
Figure 3 shows an anterior planar image of the neck and chest.
Many imaging protocols utilize anterior images obtained with a
parallel-hole collimator. This image provides an overview of the
entire neck and chest. Further imaging with SPECT would be
necessary for optimal three-dimensional localization if an ectopic
adenoma is discovered on this image. If no ectopic adenoma is
identified, more focused images of the neck are required. The
perithyroidal region should be interrogated with converging and
lateral pinhole images for the most complete examination.
Unilateral neck exploration
Many surgeons perform unilateral neck exploration (UNE) based on
parathyroid scintigraphy directing the exploration posterior to the
right or left thyroid lobe. Parathyroid scintigraphy has been found
to have an approximate 90% lateralization success for this type of
In general, anterior images over the thyroid on a dual-phase
protocol are used for correct lateralization. Lateral pinhole
images over the neck are also recommended, as additional
information can often be gleaned from these images. For example,
the depth of the adenoma within the neck (eg, in the
tracheoesophageal groove) can be reliably demonstrated on these
Figure 4 presents a case of anterior images being insufficient
for correct lateralization, with lateral pinhole images
demonstrating the adenoma. Single-photon CT images can also provide
this information, although SPECT may be superfluous (and
time-consuming) if the lateral pinhole images are sufficient.
Minimally invasive parathyroidectomy
The decision to perform MIP is based on normal anatomic findings
in the neck: Inferior parathyroid glands are invariably located
ventral to the recurrent laryngeal nerve, which is rarely
anomalous. Therefore, the surgical incision for inferior and
superior parathyroid adenomas differs to accommodate optimal
intraoperative localization without crossing the anatomic plane
that this nerve delineates (Figure 2). This minimizes dissection
and reduces potential recurrent laryngeal nerve damage.
Additionally, dissection across the plane of the thyrothymic
ligament in this region can be difficult.
Superior and inferior adenomas can be found in various positions
in relation to the posterior aspect of the thyroid gland (Figure
2), as seen on lateral pinhole images. Interestingly, superior
parathyroid glands can fall posteriorly and inferiorly when
extracapsular and enlarged. Because these lie in the same axial
plane on planar images, these superior adenomas are often mistaken
for inferior parathyroid adenomas when anterior-only planar images
are obtained on scintigraphy.
Valuable information can often be gleaned if lateral pinhole
images are obtained, such as depth of the adenoma within the
anteroposterior plane. If lateral pinhole images localize the
adenoma adequately, SPECT imaging does not need to be performed.
What appears to be an inferior parathyroid adenoma on anterior
images turns out, on lateral images, to be a superior adenoma that
is displaced posteriorly and inferiorly (Figure 5). With the MIP
performed at our center, this inadvertent call leads to a larger
incision, increased dissection, longer operative time, and often
creates a need for general anesthesia.
Planar versus SPECT images
Many authors advocate the use of SPECT imaging for optimal
localization, particularly for localization within the
Several authors directly compared anterior planar views with SPECT
and found that SPECT further increases the sensitivity of
However, lateral pinhole images are not included in their
protocols. At our institution, the addition of 2 lateral pinhole
planar images over the region of the thyroid increases total
imaging time by 20 minutes. Compared with SPECT imaging at
approximately 45 minutes, we find that lateral images provide
anterior/posterior localization without the added imaging time,
cost, and the patient compliance issues related to the use of
However, if no adenomas are localized with early anterior and
lateral planar scintigraphy, immediate SPECT imaging should be
employed for possible identification of a small adenoma (whether
perithyroidal or ectopic), an adenoma that is being attenuated by
the tracheoesoph-ageal groove, or an occasional adenoma that is not
localizable without the improved contrast of SPECT. Poor or no
uptake in an adenoma on planar imaging is a common indication in
the literature for SPECT.
Early SPECT imaging, particularly before tracer has washed out of
the thyroid, allows for localization of an adenoma in relation to
the thyroid, which is useful for surgical planning.
Thyroid-specific radiopharmaceuticals, such as Tc-99m
pertechnetate and I-123, can delineate the thyroid parenchyma if
needed after dual-phase imaging. This is helpful as a second-line
"visual subtraction" method when no parathyroid adenoma is apparent
on dual-phase parathyroid scintigraphy.
Unilateral cervical exploration and minimally invasive
parathyroidectomy after preoperative localization with parathyroid
scintigraphy have replaced bilateral neck exploration for primary
hyperparathyroidism at many medical centers. Depending on the type
of surgery performed, scintigraphic imaging protocols can be
tailored for optimal preoperative localization. Screening for
ectopic adenomas with anterior planar images over the neck and
thorax is prudent. Planar images that include anterior converging
and lateral pinhole images over the thyroid provide lateralization
as well as anterior/posterior localization. Single-photon emission
CT is indicated if planar images are unrevealing.