Download Magnetic resonance imaging anatomy of the prostate

Radiotherapy and Oncology 76 (2005) 99–106
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Educational review
Magnetic resonance imaging anatomy of the prostate and
periprostatic area: a guide for radiotherapists
Geert M. Villeirsa,*, Koenraad L.Verstraetea, Wilfried J. De Neveb, Gert O. De Meerleerb
a
Department of Radiology, Ghent University Hospital, Ghent, De Pintelaan 185, 9000 Gent, Belgium, bDepartment of Radiotherapy,
Ghent University Hospital, Ghent, De Pintelaan 185, 9000 Gent, Belgium
Abstract
Magnetic resonance imaging (MRI) offers superb soft tissue contrast on T2-weighted images and allows direct multiplanar
image acquisition. It can show the internal prostatic anatomy, prostatic margins, and the extent of prostatic tumors in much
more detail than computed tomography (CT) images. The present article reviews some key prostatic and periprostatic
radiologic landmarks that can be helpful for the radiotherapist using T2-weighted MRI as an adjunct to CT in treatment planning
for prostate cancer.
q 2005 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 76 (2005) 99–106.
Keywords: Prostate anatomy; Prostate cancer; Magnetic resonance imaging.
With the advent of conformal radiotherapy and especially
intensity modulated radiotherapy for prostate cancer, it has
become increasingly important to delineate the target
volume (prostate C/K seminal vesicles) as accurate as
possible. With computed tomography (CT), the delineated
volume is likely to be imprecise to a certain extent, because
of the low organ discriminating power based solely on
differences of attenuation coefficients and the restriction to
acquire images only in the transverse plane [25,28]. On the
other hand, magnetic resonance imaging (MRI) can demonstrate and characterise soft tissues by providing superb soft
tissue contrast on T2-weighted images, and by allowing
direct multiplanar image acquisition without loss of spatial
resolution [14]. MRI can therefore show in much more detail
the internal prostatic anatomy and prostatic margins, and
the extent of prostatic tumors, leading to more accurate
delineations of both prostate and critical structures, with
improved target coverage [6,15] and even definition of
subtargets within the prostate [18,23].
This article reviews some important prostatic and
periprostatic radiologic landmarks that can be helpful for
the radiotherapist using MRI in treatment planning for
prostate cancer.
Imaging technique
All images in the present study are taken from patients
examined in the treatment position, supine on a flat table
and legs gently bent on a knee-fix (Sinmed Kneefix cushion,
Cablon Medical, Leusden, The Netherlands), with the pelvis
positioned in the isocenter of the magnet [5]. An ankle-fix
(Sinmed) is additionally used to prevent rotation of the legs.
Immediately before the examination, a spasmolytic drug
(Visceralgine Fortew, Exel Pharma, Brussels, Belgium) is
administered intraveneously to avoid any bowel movement
that could degrade image quality and to enhance patient
tolerance for insertion and presence of an endorectal coil
during image acquisition. On a 1.5 Tesla MR scanner
(Magnetom Symphony, Siemens, Erlangen, Germany) fastT2-weighted images (TR/TE 4400/139; section thickness
4 mm with no interslice gap; field of view 28 cm; matrix,
512!179; bandwidth 100 Hz) are acquired in the transverse,
sagittal and coronal plane. In case an endorectal coil is
combined with a pelvic phased-array coil, at least one set of
fast-T2-weighted images (preferably three sets and always
including the transverse plane) is obtained before insertion
of the coil, because the latter may substantially deform the
prostate, making endorectal coil images useless for target
delineations. Subsequently, a balloon-covered expandable
endorectal coil (MRInnervu, Medrad, Pittsburgh, USA) may
be inflated with 60 ml of air, in order to obtain higher
resolution images and to perform MR-spectroscopy. A
volume of 60 ml is well tolerated and does not allow
movement of the coil within the rectum (unpublished
data). All scans are acquired with a moderately filled
bladder (the patient is instructed to drink 750 ml of water
within 1 h before, void 15 min before and drink an additional
250 ml immediately before image acquisition) and emptied
rectum (the patient is instructed to empty the rectum
15 min before image acquisition using a rectal laxative).
0167-8140/$ - see front matter q 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2005.06.015
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Magnetic resonance imaging anatomy of the prostate and periprostatic area: a guide for radiotherapists
Normal anatomy
Prostate gland
The prostate gland extends from the bladder base to the
urogenital diaphragm like an inverted pyramid and envelops
the prostatic urethra and the ejaculatory ducts. In young
men, the prostate can be divided into five zonal components
(Fig. 1): the nonglandular anterior fibromuscular stroma and
four glandular components: the peripheral zone (about 70% of
the glandular prostate), central zone (25%), transition zone
(5%) and periurethral glandular tissue (!1%) [20]. With aging,
the periurethral glandular tissue and the transition zone may
considerably hypertrophy, gradually compressing the central
zone and stretching the peripheral zone (Fig. 2). This
hyperplasia essentially does not involve the peripheral zone
and therefore only two areas are considered from a radiologic
point of view: the central gland (consisting of the hypertrophied periurethral glandular tissue and transition zone and
the compressed central zone) and the peripheral zone (Fig. 2)
[3]. The central gland usually consists of nodular areas of
varying signal intensity, depending on the relative amount
of glandular and stromal hyperplasia [13,24]. Glandular
hyperplasia contains relatively more ductal and acinar
elements and secretions, with resulting high signal intensity
on T2-weighted MR-images. Stromal hyperplasia contains
more muscular and fibrous elements, resulting in lower signal
intensity (Fig. 3). According to the origin of hyperplasia, the
terms ‘lateral lobe hyperplasia’ and ‘median lobe hyperplasia’ may be used to denote hyperplasia of the transition zone
and of the periurethral glands, respectively. The compressed
central zone (also called ‘surgical pseudocapsule’) may be
imperceptible or visible as a faint dark rim, separating the
central gland from the peripheral zone (Fig. 4). The latter,
containing numerous ductal and acinar elements with
sparsely interwoven smooth muscle, is normally of high signal
intensity and is surrounded by a dark fibromuscular rim
representing the prostatic capsule [3]. At the level of the
apex, the prostate gland merely consists of high signal
intensity peripheral zone tissue (around the distal prostatic
urethra). The ratio of peripheral zone to central gland tissue
then gradually decreases upward to the prostatic base, at
which level the prostate gland almost entirely consists of
mixed signal intensity central gland tissue (Figs. 1 and 4).
Urethra
The prostatic urethra traverses the prostate gland from
the bladder neck to the apex. The proximal (also called
preprostatic) part lies within the central gland and is
generally imperceptible on transverse MR-images. At the
level of the veru montanum, midway between the prostatic
base and apex and at the verge of the central gland and
peripheral zone, it curves 35 degrees anterocaudally into the
distal (also called prostatic) part (Fig. 5a), that is generally
visible as a small black dash surrounded by high signal
intensity peripheral zone tissue (Fig. 6). At the prostatic
apex, the distal urethra becomes surrounded by a low signal
intensity cone of striated muscle (external urethral sphincter) extending downward to the penile bulb (Figs. 5b–6). This
area has traditionally been called the ‘urogenital diaphragm’, although the concept of a true diaphragm
(transversely oriented muscle covered by a superior and
inferior fascia) has been matter of debate [21,22].
Seminal vesicles and ejaculatory ducts
Fig. 1. Zonal anatomy of the prostate: (A) The urethra is represented
in yellow and the seminal vesicles and ejaculatory ducts in blue. The
periurethral glands (pale green stripe) extend from the bladder base
to the veru montanum in the submucosal layer of the proximal
prostatic urethra. The transition zone (dark green body) consists of
two independent lobes extending from the bladder neck anterosuperiorly toward the verumontanum posterolaterally; (B) the
central zone (orange) bounds on the posterior surface of the
transition zone and proximal urethra and encloses the ejaculatory
ducts. The periurethral glands, transition zone and central zone are
jointly called ‘central gland’; (C) the peripheral zone (red) is the
most posterolateral glandular component of the prostate. The ratio
of peripheral zone to central gland tissue gradually decreases
upward from apex to base (D) Anteriorly, the prostate is covered by a
thick non-glandular anterior fibromuscular stroma (brown).
The seminal vesicles are paired grapelike pouches filled
with high signal intensity fluid (Fig. 7). They lay caudolateral
to the corresponding deferent duct and are placed between
the bladder and the rectum. Their position may be variable,
being directed upward to the level of the ureteral
termination, or backward around the anterolateral aspect
of the rectum. Their size may vary, depending among others
on age and postejaculatory condition [9]. The caudal tip of
each seminal vesicle joins the corresponding deferent duct
to form the ejaculatory duct, which is enveloped in a thick
low signal intensity muscular coat and traverses the central
zone of the prostate to terminate at the veru montanum
(Fig. 8).
Deferent ducts
The deferent ducts traverse the inguinal canal and enter
the pelvic cavity between the peritoneum and the lateral
wall of the pelvis. They cross the ureter and then bend
G.M. Villeirs et al. / Radiotherapy and Oncology 76 (2005) 99–106
101
Fig. 2. Central gland changes in prostatic hyperplasia (transverse plane). (a) Normal prostate in 34-year old man: compact central gland (CG) and
broad peripheral zone (*); (b) hypertrophied central gland in 62-year old man. RZrectum (with endorectal coil in (a), not in (b)); arrowheadsZ
prostatic capsule; LZlevator ani muscle.
downward to adjoin the medial side of the corresponding
seminal vesicles. They are thin tubular structures with low
signal intensity and should not be confused as part of the
seminal vesicles (Fig. 9).
Other periprostatic structures
The prostate is surrounded by a thin and firmly adherent
nonglandular fibromuscular band (‘prostatic capsule’), that
is continuous internally with the stromal septa subdividing
the glandular tissue within the peripheral zone and
externally with the periprostatic connective tissue [1]. It is
usually visible as a sharply demarcated dark rim at the
posterolateral aspects of the prostate on T2-weighted MRimages (Figs. 2 and 4). Only at the prostatic apex and base,
this demarcation is less distinct. At the apex, the
fibromuscular band blends with the adjoining external
urethral sphincter and surrounding fibrous tissue (Fig. 5b)
and occasionally becomes sparsely intermingled with loose
glandular elements from the apical peripheral zone [3]. At
the prostatic base, the fibromuscular band blends imperceptibly with the bladder musculature and therefore cannot
be differentiated from the low signal intensity bladder wall
[3]. In prostatic hyperplasia causing protrusion of hypertrophied tissue at the bladder base, the bladder wall is
elongated and hardly visible (Fig. 10). Anteriorly, the
prostate is covered by the thick anterior fibromuscular
stroma, which contains no glandular elements and has low
signal intensity (Figs. 1 and 4). It is separated from the pubic
Fig. 3. Central gland (CG) changes in prostatic hyperplasia (transverse plane). Images at prostatic base from two different patients. (a) Stromal
hyperplasia: predominantly low signal intensity; (b) Glandular hyperplasia: predominantly high signal intensity areas (*). BLZbladder;
RZrectum (with endorectal coil in (b), not in (a)).
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Magnetic resonance imaging anatomy of the prostate and periprostatic area: a guide for radiotherapists
Fig. 4. Normal prostate from apex to base (transverse plane). (a) The apex consists of the distal part of the prostatic urethra (white arrow)
surrounded by high signal intensity peripheral zone tissue (*); (b) At the midprostate, the mixed signal intensity central gland (CG) is
posterolaterally surrounded by high signal-intensity peripheral zone tissue (*), subdivided by several stromal septa (horizontal black arrows).
The peripheral zone is surrounded by a dark fibromuscular rim representing the prostatic capsule (white arrowheads). Note the anterior
fibromuscular stroma (AFS), the compressed central zone (black arrowheads, not to be confused with the true prostatic capsule) and the
neurovascular bundles (vertical black arrows); (c) The prostatic base is composed almost entirely of central gland (CG) tissue, with only a narrow
posterior band of peripheral zone (*). BLZbladder; RZrectum (with endorectal coil).
symphysis by Santorini’s venous plexus (draining the dorsal
veins of the penis) and some ligamentous and fibroadipose
tissue in Retzius’ space (Fig. 6) [21].
The prostate rests within a muscular funnel composed of
the anterior aspects of the levator ani (caudally) and
obturator internus (cranially) muscles (Figs. 2, 5, 6 and 10)
[21]. The levator ani muscle is a broad muscular sheet
extending from pubis to coccyx and forming most of the
pelvic floor. Its thickest part (inferomedially) embraces
the lower half of the prostate (so-called ‘levator prostatae’)
and the external urethral sphincter, and the thinner
superolateral part attaches to a tendinous arc on the medial
surface of the obturator internus muscle, which embraces
the upper half of the prostate [21]. The prostatic capsule is
separated from this muscular funnel by loose connective and
adipose tissue containing the periprostatic venous plexus
intermixed with arteries, nerves and lymphatics. At the
posterolateral aspects of the prostate, some of these vessels
Fig. 5. Prostatic urethra. (a) Sagittal view of the urethra (arrowheads) traversing the prostate from base to apex. Note the anterocaudal bend at
the level of the veru montanum (white arrow); (b) Coronal image through the prostatic apex, the distal urethra becomes surrounded by the low
signal intensity external urethral sphincter (*) that extends downward to the penile bulb (PB) and is embraced by the inferomedial aspect of the
levator ani muscle (L). BLZbladder.
G.M. Villeirs et al. / Radiotherapy and Oncology 76 (2005) 99–106
103
Fig. 6. Prostatic urethra (transverse views). (a) At the prostatic apex, the distal urethra (arrowhead) is surrounded by a low signal intensity
muscular ring (arrows). Also note Santorini’s plexus (*); (b) One centimetre cranial to (a) the urethra (arrowhead) is visible as a small vertical
dash. LZlevator ani muscle; RZrectum.
are jointly called the neurovascular bundles, containing
cavernous nerve fibres that are important to normal erectile
function (Fig. 4) [3].
Prostate cancer
Prostate cancer tissue usually has low signal intensity on
T2-weighted images. Since 70% of all prostate carcinomas
arise in the peripheral zone [19], many of them can be
readily detected within the high signal intensity background
of normal peripheral zone tissue (Fig. 11a). This sign,
however, is by no means specific, since chronic prostatitis,
hemorrhage, scar tissue, etc. can produce similar
findings [27]. On the other hand, hormonal ablation may
decrease the signal intensity of the normal peripheral zone,
thus reducing the visibility of low signal-intensity prostate
cancer [2]. Tumors arising in the central gland may also be
indistinct, especially when low signal intensity stromal
hyperplasia predominates (Fig. 3b). Capsular perforation
can be suspected in the presence of irregular bulging or
disruption of the prostatic capsule, obliteration of the
rectoprostatic angle, or asymmetry of the neurovascular
bundle [30]. Furthermore, an abnormally low signal intensity
within the vesicular lumen or a focal thickening of the
seminal vesicle wall is suggestive of seminal vesicle invasion
(Fig. 11b) [11]. In a recent meta-analysis, Engelbrecht et al.
calculated a joint maximum sensitivity and specificity of 71%
for overall tumor staging (cT2 versus cT3 differentiation),
64% for detection of extracapsular extension and 82% for
detection of seminal vesicle invasion [8].
Newer techniques that aim at improving this accuracy are
currently under investigation. Magnetic resonance spectroscopy (MRS) at 1.5 Tesla provides metabolic information
Fig. 7. Seminal vesicles. (a) Transverse plane and (b) coronal plane. The seminal vesicles (SV) are high signal-intensity fluid-filled pouches with a
thin low signal-intensity wall, arranged in a grapelike pattern. BLZbladder; RZrectum (with endorectal coil); PZprostate.
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Magnetic resonance imaging anatomy of the prostate and periprostatic area: a guide for radiotherapists
Fig. 8. Ejaculatory ducts (transverse views). (a) At the midprostate, the ejaculatory ducts join the prostatic urethra at the veru montanum,
which is usually visible as a high signal intensity structure (arrowhead). (b) At the prostatic base, the ejaculatory ducts (arrows) traverse the
central zone and are enveloped by a low signal intensity muscular ring. BLZbladder; RZrectum (with endorectal coil).
in addition to morphologic data and can draw the attention to
areas that seem less important morphologically [16].
Metabolites of importance are citrate (produced in normal
epithelial cells and secreted into the prostatic ducts), choline
and related compounds (important precursors in the phospholipid cell membrane synthesis and more prevalent in
tumor tissue) and creatine (involved in the cellular energy
metabolism and also more prevalent in tumor tissue). A
(cholineCcreatine)/citrate ratio O1 is considered suggestive of malignancy, and a correlation between the magnitude
of this ratio and Gleason grade has been suggested [29, 31].
This may be an interesting issue for further investigation,
because a strong correlation between Gleason grade and
probability of local relapse after radiation therapy has
already been demonstrated [10]. So far, accuracies of up to
88% have been reported for tumor lateralization (presence of
tumor in the right or left prostate half) and a more accurate
prostate cancer tumor volume measurement is possible with
combined MRI and MRS [4,26,29]. MRS on higher field strength
machines (3 Tesla) is an exciting new opportunity that is
currently under investigation.
Another imaging modality that aims at achieving a higher
accuracy for tumor detection is dynamic contrast-enhanced
T1-weighted MR-imaging (dMRI). After intravenous bolus
injection of a gadolinium-containing contrast agent, tumor
areas in the prostate enhance earlier and faster on the basis
of tumor neoangiogenesis and thus increased microvascular
density as compared to normal prostate tissue or benign
prostatic hyperplasia [7,12]. Preliminary studies have shown
promising results, but further research is needed to clarify
the exact role of dMRI and to assess its accuracy in prostate
cancer detection and staging [8].
Fig. 9. Deferent ducts. (a) Coronal view of the low signal-intensity deferent ducts (arrowheads) proceeding along the craniomedial side of the
corresponding seminal vesicles (SV). (b) Transverse view of the deferent ducts at the medial side of the seminal vesicle, between the bladder
and the rectum. BLZbladder; RZrectum.
G.M. Villeirs et al. / Radiotherapy and Oncology 76 (2005) 99–106
105
Fig. 10. Periprostatic structures (coronal views). (a) The prostate (P) rests within a muscular funnel composed of the anterior portions of the
levator ani (L) and internal obturator (IO) muscles. Loose connective and adipose tissue containing the periprostatic venous plexus and
neurovascular components (*) separate these muscles from the prostatic capsule. Caudally, the apex is separated from the penile bulb (B) by the
so-called urogenital diaphragm (UGD), containing the external urethral sphincter. Also note the penile cavernous corpora (C), that are attached
to the ischiopubic rami (I); (b) In prostatic hyperplasia protruding at the bladder base (arrowheads), the bladder wall is elongated and hardly
visible.
Use of MRI in radiotherapy planning
MRI can be used to improve treatment planning for
prostate carcinoma by providing information that not only
helps to more accurately delineate the prostate and seminal
vesicles, but also to define a subtarget within the prostate
that can be treated to a higher dose.
As opposed to CT, MRI can identify superb anatomical
landmarks for clinical target volume (CTV) delineations by
offering a clear-cut depiction of the prostatic capsule
posterolaterally (separating the CTV from the rectum and
levator ani muscle), the fibromuscular stroma anteriorly
(separating the CTV from Retzius’ space) and the transition
from high signal-intensity normal peripheral zone to low
signal-intensity fibromuscular tissue caudally (separating
the CTV from the urogenital diaphragm). These boundaries
can be employed indirectly by using MRI-derived information (on hardcopy) to improve the delineation accuracy
on CT-images (on the workstation) or by direct delineation
on the MR-images (on the workstation), using image
segmentation and image registration or correlation to
account for the lack of tissue electron density values on
MRI [14,17].
Fig. 11. Prostate cancer. (a) Transverse view of low signal-intensity tumor (*) within high signal-intensity peripheral zone tissue. (b) Coronal view
of abnormal low signal-intensity lesion (arrows) within the lumen of the left seminal vesicle (SV), indicating seminal vesicle invasion. RZrectum
(with endorectal coil); BLZbladder.
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Magnetic resonance imaging anatomy of the prostate and periprostatic area: a guide for radiotherapists
Furthermore, intraprostatic lesions detected with MRI,
with combined MRI and MRS, or with combined MRI and dMRI
can additionally be delineated as a subtarget, potentially
benefiting from the administration of a focused higher
radiation dose.
*
Corresponding author. Geert M. Villeirs, Genitourinary Radiology, Ghent University Hospital, De Pintelaan 185, 9000 Gent,
Belgium. E-mail address: [email protected]
Received 25 May 2004; received in revised form 20 May 2005;
accepted 5 June 2005; available online 14 July 2005
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