Key words
breast - technology assessment - MR imaging
Introduction
Magnetic resonance imaging (MRI) has become an accepted method in breast imaging,
joining mammography and ultrasound. Several indications are widely accepted for its
use, like screening women with a high risk of breast cancer, occult primary breast
cancer or searching for recurrent disease in inconclusive mammography and ultrasound
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]. The role of MRI in the preoperative staging of tumor extent is still under debate
[10]
[11]. MRI image analysis is based on lesion enhancement patterns in dynamic breast MRI
and on morphologic changes [12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]. With these criteria, breast MRI has a sensitivity of about 85 % to 99 % in detecting
malignant breast lesions, as has been shown in several studies [1]
[2]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]. Recently, the implementation of MRI exams in regular screening settings has been
discussed as well [19]
[25]
[26]. If MRI of the breast is to be used under screening conditions, the shortening of
protocols would be preferable. In non-fat-suppressed dynamic breast imaging, it is
a well-accepted recommendation to acquire data at or close to echo times that fulfill
the in-phase condition for fat and water, such as 4.8 ms at 1.5 Tesla, in order to
avoid chemical shift artifacts that lead to signal cancellation at fat/water interfaces.
This effect was described and analyzed, e. g., by Bedrosian et al.[1] and Fischer et al. [1]
[4]
[27] with the recommendation of using either in-phase echo time (TE) or “TE less than
1.2 ms” resulting in a phase difference of below 90° [28]. With otherwise comparable parameter settings, this would result in a decrease of
50 % of the acquisition time due to the resulting shorter repetition time (TR) of
approximately 8 ms vs. 4 ms. Meanwhile, with current gradient systems and fast imaging
sequences (now rapid changes of the magnetic gradient due to a better scanner geometry
and improved materials are possible and better coils lead to an improved signal) [28]
[29]
[30] as well as better software-based post-processing, it has become possible to achieve
such short echo times without compromising the matrix size or the bandwidth. In this
study, we have set up an interleaved protocol approach to achieve a direct comparison
of a minimum TE (minTE) vs. standard TE (nTE) acquisition within a clinical standard
protocol. The purpose of our study was to evaluate the image quality on non-fat-saturated,
subtracted T1-weighted images of the minTE acquisition compared to conventional standard
TE protocols (nTE) in patients with breast lesions. To our knowledge, this is the
first study in which the feasibility of minTE breast MRI has been evaluated in a prospective
manner.
Methods
Patients
The study was approved by the local institutional review board and all patients gave
written consent to the study and the MRI exam. The study complies with the Declaration
of Helsinki. The parameters of the study sequence ranged within the limits of breast
MRI performance guidelines [31]
[32].
From January 2014 to July 2014, 231 women underwent breast MRI at our institution
for various indications. A total of 144 women were randomly scheduled for the scanner
with the additional minTE sequences dependent on scanner slot availability. MRI indications
for these women were: to exclude preoperatively multifocal disease (n = 11) of histologically
verified breast cancer, positive family history for breast cancer (n = 40), to exclude
recurrent breast cancer (n = 39), monitoring neo-adjuvant chemotherapy (n = 2), unclear
findings in mammography (BI-RADS 0), ultrasound or clinical breast complaints (n = 47),
and cancer of unknown primary (n = 5).
MRI protocol
Data were acquired on a 1.5 Tesla MRI scanner (MAGNETOM Avanto, Siemens, Erlangen,
Germany) using a bilateral dedicated phased-array breast coil (4-channel breast array
coil, Siemens, Erlangen, Germany). Besides the dynamic T1 protocol, an axial 2 D T2-weighted
turbo spin echo (TSE) pulse sequence (TR/TE, 5250/113 ms, FOV 380 × 380 mm, matrix
512 × 358, slice thickness 3 mm, in-plane resolution 0.4 × 0.4 mm) and one pre- and
one post-dynamic contrast sagittal 3 D T1-weighted spoiled gradient echo pulse sequence
(TR/TE, 21/4.8 ms, FOV 380 × 380 mm, matrix 512 × 512, slice thickness 2 to 2.5 mm,
in-plane resolution 0.4 × 0.4 mm) for morphologic details were used. The dynamic sequence
was a bilateral, axial 3 D spoiled gradient echo pulse sequence (TR/TE, 7.3/4.8 ms,
FOV 340 × 340 mm, matrix 512 × 430, slice thickness 2 mm, in-plane resolution 0.8 × 0.8 mm)
and consisted of one pre- and four post-contrast repetitions of the whole volume of
both breasts. Contrast agent (Gadobutrol, Bayer, Leverkusen, Germany, 0.1 mmol per
kilogram body weight) was applied after the first dynamic acquisition with an MR-compatible
power injector (Spectris; Medrad, Pittsburgh, PA, USA) with a flow of 1 ml/sec followed
by a 20 ml saline flush.
In addition to the “normal” standard TE of 4.8 ms (nTE), a sequence with a “minimal”
TE of 1.2 ms (minTE) was used ([Table 1]). The very short TE of 1.2 ms was achieved by a gradient-echo sequence (VIBE) that
allows asymmetric readout windows. Thus, only an adjustable percentage of the gradient
echo is sampled. The clinical reference protocol with nTE was set up in a way that
all scan parameters were identical with the minTE protocol, except for the echo asymmetry
and the flip angle, which were adapted to the TR. The spatial resolution was 0.8 × 0.8 × 2 mm3 with a bandwidth of 445 Hz; parallel imaging was used (GRAPPA acceleration factor
of 2). This resulted in an acquisition time of 60 s for the nTE scans and 30 s for
the minTE scans, thus the resulting temporal resolution for both techniques was 90 s.
[Fig. 1] shows the interleaved scan protocol, which allows a direct comparison and [Fig. 2] shows an example of the complete protocol.
Table 1
Comparison of the technical specifications of nTE sequences and minTE sequences.
Tab. 1 Vergleich der technischen Spezifikationen der nTE und der minTE Sequenzen.
|
technical specifications
|
nTE (4.8 ms)
|
minTE (1.2 ms)
|
|
TR/TE
|
7.3/4.8 ms
|
3.6/1.2 ms
|
|
FOV
|
340 × 340
|
340 × 340
|
|
matrix
|
512 × 430
|
512 × 430
|
|
slice thickness
|
2 mm
|
2 mm
|
|
in-plane resolution
|
0.8 × 0.8 mm
|
0.8 × 0.8 mm
|
|
spatial resolution
|
0.8 × 0.8 × 2 mm3
|
0.8 × 0.8 × 2 mm3
|
|
flip angle
|
25
|
15
|
|
bandwidth
|
445 Hz
|
445 Hz
|
Fig. 1 Scan protocol of the dynamic gradient echo sequences with the interleaved dynamic
minTE sequences. After an unenhanced sequence with conventional sequence parameters
(TE 4.8 ms) and one with a minimal TE (1.2 ms), we injected contrast media and performed
four post-contrast conventional sequences. Each of these sequences was followed by
a sequence with a minimal TE, so both nTE and minTE sequences were measured alternately
during the whole dynamic scan.
Abb. 1 Scan-Protokoll der dynamischen Gradientenechosequenzen mit den zusätzlichen dynamischen
minTE-Sequenzen. Nach einer nativen Sequenz mit herkömmlichen Sequenzparameter (TE
4.8 ms) und einer mit einem minimalen TE (1,2 ms) injizierten wir Kontrastmittel.
Vier konventionelle post-KM Sequenzen wurden aquiriert. Jeder dieser Sequenzen folgte
eine Sequenz mit einer minimalen TE, beide Sequenzen (nTE und minTE) wurden abwechselnd
während der Kontrastmittelphase gemessen.
Fig. 2 MRI protocol with different sequences as well as the subtracted and non-subtracted
images of a 43-year-old woman with a 1.2 cm enhancing mass in the left breast. Histology
after breast-conserving surgery revealed a 1.2 cm invasive ductal carcinoma. nTE:
mass with round shape, circumscribed margins, homogeneous enhancement; minTE: mass
with round shape, circumscribed margins, homogeneous enhancement.
Abb. 2 MRT Protokoll mit den unterschiedlichen Sequenzen und den subtrahierten und nicht
subtrahierten Bildern einer 43 Jahre alten Frau mit einer 1,2 cm großen Raumforderung
in der linken Brust. Histologisch ergab sich nach brusterhaltender Operation ein 1,2 cm
invasives duktales Karzinom. nTE: glatt begrenzter, runder Knoten, homogene KM-Aufnahme;
minTE: glatt begrenzter, runder Knoten, homogene KM-Aufnahme.
Post-processing included subtractions of the second post-enhanced sequence from the
sequence without contrast medium using the scanner software.
Image analysis
The breast MRI images were evaluated in consensus by one senior breast radiologist
with more than 10 years of experience in breast MRI and one radiologist with 2 years
of experience in breast and MR imaging. For the comparison of the standard TE sequence
(nTE) and the minimum TE sequence (minTE), only examinations with identifiable enhancing
lesions in the contrast-enhanced sequences were chosen. The study evaluation was performed
on the second post-enhanced sequence. Dignity of the lesions was either confirmed
by core or excisional biopsy or on the basis of the clinical examination, ultrasound,
mammography and MR findings.
-
Image quality was assessed on standard PACS workstations on images in the axial plane.
For each examination the regions of interest (ROI) were placed in the enhancing lesion,
in the pectoral muscle and in the air between the right and left breast. The mean
signal intensity and the standard deviation were obtained for each measurement. Standard
deviation of the air was considered as image noise. Signal-to-noise ratio (SNR = mean
density/standard deviation of image noise) and contrast-to-noise ratio (CNR = (mean
density of the lesion – mean density of the pectoral muscle)/standard deviation of
image noise) were calculated for the ROIs of the enhancing lesions and the pectoral
muscle in the native and in the second post-contrast sequence of both the nTE and
minTE dynamic series.
-
The lesion size (longitudinal and transverse diameter) was measured in millimeters.
-
The subjective confidence of the minTE sequences was evaluated on a subjective 3-point
scale before looking at the nTE sequences (1 = very sure that I can identify a lesion
and classify it, 2 = quite sure that I can identify a lesion and classify it, 3 = definitely
want to see nTE for final assessment).
-
The subjective image quality of all examinations (nTE and minTE sequences) was evaluated
using a subjective four-grade scale (1 = sharp, 2 = slight blur, 3 = moderate blur
and 4 = severe blur/not evaluable) for lesion (= lesion surrounding) and skin sharpness
(= skin evaluation).
-
Multifocal disease was detected on both sequences (more than one suspicious lesion:
yes or no).
-
To describe single lesions, we used the ACR BI-RADS® Atlas (fifth edition 2013). Initially we distinguished the lesions between focus
(less than 5 mm in diameter) and mass/non-mass enhancement. In masses, the shape (oval,
round, irregular), the margins (circumscribed, not circumscribed (irregular/spiculated))
and the internal enhancement patterns (homogeneous, heterogeneous, rim enhancement,
dark internal septations) were evaluated. In non-mass enhancement we checked for the
distribution (focal, linear, segmental, regional, multiple regions, diffuse) and for
the internal enhancement patterns (homogeneous, heterogeneous, clumped, clustered
ring). Histologically proven multifocality was evaluated for both the nTE and the
minTE sequences.
Statistical analysis
Statistics were calculated using Excel 2007 (Microsoft, USA) and Graph-Pad Prism 4.03,
2005 (Graph-Pad Software, San Diego, CA). The difference in lesion surroundings and
skin evaluation was evaluated with the Wilcoxon signed rank test and the signal intensity
in both nTE and minTE sequences was evaluated with the paired t-test. A p-value < 0.05
was considered as significant, and a p-value < 0.01 was considered as highly significant.
Results
Patients
A total of 144 MRI exams with minTE sequences were performed. 121 patients did not
require histological interventions (78 patients with BI-RADS 1, 43 patients with BI-RADS
2). A total of 46 lesions were identified in the MRI examinations in 23 patients in
26 breasts.
MRI indications for these 23 patients were multifocal breast cancer (n = 11); family
history of breast cancer (n = 4); unclear findings in mammography (BI-RADS 0), ultrasound
or clinical breast complaints (n = 6); and exclusion of local recurrence (n = 2).
Histology revealed 32 malignant and 14 benign lesions ([Table 2]).
Table 2
Histology of the 46 lesions. 10 women had two or more suspicious lesions.
Tab. 2 Histologie der 46 Herdbefunde. Zehn Frauen hatten zwei oder mehr Läsionen.
|
Histology
|
IDC
|
ILC
|
DCIS
|
(fibrocy. dis.)
|
FA
|
atyp. prol.
|
Pap.
|
infl. cyst
|
|
Number of patients
|
22
|
4
|
6
|
7
|
4
|
1
|
1
|
1
|
|
13 women had a single lesion (4 IDC; 3 ILC, 1 DCIS; 3 fibrocystic disease, 1 papillomatosis
and 1 fibroadenoma)
|
|
2 patients had 2 lesions (patient a: 2x IDC; patient b: ILC + fibrocystic disease)
|
|
5 patients had 3 lesions (patient a: IDC + DCIS + fibroadenoma, patient b-d: 3 × IDC
and patient e: 2 × IDC + fibrocystic disease)
|
|
1 patient had 4 lesions (4 × IDC)
|
|
2 patients had 5 lesions (patient a: 2 × fibroadenoma, 1 × atypical epithelial proliferation,
1 × fibrocystic disease, 1 × inflammatory cyst; patient b: 4 × DCIS + fibrocystic
disease)
|
invasive ductal carcinoma (IDC), invasive lobular carcinoma (ILC), ductal carcinoma
in-situ (DCIS), fibrocystic disease (fibrocy. dis.), fibroadenoma (FA), atypical epithelial
proliferation (atyp. prol.), papillomatosis (pap.), inflammatory cyst (infl. cyst).
Comparison of patient images is presented in [Fig. 3], [4], [5], [6].
Fig. 3 Second post-contrast axial subtraction series of a 67-year-old woman with a 1.2 cm
enhancing mass lesion in the left breast. The lesion in the normal TE (4.8 ms) sequence
appears slightly sharper (thin arrow). Histology after breast-conserving surgery revealed
a 1.2 cm invasive ductal carcinoma. nTE: mass with irregular shape, not circumscribed
margins, heterogeneous enhancement, no adjacent vessel; minTE: mass with irregular
shape, not circumscribed margins, heterogeneous enhancement, no adjacent vessel.
Abb. 3 Axiales Subtraktionsbild einer 67-jährigen Frau mit einer 1,2 cm großen, KM-aufnehmenden
Raumforderung in der linken Brust. Die Läsion erscheint in der normalen TE (4.8 ms)
Sequenz etwas schärfer (dünner Pfeil). Die Histologie nach brusterhaltender Operation
ergab ein 1,2 cm invasives duktales Karzinom. nTE: irregulär begrenzter Knoten, nicht
glatt begrenzt, heterogene KM-Aufnahme, kein zuführendes Gefäß; minTE: irregulär begrenzter
Knoten, nicht glatt begrenzt, heterogene KM-Aufnahme, kein zuführendes Gefäß
Fig. 4 Second post-contrast axial subtraction series of a 34-year-old woman with a 1.1 cm
invasive ductal carcinoma in the right breast. The tumor-feeding vessel (thin arrow)
can be adequately delineated in the nTE and minTE sequence. nTE: mass with oval/irregular
shape, circumscribed margins, heterogeneous enhancement, adjacent vessel; minTE: mass
with oval/irregular shape, circumscribed margins, heterogeneous enhancement, adjacent
vessel.
Abb. 4 Axiales Subtraktionsbild einer 34-jährigen Frau mit einem 1,1 cm invasiven duktalen
Karzinom in der rechten Brust. Ein kräftiges, den Tumor versorgendes Gefäß (dünner
Pfeil) kann in der nTE und in der minTe Sequenz ausreichend abgegrenzt werden. nTE:
irregulär/ovalär begrenzter, umschriebener Knoten, heterogene KM-Aufnahme, zuführendes
Gefäß; minTE: irregulär/ovalär begrenzter, umschriebener Knoten, heterogene KM-Aufnahme,
zuführendes Gefäß.
Fig. 5 Second post-contrast subtraction series in axial orientation of a 24-year-old woman
with a strong family history for breast cancer. The 1.5 cm histologically verified
fibroadenoma in the left breast can be delineated equally in both sequences. The skin
appears a bit unsharp in the minTE sequence. nTE: mass with round shape, circumscribed
margins, homogeneous enhancement, no adjacent vessel; minTE: mass with round shape,
circumscribed margins, homogeneous enhancement, no adjacent vessel.
Abb. 5 Axiales Subtraktionsbild einer 24-jährigen Frau mit einer positiven Familienanamnese
für Brustkrebs. Das 1,5 cm große, histologisch nachgewiesene Fibroadenom in der linken
Brust kann gleichermaßen in beiden Sequenzen abgegrenzt werden. In der minTE Sequenz
wirkt die Haut ein wenig unschärfer. nTE: glatt begrenzter, runder Knoten, homogene
KM-Aufnahme, kein zuführendes Gefäß; minTE: glatt begrenzter, runder Knoten, homogene
KM-Aufnahme, kein zuführendes Gefäß.
Fig. 6 Second post-contrast subtraction series of a 32-year-old woman with multifocal breast
cancer – multiplanar reconstructions in axial, coronal and sagittal orientation. There
is an obvious lesion in the upper quadrants (invasive ductal carcinoma, thin arrow)
and another one in the lower quadrants (invasive ductal carcinoma, thick arrow). Between
these pathologies a third lesion with a size of 4 mm can be identified in the nTE
and minTE sequence (curved arrow).
Abb. 6 Axiales Subtraktionsbild einer 32-jährigen Frau mit multifokalen Brustkrebs – multiplanare
Rekonstruktionen in axialer, koronaler und sagittaler Ausrichtung. Es gibt eine Läsion
im oberen Quadranten (invasives duktales Karzinom, dünner Pfeil) und eine weitere
im unteren Quadranten (invasives duktales Karzinom, dicker Pfeil). Zwischen diesen
Pathologien kann eine dritte Läsion mit 4 mm in der nTE und in der minTE-Sequenz (gebogener
Pfeil) identifiziert werden.
The median age of all patients included in the study was 47.6 years (range: 22 to
72 years).
ROI measurements
The mean SNR of the lesions before contrast medium was 22.5 ± 9.1 in minTE and 23.3 ± 7.6
in nTE sequences (p = 0.6) and after contrast medium 42.5 ± 13.1 in minTE and 41.4 ± 14.4
in nTE sequences (p = 0.45). The mean CNR of the lesions was 42.8 ± 23.6 in minTE
and 47.6 ± 21.8 in nTE sequences (p = 0.17) ([Fig. 7]).
Fig. 7 a shows boxplots of the longitudinal and the transverse lesion size according to nTE
and minTE. In b boxplots of the SNR (before and after contrast medium) and the CNR are shown.
Abb. 7 a zeigt Boxplots des Längs- und Querdurchmessers der unterschiedlichen Sequenzen (nTE
vs. minTE) und b zeigt Boxplots der SNR (vor und nach Kontrastmittel) und der CNR.
Lesion size
The mean longitudinal diameter was 1.8 cm (range: 0.3 to 8.6 cm) and the transverse
diameter was 1.4 cm (range: 0.3 to 6.8 cm) in the nTE (4.8 ms) sequences. In the minTE
(1.2 ms) sequences, the mean longitudinal diameter was 1.8 cm (range: 0.3 to 8.4 cm)
and the transverse diameter was 1.4 cm (range: 0.3 to 6.9 cm). The difference was
not significant using the paired t-test (p = 0.96 for longitudinal diameter and p = 0.99
for transverse diameter).
Subjective confidence
With the minTE sequence, 26 lesions were rated with 1 = very sure that I can identify
a lesion and classify it and 20 lesions with 2 = quite sure. No lesion was rated with
3 = definitely want to see nTE sequences for final assessment.
Subjective image quality (lesion surroundings, skin evaluation) and multifocality
In minTE sequences, lesions and the skin were rated to be significantly more blurry
than in the normal TE sequences with a p-value of 0.0003 (lesions) and 0.047 (skin);
see [Table 3], [4]. Nevertheless, neither the skin nor the lesion was rated as having severe blurring
in nTE or in minTE sequences. There was no difference between the nTE and the minTE
sequences in the detection of histologically confirmed multifocal disease.
Table 3
Evaluation of the lesion borders.
Tab. 3 Beurteilung der Läsionsbegrenzung.
|
Lesion border
|
nTE (4.8 ms)
|
minTE (1.2 ms)
|
|
Sharp
|
26 [0.57]
|
12 [0.26]
|
|
Slight blur
|
19 [0.41]
|
24 [0.52]
|
|
Moderate blur
|
1 [0.02]
|
10 [0.22]
|
|
Severe blur
|
0 [0]
|
0 [0]
|
|
Number of lesions (n)
|
46
|
46
|
Using the Wilcoxon signed rank test, there was a significant difference between the
two types of sequences (p < 0.01). Relative frequency is shown in brackets [..].
Mit Hilfe des Wilcoxon Rangsummentest konnte ein signifikanter Unterschied festgestellt
werden (p < 0.01). Die relative Häufigkeit wird in den eckigen Klammern angegeben
[..].
Table 4
Evaluation of the skin.
Tab. 4 Beurteilung der Hautschärfe.
|
Skin
|
nTE (4.8 ms)
|
minTE (1.2 ms)
|
|
Sharp
|
22 [0.96]
|
13 [0.57]
|
|
Slight blur
|
1 [0.04]
|
10 [0.43]
|
|
Moderate blur
|
0 [0]
|
0 [0]
|
|
Severe blur
|
0 [0]
|
0 [0]
|
|
Number of patients (n)
|
23
|
23
|
Using the Wilcoxon signed rank test, there was a significant difference between the
two types of sequences (p < 0.05). Relative frequency is shown in brackets [..].
Mit Hilfe des Wilcoxon Rangsummentest konnte ein signifikanter Unterschied festgestellt
werden (p < 0.05). Die relative Häufigkeit wird in den eckigen Klammern angegeben
[..].
Lesion characterization
7 lesions were rated as a focus in nTE and in minTE sequences. No non-mass enhancement
was seen in both sequences. Therefore, 39 lesions were evaluated as mass enhancement.
In both sequences the shape of this mass enhancement was rated as round in 10 lesions,
as oval in 6 lesions and as irregular in 23 lesions. There was also no difference
in the evaluation of the margins (12 × circumscribed, 13 × not circumscribed (irregular),
14 × not circumscribed (spiculated)) and the internal enhancement (11 × homogenous,
25 × heterogeneous, 1 × rim enhancement, 2 × dark internal septations). Overall no
difference in lesion characterization was detected between nTE and minTE sequences.
Discussion and conclusion
Discussion and conclusion
In our study we compared the subtracted images of two different TE sequences without
fat saturation, a minTE sequence (TE 1.2 ms) and a normal TE sequence (TE 4.8 ms),
for the enhancement kinetics. The study suggests that visualizing breast disease and
its extent is feasible with a minTE MRI sequence. Compared to the classic approach,
there was no significant difference in the detection, morphology and size determination
of breast lesions. In our approach we used a TE of 1.2 ms, which reduced the scan
time per time point by about 50 % with no significant SNR or CNR changes of the lesions.
This short TE did not lead to false-negative or false-positive findings. As there
was no significant difference in the longitudinal and transverse lesion diameter,
the minTE sequence should not lead to a clinically important under- or overestimation
of the lesion size. The drawback of the minTE measurement is an increase in skin and
lesion blurring, which can be explained by the asymmetric readout that does not detect
the full echo signal. Therefore, there could be a loss of signal information. Opposed
phase cancellation might also play a role in small lesions [28]
[33]. Also in the literature ultrashort TE sequences acquire an echo signal from the
central to the outer parts of k-space with ramp sampling and therefore are very sensitive
to small k-space trajectory errors [34]. There was no significant difference in the detection of multifocal disease in minTE
and nTE. We were also able to detect tumor vessels in both sequences which is important
because the adjacent vessel sign was significantly associated with malignancy [35]
[36].
To our knowledge, no breast MRI study has dealt with short TE sequences before. There
were some studies that showed the feasibility of ultrashort TE sequences for the clinical
routine but, unlike our study, the TE time ranged between 0.07 ms and 0.14 ms. For
example, Robson et al. showed that contrast enhancement can be identified in tissues
using ultrashort TE pulse sequences (TE of 0.08 ms) [37] or the periosteum can be visualized with ultrashort echo time pulse sequences in
health and disease [38]. In addition, ultrashort TE pulse sequences provided anatomical detail not apparent
with conventional sequences and showed patterns of both increased and decreased enhancement
in tendinopathy [39]. However, in contrast to our study, they did not use a dynamic protocol. Yamashita
et al. demonstrated that with a 3.0 Tesla MRI scanner the middle ear ossicles could
be clearly visualized on short TE images, while they were not visible in long TE images
[40]. In contrast to this study, we used a 1.5 Tesla MRI scanner and were not able to
find a difference in lesion detection in breast imaging.
Recently, it was reported that the maximum curve slope in an ultrafast protocol performed
even better in the differentiation of benign and malignant lesions compared to BI-RADS
curve types [25]. This study reported that the initial phase of the enhancement curve was more important
compared to late-phase characteristics. Typically, such ultrafast protocols are realized
using view-sharing protocols that lead to temporal blurring, while in this study,
all time points are acquired separately. The minTE technique allows acquisition of
more time points in dynamic scans because minTE last 30 s and nTE last 60 s (therefore
two minTE sequences are acquired in the time it takes to acquire one nTE sequence).
Therefore, this could lead to more detailed dynamic curves and/or to a shortened protocol
so that one option for minTE application could be a short-scan protocol with few minTE
dynamic time points for lesion detection and to perform the dynamic early curve-phase
evaluation. This is a possible scenario for a screening population in which detection
is more important than detailed morphologic lesion characterization [26]. For more detailed morphologic information, a high-resolution non-dynamic sequence
could be added to the scan protocol. Mango et al. showed that an abbreviated breast
MRI protocol allows detection of breast carcinoma with only one pre- and post-contrast
T1-weighted sequence [41]. Whether a shortening of the examination time with minTE also shows this effect
needs to be clarified in a further study.
There are some limitations to our study. The number of individuals undergoing breast
MRI and showing a suspicious lesion was small. We only evaluated the subtracted images
and not the kinetic curves because the different TE sequences were acquired consecutively
and therefore would result in slightly different kinetic curves. The impact of more
measurements on the kinetic curve has to be evaluated in the future in a separate
study. Lesion detection, classification and time curves are not only dependent on
sequence parameters but also on the contrast medium used [42]. We used just one type of contrast medium because the comparison of different contrast
mediums was not part of the study. The diagnoses of suspicious lesions smaller than
5 mm were all made in breasts with another obvious pathological lesion. Opposed-phase
lesion cancellation might play a role in affecting curve shape in small lesions, but
this was not part of the study. Malignant lesions sometimes show central necrosis
and the rim of the lesion may be too thin for correct ROI placement. In our study
we evaluated one rim-enhancing cyst where it was impossible to settle a correct ROI,
but the enhancing cyst could be detected in both sequences, and it was rated as suspicious
in both sequences. Interestingly there was no non-mass enhancement in our study despite
some findings as DCIS usually shows non-mass enhancement. We can only speculate about
this finding. Maybe the number of patients was too small in this study. Nevertheless
no non-mass enhancement was seen in both sequences and so there was no difference
between these two sequences. We focused our study on the image quality of subtracted
images. Therefore, there could be a bias in the evaluation of the skin and lesion
surroundings due to motion artifacts. Nevertheless, motion artifacts were in both
sequences, and therefore should not be statistically relevant. Another limitation
was that a subjective scale was used for image analysis, but images were evaluated
in consensus by two breast radiologist.
In conclusion, dynamic breast MRI with a shorter TE time than the in-phase condition
(nTE) is possible. Shorter TE times increase the temporal resolution, which leads
to a better in-flow curve and might decrease the overall scan time.
-
Increase of the temporal resolution for a better in-flow curve
-
Dynamic breast MRI with a shorter TE time than the in-phase condition is possible
-
Possible decrease of the overall scan time
