Yonago Acta medica 1996;39:183–187

Dynamic Study of the Tracheal Cavity by Fast MRI with Coughing inPatients with Tracheomalacia
Yuji Suto, Shuji Sugihara, Kotaro Yoshida, Takashi Kato, Masayuki Kamba andYoshio Ohta
Department of Radiology, Faculty of Medicine, Tottori University, Yonago 683, Japan
Fast magnetic resonance imaging (MRI) with coughing was performed in 3 patientswith tracheomalacia (TM) and 10 healthy volunteers, and its usefulness was evaluated.
Twenty serial images were obtained using two-dimensional-turbo fast low angle shot(FLASH) sequences and the image data were transferred to a graphic workstation
connected on-line. The maximum and minimum cross-sectional areas of the trachea
were measured during deep breathing (forced expiration and inspiration) and duringcoughing, and the collapsibility index (CI) = [(maximum cross-sectional area – minimum
cross-sectional area)/maximum cross-sectional area] × 100 (%) was calculated. In thehealthy volunteers, CI was 36 ± 15% (mean ± SD) during forced expiration andinspiration and 48 ± 15% during coughing, and the difference was not significant. Inthe TM patients, it was 68 ± 6% and 85 ± 5%, respectively, with the value duringcoughing being significantly higher (P = 0.019). CI was significantly higher in the TM
patients than in the normal volunteers both during forced expiration and inspiration(P = 0.006) and during coughing (P = 0.002). Fast MRI with coughing may facilitate theevaluation of the tracheal collapsibility in TM.
Key words: dynamic study; fast MR imaging; trachea; tracheomalacia

Tracheomalacia (TM) is defined as marked
respiratory collapse of the trachea and main
bronchi due to their increased collapsibility(Baxter and Dunbar, 1963). This condition isoccasionally misdiagnosed as bronchial asthma
because of stridor or neuropathy due to a suspicion that there is laryngeal obstruction and occasional syncope (Kwong et al., 1993). Occurrence of marked collapse of the trachea in TM
patients during coughing has been known(Johnson et al., 1973). With the recent introduction of fast MRI with a scanning time of
0.2 s or less, dynamic imaging of mobile partsof the body has become possible (Jaeger et al.,1995). In this study, we examined the usefulness of fast MRI with coughing in the evaluation of TM.

The subjects were 10 healthy volunteers and 3
patients who were suspected of having TM on
the basis of clinical symptoms and chest X-ray
findings, and were definitively diagnosed bybronchoscopy with coughing. They consistedof 8 males and 5 females and ranged in age from17 to 68 years (mean 48 years). The clinicalbackground and the data of spirograms of the 3patients with TM are summarized in Table 1.
All subjects gave written consent to participate
in the studies after full explanation of its nature.

Abbreviations: CI, collapsibility index; CT, computed
tomography; FLASH, fast low angle shot; FOV, fieldof view; MRI, magnetic resonance imaging; TE, echotime; TI, inversion time; TM, tracheomalacia; TR,repetition time

Imaging procedure
MRI was performed using a 1.5 Tesla superconducting MR system (Magnetom Vision,Siemens, Erlangen, Germany) using a body coil.

Subjects and Methods

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The subjects were placed in the supine position,and transaxial images perpendicular to theposterior wall of the trachea were obtained bytwo-dimensional-turbo-FLASH sequences. The
scanning level was fixed at 1 cm below the upper margin of the aortic arch, and serial images
were obtained 10 min after the administrationof Gd-DTPA. Scanning parameters were as follows: repetition time (TR), 3.5 ms; echo time(TE), 1.7 ms; inversion time (TI), 2 ms; flipangle (FA), 8˚; field of view (FOV), 250 × 250
mm; imaging matrix, 128 × 128; slice thickness, 6 mm; signal averaging, 1; and temporal
resolution, 10 images per 2 s. Twenty serial
images were obtained.
Analysis of MR images
The image data acquired were transferred toa graphic workstation (Titan II, KGT, Tokyo,Japan) connected on-line with the MRI system,and the software used for imaging processing
was an application visualization system (AVS:AVS Inc., Waltham, MA).
Basic evaluation: A phantom experiment
was performed to adjust the image display conditions incorporated in the graphic work station.
The signal intensity of MRI was normalized to
1-byte data (0–255), and signal intensity levels
were determined. The maximum intensity wasdefined as level 255, and the minimum intensity

as level 0, and signals of level 0 to level 50 weredisplayed as images. The cross-sectional areasof the lumens of 5–55 cc injectors with known
cross-sectional areas were measured under theabove image display conditions, and the accuracyof this technique was tested. A regression linewith a correlation coefficient of 0.987 was obtained.
The maximum and minimum cross-sectional
areas of the tracheal lumen were measured in
the subjects under the image display conditions
tested by this phantom experiment.
Clinical evaluation: The maximum andminimum cross-sectional areas of the tracheallumen were measured during forced expirationand inspiration and during coughing. The collapsibility index (CI) was calculated as [(maximumcross-sectional area – minimum cross-sectionalarea)/maximum cross-sectional area] × 100(%). Differences in CI in the normal volunteers
and TM patients between the period of forced
expiration and inspiration and the period ofcoughing were examined by Student’s t-test. A
P value of less than 0.05 was regarded as significant.
Results
Among the 10 normal volunteers, CI (mean ±SD) was 36 ± 15 % (10–63%) during forcedTable 1. The clinical background and the data of spirograms in the patients with tracheomalacia
1Case number23

Age(year)
4467
68SexMaleMaleFemale
Smoking history
1 pack/day × 20 years 1 pack/day × 45 years
Nonsmoker
VC( L)
2.97
3.39
1.87%VC(%)
95.5
112.6
83.5
FVC( L)
2.52
3.05
1.55%FVC(%)
86.0
101.3
69.2
FEV1.0( L)
1.25
1.41
0.92
FEV1.0%
(%)
49.6
46.2
59.4
Cardiorespiratory disease Asthma
Asthma, hypertension
Asthma, angina pectoris
FEV1.0 , forced expivatory volume in one second; FEV1.0%, FEV 1.0/FVC × 100; FVC, forced vital capacity;%FVC, FVC percent of predicted; VC, vital capacity; %VC, VC percent of predicted.

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Fig. 1. A 44-year-old male with tracheomalacia. a:During forced inspiration. b: During forced expiration. c: During coughing. Narrowing of the trachea(arrow) is clearer during forced expiration than during forced inspiration but is even more notable during coughing.

Discussion

expiration and inspiration and 48 ± 15% (25–83%) during coughing with no significant
difference. Among the TM patients, it was 68 ±
6 (63–75%) during forced expiration andinspiration and 85 ± 5% (80–90%) duringcoughing, with the value during coughing being
significantly higher (P = 0.019). CI was
significantly higher for the TM patients than for
the normal volunteers during both forced
expiration and inspiration (P = 0.006) andcoughing (P = 0.002).

A definitive diagnosis of TM has been made by
dynamic radiography of the trachea (Johnson etal., 1973 ), bronchoscopy with coughing (Ikedaet al., 1992), or computed tomography (CT)(Gamsu and Webb, 1982, 1983; Vock et al., 1984;
Stern et al., 1993; Kwong et al., 1993; Trigaux etal., 1994).
Dynamic radiography has a high temporal
resolution but, unlike tomography, causes overlapping of images. There is also the problem ofradiation exposure.
Bronchoscopy with coughing is an invasive
examination and requires tedious preparation
including local anesthesia. Also, as the bronchoscope is a single-eye device, measurement of
the depth is impossible, and quantitative evaluation based on objective measurement of theimages is difficult (Ikeda et al., 1992).
In many reports using dynamic CT, thetomographic plane and level were fixed, andserial images of the trachea were obtained fromforced expiration to forced inspiration (Kimura185

Y. Suto et al.

Fig. 2. A 34-year-old healthy volunteer. a: During forced expiration. b: During coughing. No marked
difference is observed in the cross-sectional area of the trachea (arrow) during forced expiration as comparedwith during coughing.

et al., 1990; Stern et al., 1993; Trigaux et al.,1994). CT of the trachea during coughing has
not been reported. As demonstrated in thisstudy, tracheal narrowing during forced expiration and inspiration is less notable than thatduring coughing. Therefore, collapsed lesions
may be masked in scanning only during forced
expiration and inspiration (Ikeda et al., 1992).
Also, CT is appropriate for measurement of thetracheal diameter (Vock et al., 1984; Stern et al.,1993), but it has the problems of X-ray exposureand limitation of the tomographic plane to thetransverse view (Suto et al., 1993).
Fast MRI has excellent temporal resolutionin addition to advantages such as the freedom of
selection of the imaging plane, high contrast
resolution, and minimum invasiveness (Suto etal., 1993). In this study, the scanning time was
0.2 s per scan, and images could be obtained
without blurring even during coughing.
In this study using fast MRI, the difference
between the collapsibility during forced expiration and inspiration and that during coughing was not significant in the normal volunteers
but was significant in the TM patients (Figs. 1and 2). CI was significantly higher in the TM
patients than in the normal volunteers both dur-ing forced expiration and inspiration and duringcoughing.
These findings indicate that marked expiratory collapse may be induced by coughing in
TM patients due to weakness of the trachea
unlike normal individuals. Also, fast MRI withcoughing may reveal slight differences between
TM patients and normal individuals that escapedetection by MRI during forced expiration andinspiration.
In this paper, a dynamic study was performed in the same transverse plane as CT because,in the coronal plane, which allows evaluation of
the entire length of the trachea, the trachea may
be displaced out of the scanning area due toanteroposterior movement of the thorax if thinslices are used, and because saber sheath typelesions such as in Case 1 are difficult toevaluate in the sagittal plane. Therefore, by ourpresent method of dynamic MRI studies using
the transverse plane, similar to the CT studies,
we don’t make the best use of the freedom ofselection of the scanning plane permitted in
MRI, and further evaluation may be needed considering technical aspects such as the scanning
procedure and selection of the scanning plane.
Furthermore, how quantitative data such as

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those of the CI used in this study are related tovarious clinical data of TM remains unknown
because of the insufficiency of the number ofcases. Further accumulation of cases may beneeded to solve these problems.
In conclusion, fast MRI with coughing is expected to facilitate evaluation of the collapsibility of the trachea in TM.

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(Received August 19, Accepted September 14, 1996)

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